Sexual Selection and Parental Care


It is clear that in sexually reproducing species, we see both physiological and behavioral differences between the sexes. For example, in many bird species, the male is more brightly colored than the female, and in mammals, we see males possessing very large canine teeth, manes, or horns/antlers/tusks that females do not have.

Some of these differences between the sexes cannot be explained from the simple physiological necessities of successful reproduction (which could be explained by natural selection). For example, a male bird of paradise could father a successful clutch if his elaborate plume was plucked and a male red deer could sire many offspring if his antlers were sawn off. Why do males of so many species have elaborate characteristics?

Male Bird of Paradise video

These traits are adaptations in that they allow for gaining mating access to females; that is the plumes of a bird attract the female and the antlers of a deer allow him to fight for access to a harem of females. Over time, evolution has selected and exaggerated traits that already existed in a species, and made them more and more noticeable; however since the purpose of these exaggerations is to either attract or compete for a mate, even very differing sets of characteristics can be compared in terms of selective processes that produced the traits between species (the assumption that similar selective forces will result in similar adaptations). For example, the antlers of a red deer and the claws of a fiddler crab are essentially the same thing; that is, they are weapons that allows access to females because of male-male competition.

Male Fiddler Crab

Red Stag

In order to talk about the selective forces that are implicated in sex differences, we need to differentiate between natural selection and sexual selection.

Natural selection has to do with traits that organisms clearly need to survive and to reproduce as males or females. Testes, ovaries and associated plumbing are accepted as having evolved, at least initially, through natural selection. However, sex differences cannot be explained so easily and we must be careful to make a distinction between differences that have evolved through natural selection and those that have evolved through selection in relation to sex.

The distinction, although somewhat artificial is valuable. For example, a male crab may need claws large enough to hold a female during copulation. If some crabs copulate in rougher waters than others, their claws may need to be larger to hold on to the female. Those claws would be organs necessary for reproduction and would be the result of natural selection. However, some crabs, like the fiddler crab have claws which are far larger than necessary to clasp the female, and which may hamper their survival.

These sex differences are of interest because they do not have a ready explanation.

For example, what causes variation among species in the degree of enlargement of male claws?

Or, when only one sperm is necessary for an egg to be fertilized, why do the males of some species have a much larger sperm-producing capacity than their close relatives?

We might suspect that these are the males of species where the females need to fertilize more than one egg or where the male gets the opportunity to fertilize the eggs of more than one female. We would be wrong. If we were right, the differences between related species would follow as a result of what Darwin called natural selection, rather than what we call sexual selection.

Historically, sex differences arising from sexual selection have been considered to be of two types:

  1. First, differences can result from mating preferences: if females will mate only with males that have a particular characteristic such as bright colors, then males may evolve bright colors (mate choice).

  2. Second, differences can result from selection for success at combat among members of one sex to gain mating access to members of the other sex; if females are a limited resource, then males that can win fights for access to them will be the ones that leave offspring (combat).

  3. More recently, a number of sex differences have been ascribed to a more subtle form of combat involving sperm competition. Whenever females typically mate with several males, then those males might have evolved characteristics to help them win at fertilizing the eggs (sperm competition).


Which sex will diverge farthest from the natural selection optimum?

The answer depends on which sex is a more limited resource for reproduction by the other.

Often, especially in mammals, males provide very little for their young, and their rate of reproduction is limited only by the number of eggs they can fertilize.

The potential reproductive rate of females, on the other hand, tends to be limited by the number of eggs they can produce or by the number of young they can raise.

If females are a limited resource for males, Darwin argued that the consequent 'law of battle' among males was bound to favor the elaboration of behavioral displays and ornaments to attract females, as well as weapons and greater body size to fight with other males in order to gain access to females.

The evolution of sexually selected traits will then depend on genetic variation for precursors in the lineage and proceed to the point where the cost in terms of reduced viability is offset by the gains in reproductive success (adaptation).

Where females contribute little or no parental care relative to the male, and male care cannot be shared easily among the offspring of many females, males become a limiting resource. Females are thus more competitive, and typically evolve more elaborate courtship displays and weaponry. Well-known examples include phalaropes and some species of pipefishes, midwife toads, giant water bugs and bush crickets.

Male Wilson's Phalarope

Female Wilson's Phalarope

Male and female pipefish

Male midwife toad carrying eggs

male and female midwife toad mating (eggs deposited around males legs)

Male giant water bug with eggs glued to his back

Giant water bugs hatching

Great Green Bush Cricket (katydid)

Sexual selection can be important in sexually reproducing species, including plants, and it can have profound effects extending beyond reproduction. For example, if males evolve a larger size than females, we should expect them to have a different diet and generally to behave in ways that accord with their larger size. In other words, they will come to occupy a different ecological niche from females.

More in-depth exploration of mate choice, combat and sperm competition:

  1. Mate Choice:

    Do females prefer ornamentation simply for the fact of ornamentation? Males of many species spend a large amount of time displaying rather ornate characteristics, such as bright colors or long tails. Why have these evolved? Darwin suspected that such traits might appeal to an esthetic sense of females. Darwin, however, lacked experimental evidence for this supposition, although he did talk a pigeon fancier he knew into dyeing the feathers of male birds magenta. However, the female pigeons were not impressed, nor were other theorists in general impressed with the idea that females should prefer non-adaptive ornaments and displays.

    However, Semler and Andersson noticed that male three-spined sticklebacks in a Washington lake varied greatly in the extent of coloration on their throats; females were silver-brownish, and preferred to mate with red-throated males. They then painted dull-colored males with red lipstick and others with transparent lip gloss (controls). In lab preference tests, females preferred the red males, suggesting that female choice may be at least partly responsible for the evolution of red colors in male sticklebacks (although the red color may indicate something else and thus not simply be a preference for red by the female).

    male three-spined stickleback

    another pic of male three spined-stickle back

    Andersson then used the African long-tailed widowbird to test for female choice. The females are dull with short tails, while the males are black with red wing epaulets and tails measured about 50 cm. Anderssson cut the tails of some males and used the feathers to extend the tails of other males. The result was that the females nested in the territories of males with elongated tails at the expense of males with normal or shortened tails.

    African Widowbird

    Similar experiments have been performed on a variety of species. For example, in barn swallows, the females prefer the males with elongated tails and if the male they are paired with does not have a long tail, they will engage in extra- pair copulations with the long-tailed males.

    Barn Swallow

    Although female choice may be at least partly responsible for the evolution of elaborate male display traits, it is still not obvious why females prefer mating with such males. Several possibilities have been discussed in the literature, but at the moment, there is no consensus.

    1. R. A Fisher suggested that if females develop a slight preference for males of a particular sort, say with tails longer than average, then those males will father a disproportionate number of offspring and at the same time, the characteristic will become exaggerated.

      Fisher envisaged that females were selected to prefer males with longer tails because, originally, longer tails had a natural selection advantage, but that tail size would increase due to the mating advantage it conferred on sons until it reached beyond its optimum and had a natural selection disadvantage (this would occur only if there were no cost associated to the female with the choice of a longer tailed male) (this is known as runaway selection).

      Thus, females that mated with preferred males would produce offspring that inherited the genes for the preference from their mothers AND the genes for the attractive male characteristic from their fathers. Sons that expressed the preferred trait would have better fitness and daughters that respond to the trait would gain by producing sons who are preferred. Thus, female mate choice genes and genes for the preferred male trait can be inherited together. This could begin a runaway process in which more extreme female preferences and male traits spread together as new mutations occur.

      The runaway process would end only when natural selection against costly or risky displays balanced sexual selection in favor of traits that are preferred by females.

      Models of sexual selection have also demonstrated that there not need be any underlying selection advantage to the trait, but that females would simply have to demonstrate some kind of underlying sensory process that favored a particular sensory stimulus. Thus, the trait might be a disadvantage to the sex that has it, and the only advantage is that females prefer it (a color preference could have first evolved due to natural selection because of:

      • Differences in signaling to a predator; for example, some traits may signal to predators that this will be 'unprofitable prey' because of the animal's ability to run or hide, or they signal poison or a bad taste (e.g., the color of monarchs signals that they taste awful);
      • Color may develop unless it is prevented; those species for whom a color would be costly in terms of attracting predators may not develop them, while those for whom the cost is low do develop them and this may have nothing to do with mate choice at all. The corollary to this is that costly ornaments will not develop even if there is sexual selection if the costs outweigh the benefits.
      • It could be that a species has a preexisting bias in a sensory system due to some other pressure. For example, if in the past, a preferred food was highly colored, a preference that has nothing to do with mate choice might have developed. Some studies of frogs and swordtail fish have confirmed that females may have pre-existing preferences for traits which males of their own species may not possess. The implication is that if such traits were to arise, they may evolve rapidly by female choice, perhaps because initially their bearers are easy for females to detect and later, as the traits become costly, they indicate male quality.


    2. Others have argued that the displays indicated that their bearers possess 'good genes' of some kind.

      Zahavi suggested that if a male can survive with a viability handicap, such as a long tail, females should choose to mate with him, since he has demonstrated his vigor in the face of adversity. Although this idea was treated with a great deal of scorn for many years, it has gained increasing acceptance, and has produced some innovative spinoffs.

      For example, Hamilton and Zuk argued that parasitized birds cannot develop extravagant secondary sexual characteristics, thus if resistance to parasites is heritable, by choosing showy males as mates, females will produce offspring that are resistant to parasites.

      More recently, Moller and Thornhill have argued that the degree of bilateral symmetry in sexually selected traits might indicate quality. During development of a fetus, it is physiologically difficult for animals to produce symmetry, and any genetic abnormality or environmental insult such as toxin exposure, malnutrition, stress, etc. might be reflected in differences in the lengths of the left and right elongated tail feathers of barn swallows or in the size and shape of patches of color on each side of a fish's body (asymmetry). Many studies have supported this idea.

    3. A third possibility is that extravagant male characteristics could reliably indicate direct benefits to females or their offspring.

      In species in which males contribute parental care or territorial resources, perhaps some male sexual traits reliably reflect such direct benefit in addition to any good genetic quality they might indicate.

      Again, bilateral symmetry of ornaments is a potential cue whereby animals could assess either genetic or environmentally caused variation in the quality of prospective mates, and hence, the resources they will provide for offspring.

      Ornaments might also indicate more subtle direct benefits, to mates, even in species where members of the opposite sex appear to contribute little besides genes to the offspring. For example, if long tails demonstrate that males are healthy and dominant over rivals, females might choose such males because they run a lower risk of catching a sexually transmitted disease or of being harassed by other males when mating. If the costs of choice are low, then these slight benefits could augment genetic benefits and explain why females are often choosy even in species where males have little to offer.


    Currently there is a compelling body of experimental evidence that shows the importance of mate choice in the evolution of sexually dimorphic traits. But, so far, few solid predictions have been upheld by comparisons among species of display characteristics, and correlations of selective forces and extravagance are weak.

  2. Combat:

    In many species, the struggle to obtain mates can be rather violent, and Darwin suggested that fights between members of the same sex for dominance or resources which allow them to attract members of the opposite sex may be responsible for the evolution of many costly traits, such as antlers, tusks, or large body size. The potential gains in reproductive success are likely to be particularly high whenever there is the opportunity to mate with numerous members of the opposite sex. Since these are two sided contests, they constitute the essential components of an arms race, which encourages an escalation in the development of weapons. Selection, in the long run, will favor the evolution of larger and more effective weapons, as long as the functional benefits outweigh the overall costs of developing and carrying such an armory.

    A brief survey across mammals reveals many familiar examples of specialized weaponry:

    • Tusks (walrus, elephant, bush pig and hippopotamus)
    • Spiral or curved horns (antelope, sheep, goats, cattle)
    • Skin covered pedicles or ossicones (giraffe; ossicones are ossified cartilage covered with skin; males have generally lost the hair on theirs due to fighting)
    • Antlers (deer, elk, reindeer)


    Elephant tusks

    Antelope horns

    Reindeer antlers

    Giraffe (both males and females have ossicones, but the females' are thinner; males can grow 2 sets of horns)

    Gemsbok

    In most, if not all of these cases, it appears to be competition between individuals within the species (intraspecific selection) that accounts for the evolution of the weaponry. In particular, it is the competition between males for social dominance, harems or territory (or all three) that favors the development of weapons. Thus, males have bigger teeth, horns and antlers than females, and males of polygynous (one male mating with many females) species, where competition for mates is most intense, have the biggest weaponry of all.

    In the case of females who develop large weaponry, the functional significance of such equipment usually relates to its value in defense from predators, particularly defense of young and in intraspecific competition when females compete with other females (and/or maybe males) for food, space and shelter. In most social species there is a clearly defined dominance hierarchy, with dominance allowing access to the best feeding and resting places. The best example of females with well-developed weaponry are in species that live in large mixed sex groups where competition for food is particularly intense (e.g., gemsbok and reindeer; both species in which food supply can be very patchy and seasonal).

    In a comparative study, Craig Packer attempted to distinguish between the effects of sexual and natural selection on the horns of African antelopes. Male antelopes use their horns in vigorous head to head clashes when competing for access to females. Both sexes use their horns for defense against predators and females use their horns to defend their young. Packer compared three components of horn morphology: total length, basal area and shape. Among those species where both sexes possess horns, males and females have horns of equal length, but male horns have double the basal area and tend to be more curved with tips pointing back to the base rather than forward as in females.

    Drawing of African Antelope Horns (male)

    Packer interpreted his results, as follows:

    1. The increased basal area of male's horns allows them to withstand twice the force without breaking during a butting or shoving match (they are twice as strong as female horns).

    2. The more complex horn shape in the male serves to catch the blows of an opponent.

    3. Since females use their horns to ward off attacks by predators, they do not need such thick horns.


    A study of the structure and function of male antlers revealed that the antler is strengthened mechanically by the deposition of extra bone around the main beam and tines and in all species studied, the actual strength was many times greater than that necessary to support the weight of the antlers. The conclusion is that antlers (and by implication horns) are designed to fulfill their role as fighting weapons.

    This and other studies suggest that to understand differences among species in weapons and body size, we should pay attention to the details of how animals fight with one another. Shoving, biting, grappling or stabbing can each select for different combative and defensive traits. Whichever traits are favored, they tend to be more pronounced in males, compared with females, in mammal species where females live in groups and where males have potential mating access to more than one female.

    Since the 1970s, many studies have demonstrated clear relationships between the extent of male weaponry and the degree of polygyny, as measured by female group size. For example, species of deer with a highly polygynous mating system have bigger antlers relative to body size than deer that form small mating groups. This is consistent with the idea that increased competition between males, which occurs with increasing polygyny, favors the evolution of larger and more complex weapons, The selective pressure associated with polygyny also results in increased body size, musculature and aggressive behavior, all attributes that contribute to fighting success. Species such as red deer, wapiti (elk), fallow deer and reindeer are typical of large, social, polygynous species that fight overtly during a brief rutting season for access to females.

    Studies of deer have indicated that during the rut, stags fight frequently. Once study showed that stags on the Isle of Rhum (off the west coast of Scotland) are involved in a serious fight once every five days on average during the rutting season and injuries are common. The animals use roaring behavior to advertise their presence and to intimidate other stags. A challenge between two rivals involves a ritualized display. The two animals parade side by side a few yards apart. If neither animal withdraws, a fight will develop as the opponents turn to face each other. The antlers are interlocked and animals viciously attempt to force each other backwards and off balance. The sharp points of the antlers act as dangerous weapons, while the curved times form a shield to block the opponent. The outcome of a fight is usually decisive and the vanquished animal is chased from the scene. Virtually all matings are achieved by the stags that can exclude rivals and can monopolize a harem during the main period of rut when the hinds are coming into estrus and ovulating.

    A number of studies in deer have experimentally manipulated the size of antlers. In one study, the main portion of the antler was removed from the dominant stag shortly before the rutting season. He was immediately challenged by the number two stag in the bachelor group. There was a brief and vicious fight, which the dominant stag won by virtue of his greater body weight, although he clearly had to compensate for the missing antlers (he was fighting for a while with imaginary antlers and making judgment mistakes about when contact would be made). Although he won that fight, a series of other challenges ensued by other subordinate stags and after a series of defeats, the experimental animal was relegated to near the bottom of the social hierarchy. During the ensuing rutting season, he was excluded from the company of the hinds by the other stags and never formed a stable harem. His lowly status continued throughout the winter until the following year when he redeveloped his normal set of antlers and regained his dominance and regained a harem during the rut.

    Isle of Rhum Stags fighting (about 6 min)

    Red Stags Fighting (@ 2 min)

    Red Deer Stag Rescue

    Another deer rescue

    Red Deer Stag Fight Isle of Rhum (@ 1 min)

    deer fight in town

    giraffes fighting

  3. Sperm competition:

    In an influential series of papers in the 1970s, Roger Short suggested that differences in testes size among the great apes (gorillas, chimps, bonobos) and humans might result from sperm competition.

    Gorillas are 4-5 times the weight of chimpanzees (@550 lbs vs 110 lbs), but chimpanzees have testes that are much larger than those of gorillas (2.11 oz vs. 0.3 oz or 7 times larger). Since female gorillas mate with just one male per estrus, females will only mate every 3 to 4 years and a harem contains only 2 to 4 females, a male gorilla may only be mating once a year (if that). Thus, the male only needs to produce enough sperm to ensure fertilization of the egg.

    In contrast, while female chimpanzees only give birth every 5 years or so, they go into estrus about a year after giving birth (even though they won't get pregnant for years), will mate with several males during their estrus, and they live in larger groups than do gorillas, so there are more females to mate with at any one time than in gorillas; thus, sperm competition occurs in chimpanzees. As Roger Short says, since chimpanzee males do not know who the father is for any offspring, one way to ensure fitness is to increase the chance of fathering offspring by 'having more tickets in the lottery', that is to produce more sperm (hence the need for larger testes).

    Similar patterns have been found in other mammals and birds. For example, in birds, the largest testes occur in polyandrous (females mate with many males) species where females attempt to form pair bonds with more than one male per season, and hence the potential for sperm competition is particularly high. In contrast, in species in which females mate with only one male, even though males mate many times with many females, testes are small; e.g. lek mating systems in birds.


Mating Patterns:

It is important to realize that the behavioral patterns expected from sperm competition, a form of post-mating sexual selection, are different from those expected from pre-mating sexual selection. For example, there is more potential for male combat to be advantageous in polygynous primates than in monogamous ones, but among the former, only those species where females mate with multiple partners will be selected to have large testes. As a consequence, we find the following mating patterns:

Blank Mating Strategies Chart

Mating Strategies Chart

Monogamy (some species of birds, prairie voles; more than 1 offspring in a litter/birth):

In some long-lived birds and a few mammals, the male is mated monogamously to the same female for life. In this kind of breeding system, a number of things will be true:

  1. First of all, the kind of breeding system a species has is assumed to be related to the energy investment that each parent must make in raising the young. Thus, bird species are more often monogamous than mammals, since bird parents feed their young equally, while mammalian mothers must lactate and invest an inordinate amount of energy (in comparison to the male) in raising the young (thus, monogamy = equal parental investment).

  2. In a monogamous system, it is assumed that the number of adult females and males is approximately equal (that is, both enter the mating pool at the same time) and that both sexes have to compete for mates to approximately the same extent. Thus, there is little intraspecific competition for mates, leading to # 3:

  3. No male-male or female-female combat to obtain mates, leading to #4:

  4. Males and females are likely to be the same size (little sexual size dimorphism) since males don't have to compete with other males for access to females and engage in combat (combat usually = larger body size); leading to # 5:

  5. Male mating should not be delayed since males do not have to attain a certain body size or competence to fight for access to females.

  6. Since males are mated permanently to a particular female, a male's breeding success depends largely on the breeding success of his mate and lifetime reproductive success (the closest measurement of fitness we have) is usually equal in the two sexes (although it may be greater in the male if he engages in extra-pair copulations).

  7. Since males and females are copulating within a pair bonded couple, females should not need to advertise when they are in estrus or ovulating, so ovulation should be hidden.

  8. Since males and females are monogamous, there should be no sperm competition, since males are not copulating with many females that many males have access to, leading to #9:

  9. There shouldn't be sexually selected characteristics (ornamentation or exaggerated characteristics) because: 1) there are equal numbers of males and females, 2) neither sex is choosy since parental investment is roughly equal, 3) males are not competing so there is no combat and 4) there is no sperm competition, leading to #10:

  10. Testes size should be small, since there is no sperm competition, leading to #11:

  11. High numbers of abnormal sperm, because there is no sperm competition.


Polygyny: (one male, many females; females only mate with one male; e.g. gorillas; only 1 offspring at a time)

  1. Parental investment is greater in females than males, females do almost all of the child care.

  2. The ratio of receptive females to sexually active males (the operational sex-ratio) typically shows a strong bias towards males (females enter the mating pool earlier than males and there is a long period of gestation and lactation before females are ready to breed again).

  3. Male-male combat for access to a female or a group of females; one male does all or most of the mating with a group of females.

  4. Where large body size has a greater effect on the breeding success of males than of females, males are likely to evolve to be larger than females, and this is presumably why a pronounced sexual size dimorphism is a common feature of polygynous vertebrates. In contrast, where body size has little effect on male success, little or no sexual dimorphism is seen. So, for example, in two closely related species, the monogamous prairie vole has little body size dimorphism; while the meadow vole, which is polygynous, has a pronounced body size dimorphism.

  5. Male mating is delayed until the male is large enough and experienced enough to challenge other males to combat, thus, mating may be delayed for quite some time after sexual maturity is reached. For example, in red deer, males rarely mate successfully before reaching adult weight at 7 or 8 years of age, while bull elephant seals rarely breed before the age of 9 years.

  6. Males' fitness can be much greater than that of females, since they are mating with many females. For example, in red deer, the most successful stags produce about 30 offspring in a lifetime, while the most successful does produce about 9 young. However, males in polygynous species rarely have a prolonged prime; red deer males rarely breed past the age of 12 and bull elephant seals past the age of 13; in contrast the effective breeding life-span of females of these species is about twice as long as the males.

  7. This compression of breeding life-span varies with the form of inter-male competition which exists in a species. For example, in some primate species and in dolphins, males commonly assist each other in competition for females and, at least for primates, a male's status and breeding success is heavily influenced by the rank of the assisting males. Thus, in those species, breeding life-span is affected less by age than in a more intensively competitive species like deer. In some polygynous species, such as some human societies, breeding success is actually increased by age, since as a male accumulates wealth and power he becomes more attractive as a mate and may acquire multiple wives and have a very long reproductive life-span.

  8. There should be hidden estrus, since females are not advertising to attract males to mate with them.

  9. There should be no sperm competition, since once a male has access to the female, she is not mating with other males.

  10. There should be sexually selected characteristics related to combat, such as large body size, manes, antlers or horns, teeth and hair.

  11. Testes size should be small, since males are not competing with other males in sperm competition (e.g., gorillas = 0.3 oz).

  12. High numbers of abnormal sperm, because there is no sperm competition.


Polygyny + Polyandry = Polygamy (Chimpanzees; usually only 1 offspring at a time? E.g., chimpanzees)

  1. Unequal parental investment; females do all the child care.

  2. The ratio of receptive females to sexually active males (the operational sex-ratio) is roughly equal.

  3. Since the numbers of males and females are roughly equal, there is no male-male combat.

  4. Because there is no male-male combat, body size of males and females is roughly equal.

  5. Male mating is not delayed since males do not have wait to attain a certain body size or competence to fight.

  6. Fitness between males and females should be roughly equal, since males are competing with sperm competition.

  7. Females advertise estrus so that many males will mate with them when they are ovulating.

  8. Since males are not competing with combat, they compete through sperm competition.

  9. Yes, sexual selection occurs in the form of sperm competition.

  10. Males have large testes because of sperm competition.

  11. There are low numbers of abnormal sperm because of sperm competition.


Comparison of gorillas and chimpanzees

Gorillas are characterized by intense male-male competition for access to females and their breeding system is polygynous. As predicted by this competition, male gorillas are considerably larger than females (250 kg/550 lbs male body weight, 10 g/0.3 oz testes; body size is twice that of females). However, copulatory frequency is low (few females - a harem of 2 to 4 females, long gestation [almost 9 mo; females breed on average every 3 to 4 years] and lactation), thus the testes, whose size is determined almost exclusively by the volume of the seminiferous tubules (which produce the sperm; the testosterone-producing Leydig cells account only for a small proportion of overall testicular size) are small and there is a high percentage of abnormal sperm; since an adult gorilla does not mate until age 15 or so and then may mate only once a year.

In contrast, chimpanzees compete less intensively for access to females and are cooperative maters and thus, there is little sexual dimorphism in body size (50 kg/110 lbs body weight) are only slightly bigger than females, but copulatory activity is frequent, so they have 60 g/2.11 oz testes, because they have sperm competition (instead of inter-male competition). Thus, males are not competing for access to mate with females, because many males will mate with an estrous female, rather they must produce lots of viable sperm, since the competition occurs in which sperm fertilizes the egg.

Thus, you find this interaction of breeding system, body size, and testicular size. Selection for body size is concerned with successful inter-male aggression and competition for mates, and you find little or no somatic dimorphism in monogamous species that pair for life, and since copulatory frequencies are also low in monogamous species, the testes are relatively small.

In polygynous species, however, where one male has exclusive access to several females, enhanced inter-male competition will result in the males becoming much larger than the females (like the gorilla). But, if copulatory frequency is still low, as in the gorilla, the testes may remain relatively small. In the chimpanzee, you have a less dimorphism in body size because the males don't compete for access for females, through combat, but large testes, because mating opportunities are frequent.

What about female advertisement of estrus? In the monogamous breeding system, the female does not have to advertise her sexual state to all-comers by developing pronounced sexual swellings because the male is with her at all times. In the polygynous gorilla situation, where there is one male in constant attendance on the females, they also do not need to advertise, since the male is constantly with them. In the polygynous, polyandrous chimpanzee situation, the females advertise their sexual readiness since she is going to mate with many males and it pays for her to advertise that she is about to ovulate.

What about humans? Men are 15 - 20% larger than females, suggesting that we are not inherently monogamous (80 kg/175 lbs). The relatively small size of the male's testes (about 20 g/0.6 oz), the high proportion of abnormal spermatozoa, and the lack of any cyclical sexual swellings in females suggests that neither are we adapted to a multi-male promiscuous mating system such as the chimpanzee's. A woman's ability to initiate copulation at any stage of her reproductive cycle (not tied to hormones), rather than just around the time of ovulation, has greatly increased the frequency of copulation. But, since the vast majority of human acts of intercourse are unrelated to reproduction and are probably for social bonding, the size of males' testes has not increased accordingly. The best guess would be that we are basically a polygynous primate in which the polygyny usually takes the form of serial monogamy. Since there is no male menopause, men have a longer fertile life than women, so a man is likely to produce more offspring in his lifetime than a woman. This is accentuated by the fact that if older men remarry, almost invariably they marry a younger woman.

Humans have developed a variety of secondary sexual characteristics like a large penis and well-developed breasts that further enhance inter-sexual bonding and woman are unique amongst mammals in being covert ovulators, concealing the event not only from males, but also often from themselves.

Humans, as a matter of fact, have the largest penises (per body size) and largest breasts (per body size) of any mammal on earth (although Franz deWall would argue this point re: bonobos). Women don't need large breasts to adequately feed their young, nor do men need large penises to impregnate females. These characteristics originally evolved through natural selection and through the process of sexual selection have evolved as exaggerated characteristics that are attractive to potential mates.

So, does this mean that both human males and females have exaggerated characteristics?

If the answer is yes, does this suggest sperm competition occurs in humans? Or is mate preference or combat behind the development of sexually selected characteristics?

Do humans have equal parental investment?

Who is the limited sex in humans?

Are there costs associated with mating in both sexes, not just females (pregnancy, lactation, long childhood of offspring)?

Note: The finding that many species considered to be monogamous actually engage in extra-pair matings has made determining whether or not a trait is due to sexual selection more difficult. We cannot assume that males sire every offspring born to their mates, since it is clear that many offspring are born from matings with other males.

human penis shape

Article with penis shapes of monogamous and polyandrous species

Human penis shape

Robin Baker's website with videos about sperm competition

some primate sperm swim faster

Picture from Eberhard's article in Exploring Animal Behavior

Pictures to go along with page 160 in Exploring Animal Behavior (first five are the left hand pictures, second five are the right hand pictures)

Brown Greater Galago

Garnett's Galago

Golden Potto (lemur)

Southern Needle-clawed Bush Baby

Sunda Slow Loris

Mantled Guereza

Common Marmoset

Mandrill

Patas Monkey

Cotton Top Tamarin



Parental Investment

Many of these notes verbatim from Shenk, M. K. (2011). Chapter 2: Our Children: Parental Decisions-How Much to Invest in Your Offspring. In: U.J. Frey et al., (eds.), Essential Building Blocks of Human Nature. Berlin: Springer-Verlag.

Problems of parenting:

From an evolutionary perspective, offspring are the vehicle by which their parent's genes get transported to succeeding generations. Given the importance of offspring as genetic vehicles, it is reasonable to expect that natural selection would favor powerful mechanisms in parents to ensure the survival and reproductive success of their children.

Given the importance of offspring, it is interesting that many species do not engage in parenting at all. Instead, they take the tactic of producing many offspring and simply releasing them into the environment, hoping that by producing huge numbers of offspring, some will survive. These parents use their time and energy to make offspring, not to raise offspring. Given the costs of parental care (loss of own reproductive output, for example, or even their life), when we do see parental care in a species, we assume the reproductive benefits of it are great enough to outweigh the costs.

How many offspring should parents have, and how much time and resources should they invest in each of them? These are two of the central questions at the heart of evolutionary approaches to parental investment. Each question implies a tradeoff; ol>
  • The first tradeoff is between current and future reproduction

  • The second tradeoff is between offspring quantity and offspring quality.

    The study of parental investment centers on the ways in which different ecological and cultural circumstances change the costs and benefits of investment, creating different answers to these two questions.

    Parental care was redefined by Robert Trivers (1972), who laid the foundations of current thinking about the evolutionary consequences of parental care. He emphasized the need to consider all ways in which parents contributed to the fitness of their offspring, combining these into a new term, parental investment.

    In his seminal 1972 article, Trivers defined parental investment as "any investment by the parent in an individual offspring that increases the offspring's chance of surviving . . . at the cost of the parent's ability to invest in other offspring" (Trivers 1972, p. 139).

    Parental investment includes most types of direct care, including:



    Complex human cultural/social systems also create opportunities for additional forms of parental investment not typically thought of as parental care, such as arranging marriages, transferring social connections (networking), and endowing offspring with wealth (Alexander 1990; Trivers 1972).

    The benefits of parental investment to offspring can take many forms, including: effects on survival, growth, health, immune function, social status, and the number of offspring (or other close kin) surviving in future generations.

    Trivers' ideas:



    Where females care for the young and males play little or no part in parental investment, as in most mammals, Trivers' argument provides a satisfactory explanation of competition between males for females.

    However, it is more complicated when males are heavily involved in parental care. For example, in the 3-spined stickleback, where females lay eggs in nests built by males who subsequently guard and fan the eggs, who is investing the most? Females, who produce the eggs or males who take on the fanning and guarding duties (combined with reduced food intake and increased danger from predators)?

    Once parental care has evolved, it turns out that it is the potential reproductive rate, measured in terms of the number of independent offspring that parents can produce per unit time, calculated over periods when both sexes are reproductively active that determines who is the limited sex.



    When males are able to 'process' clutches of eggs more rapidly than females can produce them (as in the stickleback), males are likely to be the primary competitors for access to the opposite sex, despite their involvement in parental care (females are the limited species because their rate of reproduction is slower). So, we should see sexually selected characteristics develop in males where females are the limited sex and sexually selected characteristics develop in females when males are the limited sex (in the three spined stickleback, it is the red throat color in males that is the sexually selected characteristic; females are the limited sex due to their slower rate of reproduction).

    Why is it more likely in mammals that females are the major caregivers?

    1. Paternity Uncertainty Hypothesis:

      Mothers throughout the animal kingdom are 100% 'sure' (no conscious recognition implied) that their offspring are their own; they gave birth to them, thus they always 'know' that their offspring contain 50% of their own genes. Males, on the other hand, since they don't give birth, have no way of knowing for sure if offspring are theirs or not. From a male perspective, there is always some uncertainty that the offspring they are investing in are theirs.

      Paternity uncertainty is greatest in species with female internal fertilization, including many insects, birds and all mammals, including humans. Because of internal fertilization, the male never knows if the female is already impregnated. Males would suffer tremendous fitness costs if they channel their resources to offspring that do not carry their genes. Thus, because of the costs that males incur as a result of misdirected parental effort, in mammals, it is always more advantageous for females to invest their resources in parental care than males.

      Alcock however, says that male paternal care will still develop even in the face of paternity uncertainty in cases where caring for young, even if some of them are not your own, increases your direct fitness more than the tactic of inseminating several females but having few young survive because they received no paternal parental care.

      Example:

      • A male's paternity probability is only 60% and females bear 10 offspring at a time (so 6 out of 10 are the male's offspring).

      • The survival rate of offspring with no male parental care is 20%, but is 50% with male parental care.

      • Males who do not give parental care can use their extra time and energy to gain sexual access to twice as many females as those who do give parental care.


      This example yields the following outcome:

      • Males who do not care for their offspring will sire 12 offspring (6 with each female) but only 2.4 of them will survive.

      • Males who do care for their offspring will sire fewer offspring, six on average, but 3 of them will survive.


      In this example, males investing in parental care end up leaving more surviving offspring they are related to, even though they raise some they are not related to.

    2. The Abandonability Hypothesis:

      This is based on the order in which sperm and egg are released. If parental care benefits offspring, then the first one who can abandon the offspring does so, putting his or her mate in a difficult position. The mate left behind can either invest in offspring or abandon them. If the offspring benefit from parental care, selection will favor a parent who sticks around, even after being abandoned by one's mate. In short, the one left behind is forced to care for the young.

      According to this hypothesis, internal female fertilization should be linked with heavy female parental care because the male is free to leave after conception. In species in which there is external fertilization and females deposit eggs or young and leave them to male care, females should be free to leave and should abandon young first. Thus, female parental care should be more prevalent in internally fertilized species and male parental care in externally fertilized species.

      There is some data to support this. In fish and amphibian species with internal fertilization, 86% have more maternal than paternal care; and in those with external fertilization, 70% show more male care than female.

      One of the problems with this hypothesis is that it predicts that in species with simultaneous release of gametes, males and females should have an equal chance of abandonment. In a study of 46 species however, 78% showed male parental care, not the 50% predicted by the abandonment hypothesis.

    3. Mating Opportunity Cost Hypothesis:

      The third hypothesis stems from sex differences in mating opportunity costs, which are missed additional matings as a direct result of effort devoted to offspring. Females and males both suffer such costs. While a mother is gestating or breast-feeding her child or a father is fending off predators, neither has a high probability of securing additional mates.

      The costs are higher for males than for females, however, since the reproductive success of males tends to be limited primarily by the number of receptive females they can successfully inseminate. Since this is true, males should be less likely than females to take on parental care and instead focus most of their energies towards securing additional mates.

      According to this hypothesis, male parental care should be rare when the opportunity costs of missed matings for males are high. When they are low, then males should engage in parental care. Precisely such a condition occurs in fish species in which the males stake out and defend a specific territory. Females investigate the territories of various males and select one in which to lay their eggs. Males can guard and even feed the eggs while at the same time guarding their own territory. In this case, the male's mating opportunities will not suffer as a result of parental investment. Actually, the presence of eggs laid by other females in a given male's territory appears to make males attractive to females, prompting them to lay their eggs in territories already containing eggs. Perhaps the presence of other eggs indicates to a female that the territory is safe from predators, or that another females has judged the resident male acceptable.


    These 3 hypotheses are not mutually incompatible and may all, to a certain extent, play into predicting which sex should engage in parental care. The two most viable, however, are the paternity uncertainty hypothesis and the lost mating opportunities hypothesis.

    What can we say about humans?

    Trivers (1972) argued that, within a species, the sex that invests more heavily in offspring will become a limited resource for the other sex. Among mammals, the higher investing sex is most often females since they bear a much larger burden of obligatory parental investment (Clutton-Brock 1989).

    In humans, the minimum amount of male parental investment is the sperm necessary for conception.

    For women, in contrast, the minimum amount of parental investment required is the production of a much larger gamete, the egg, 40 weeks of gestation, and a period of lactation that in traditional human societies can last for several years.

    This suggests that, in general, humans should follow the stereotypical pattern of high female parental investment and male competition for choosy females (thus, males should show sexually selected traits determined by female choice, combat or sperm competition, and females should not).

    Mothers' investment in offspring:

    It is clear that mothers have a tremendous effect on the survival of offspring, also supporting the idea that men should compete for high investing women since mothers are the most important caretakers of young children in most human societies. In a review article entitled Who keeps children alive?, Sear and Mace (2008) found that mothers have a virtually universal-and often profoundly-positive effect on child survival, while maternal death is often associated with extraordinarily high rates of mortality for young children. Since maternal care is so pervasive, it shows less cross-cultural variability than care by fathers or other kin or group members.

    Yet how, and how much, mothers invest does vary between societies and individuals, often in ways that are consistent with levels of resources and risk.



    Father's investment in offspring:

    Across cultures, men, on average, have greater variance in reproductive success than women, suggesting that men should compete for women who invest more in their offspring. Monogamous societies generally have smaller differences between male and female reproductive success than polygynous ones, indicating a greater motivation for male parental investment in monogamous groups (however there is tremendous variation in male parental investment across both polygynous and monogamous societies, so no one rule has been found to predict the levels of male investment based on variations in culture).

    In fact, Trivers' model predicts that, in species with high paternal investment, females should compete for access to males since it is male investment that may be the limiting factor on female reproductive success/fitness (Trivers, 1985).

    While human mothers perform the bulk of care for infants and children, humans are unusual among higher primates (Old World monkeys and apes) for the high amount of investment in offspring by fathers. Investment by fathers has long been touted as one of the hallmarks of the human species, and the importance of fathers in provisioning and/or protecting mates and children has figured prominently in many conceptions of human evolutionary history.

    Both classic and recent models suggest that paternal aid in the provisioning and care of children has helped to drive the high reproductive rates of humans as compared to other primate species.

    Some also argue that paternal investment is a key to the development of important human cognitive and social abilities.

    While there is disagreement about just how important paternal investment has been in our evolutionary history, most researchers agree that our capacity for high levels of paternal investment is a key evolved feature of human behavior.

    It is clear that paternal investment varies according to culture. Among the highest levels of direct paternal investment are found among the Aka peoples of the Central African Republic. In these groups, fathers spend a great deal of time holding infants and playing with young children, in some cases nearly rivaling the time spent by mothers. The same pattern appears to be found in many other forager groups as well.

    Fathers also appear to be highly important in monogamous societies with high levels of parental investment, such as modern nations in Europe and North America.

    In other cases (most frequently among horticulturalists) the role of fathers is more flexible, with fathers being key players in some families but not others.

    Investment in children by others than mothers and fathers (alloparenting, commonly seen in traditional societies):

    Grandmothers

    The most famous of the perspectives on the contribution of non-parental kin to offspring survival has been research on the importance of grandmothers. Based on their work among the Hadza foragers of Tanzania, Hawkes and colleagues (1997) developed the 'grandmother hypothesis' as an explanation for the evolution of menopause in humans.

    Menopause is a very rare event among animals, and its origins are hotly debated. Hawkes et al. argued that humans evolved the ability to have multiple dependent offspring at one time because the additional work it took to provision and care for simultaneous children was subsidized by post-menopausal women working alongside their daughters or daughters-in-law to feed and care for their grandchildren.

    The crux of this model is the suggestion that older women get a better reproductive payoff by investing in grandchildren than they would get from continuing personal reproduction into an increasingly feeble old age. The grandmother hypothesis has been both influential and controversial.

    An alternative perspective is provided by Williams (1957) who suggested that, given the very long dependency periods of human children, it was just as likely that women ceased reproducing in order to successfully rear their own offspring to adulthood.

    A great deal of work has been done by numerous researchers to test the implications of the grandmother hypothesis in societies around the world.

    Sear and Mace (2008) found ssubstantial cross-cultural support for the importance of grandmothers in the survival of offspring, although since their impact was not always positive, they are not universally or uniquely important.

    While many authors have come to question Hawkes et al.'s (1997) perspective on the primacy of grandmaternal investment, there is increasing agreement that cooperative breeding strategies (of which grandmothers are in many cases a key part) are in fact related to the evolution of menopause.

    Early attempts to model or predict menopause suggested that the inclusive fitness benefits of mothering or grandmothering were not sufficient to offset the potential benefits of continuing to reproduce.

    Cant and Johnstone (2008), however, argue that if one takes the costs of reproductive competition into account, one reaches a different conclusion. They argue that, in the context of female dispersal (females leaving their natal home), which likely characterized early human societies, older women will compete with younger, immigrant women who have a competitive advantage, because they are insensitive to the reproductive costs of older females and have less to gain from cooperation. In these circumstances, the benefits of continued reproduction may become low enough that there would be selection for reproductive cessation. If post-menopausal women are able to focus investment on older children and grandchildren, this would only strengthen the benefits of cessation.

    Alloparenting by siblings:

    Various authors have considered the importance of siblings to parental investment. Kramer and Boone (2002) argue that the high fertility seen among intensive agriculturalists is underwritten by the labor of children on the farm. Using data on Maya children in rural Mexico, they showed that while children may not be self-sufficient at early ages, the work they do reduces the workload of their parents, freeing up time and calories that can be spent on subsequent reproduction. Kramer (2005) further argues that partial self-provisioning by children is common in traditional societies following different subsistence patterns (hunter-gatherer, pastoralist, agriculturalist) and contributes to the human capacity for high reproductive rates.

    Are Humans Cooperative Breeders?

    The importance of relatives to the care of children in many societies, combined with cross-cultural diversity in terms of whom the key relatives are, has caused several authors to draw the conclusion that humans should be characterized as cooperative breeders.

    Kramer (2005) and Sear and Mace (2005) argue that our history as cooperative breeders is the best explanation for the high rates of fertility in humans, as compared to our close primate relatives. Hrdy (2009) further contends that cooperative investment has been a key in the evolution of our psychological adaptations for empathy and cooperation, which are unique among animals.

    Parent-Offspring Conflict:

    The benefits provided to offspring, however, come at costs to parents:



    Parental investment theory predicts that parents have been shaped by natural selection to maximize the difference between the benefits and the costs of parental investment, which they do by making tradeoffs in offspring number and/or the care that each offspring receives.

    Most predictions about parental care are based on the assumption that parents are free to adjust expenditures in their offspring in relation to their own interests. However, a parent's level of expenditure may be constrained by the behavior of its own offspring.

    Whenever parents are not genetically identical to their offspring, conflicts of interest between parents and offspring are likely to arise that may influence the level of parental investment.

    Trivers was the first to suggest that parents and offspring will eventually come into conflict at some point when offspring are demanding resources that significantly depress the parent's ability to reproduce again (or even to survive). He specifically was talking about offspring of diploid (containing 2 sets of chromosomes, one from each parent), outbred animals, where parent's and offspring have a coefficient of relatedness that is 0.50. Since the coefficient of relatedness to oneself is 1.0 (and only .5 to full sibs), Trivers predicted that individual offspring should favor the mother's continuing parental care until the costs of care to the mother's fitness exceed twice the benefit to itself.

    Where offspring can raid parental resources directly or can deceive parents into exceeding their own optimal level of investment (for example, by begging frequently for food) Trivers predicted that they should do so. Periods of evolutionary conflict between parents and their offspring might be expected to give rise to overt behavioral conflict such as when mothers and offspring 'disagree' over the time of weaning.

    Implicit in Trivers's theory is the suggestion that where offspring are able to modify their parent's behavior, they may reduce their parent's fitness by shifting the level of investment away from the parent's optimum and towards their own.

    What kinds of decisions are parents making about offspring investment?

    Life history theory focuses on key questions about reproduction, including the timing of births, the number of offspring born, how much parents invest in children, and the relationship between mortality and fertility.

    According to life history theory, an organism's lifetime is characterized by tradeoffs between investment in the reproductive effort related to either mating or parenting.

    Tradeoffs exist because any time or energy invested in one type of effort reduces the amount of time or energy that can be invested in the other.

    Many major life history tradeoffs fall into two categories relevant to reproduction:


    1. The tradeoff between current and future reproduction.

    2. The tradeoff between offspring quantity and offspring quality.


    A. Current vs. Future Reproduction

    Perhaps the most fundamental reproductive tradeoff is that between beginning reproduction versus continuing investment into the acquisition of knowledge, status, or mates (and thus, delaying mating to do so).

    Generally, when mortality risks are high, organisms do better by maturing faster and reproducing early in life. This is because the longer an organism delays reproduction, the more it increases its probability of dying before it gets a chance to reproduce.

    When mortality is low, however, organisms can afford to delay reproduction and prolong investment in other aspects of their life (body size, knowledge, status, etc.) in the hope of greater reproductive success later in life.

    An important concept in the study of this tradeoff is reproductive value (RV).

    As defined by Fisher (1958), RV is the number of offspring an individual of a given age can expect to produce in the remaining years of its life, adjusted by the probability of surviving to each of those years.

    Another definition: An organism's reproductive value (RV) is defined as its expected contribution to the population through both current and future reproduction.

    At any given age, reproductive value can be partitioned into;


    The cost-of-reproduction hypothesis predicts that higher investment in current reproduction hinders growth and survivorship and reduces future reproduction, while investments in growth will pay off with higher fecundity (future potential number of offspring produced) and reproductive episodes in the future.

    Increasing or continuing investment in current offspring reduces the resources available to future offspring, and thus RRV.

    In contrast, decreasing or withdrawing investment from current offspring increases the resources available to future offspring, increasing RRV.

    For women, RRV increases sharply until around age twenty, at which point it begins to fall off quickly, reaching zero at menopause around age 50.

    For men, reproductive value increases more slowly until the early 20s, then tails off more gradually, reaching zero around age 70.

    Females may get some benefits from delay in societies where they also compete for wealth or status, but women face more serious fertility consequences from delaying reproduction, because delaying reproduction does not delay menopause or change the steep decline in RRV with age.

    Women's decisions between current and future reproduction:

    From a woman's perspective, two contexts that affect the decision to reproduce or not are (1) age and (2) partner status (e.g., marital status).

    Human parents often have to make difficult decisions about how to divide resources between existing older offspring, new infants, and potential future offspring. One of the most widely studied examples of how parents manage this tradeoff is infanticide, a phenomenon that exists in some form in many animals, including many species of primates and most human societies.

    Women's age and infanticide:

    Young women have many years, presumably, to reproduce, older women do not (younger women have higher RRV). Thus, one decision rule that should have developed is that younger women are more likely to engage in infanticide than older women. Daly and Wilson have suggested two hypotheses related to age of women and infanticide:



    Both types of motivations can be found for abortion in modern Western societies. Young, unmarried women are often the most common abortion patients, especially when they do not have a supportive partner or they feel as though having a child would put their education or job opportunities at risk.

    Women in their 30s or older, on the other hand, usually cite poverty or responsibilities to older children as their primary reasons for seeking an abortion.

    Abortion rates among single women drop as they become older. The suggestion is that as the likelihood of finding a better future circumstance for childbearing becomes lower, current reproductive effort becomes more valuable than future effort.

    In a study of a hunter-gatherer group in Paraguay (Ayoreo), the proportion of births leading to infanticide was highest among the youngest women (15 to 19) and lowest in the oldest age of women (more than 39).

    Wilson and Daly also collected information on infanticide in Canada (1988) and found that young Canadian women commit infanticide far more frequently than do older Canadian women, with teenage mothers shown rates of infanticide 3 times higher than any other age group.

    Women's partner/marital status and infanticide:

    Women who give birth outside of marriage (or some other kind of stable partnership) face several choices: raise the child by themselves, abandon the child, give it up for adoption or commit infanticide.

    In a sample of Canadian women, two million babies were born between 1977 and 1983, with 12% delivered by unwed mothers. These women were responsible for more than half of the 64 maternal infanticides that were reported to the police. (Daly and Wilson also examined the effects of age in this group and found that the younger the unmarried woman, the more likely she was to commit infanticide, so that youth and unmarried status interact in their effect on infanticide).

    Daly and Wilson do not argue that infanticide is an adaptation, but they regard infanticide as a reverse assay of parental care, that is, a behavior that reveals an underlying psychology of decision rules about parental investment that are sensitive to specific contexts.

    Because they do not undergo menopause, men can afford to delay reproduction longer than women without significantly reducing lifetime reproductive success. Moreover, in low mortality settings with large wealth differentials, delaying reproduction can increase male reproductive success if that time is used to increase wealth or status (Miller 2000).

    Men's decisions between current and future reproduction:

    Fathers as well as mothers should be sensitive to the contexts surrounding parental investment and should be especially sensitive to their genetic relatedness to the children. In species with internal female fertilization maternity certainty is 100%, but paternity is always in doubt, thus paternity uncertainty is an important evolutionary consideration and men may limit or terminate investment in offspring they believe are not their own.

    Resemblance to father:

    In a cross-cultural analysis of infanticide using the 60 societies in the HRAF Probability Sample, Daly and Wilson (1984, 1988) report non-paternity was given as a reason given for infanticide in 20 societies, with specific concerns ranging from adulterous conceptions (the most common), to non-tribal sires, to the fact that the children were from a woman's first marriage.

    How can men possibly assess their possible paternity?

    1. One, he can collect information about the likelihood that he is the genetic father by considering how likely it was that his partner was faithful to him during the conception period.

    2. Second, he can assess how much the child resembles him. It is reasonable to presume that males will have developed EPM to be sensitive to both sources of information. We might also assume that women will attempt to influence the man's perceptions of these issues.


    Daly and Wilson (1982) have suggested that mothers should be motivated to promote a father's certainty of paternity by remarking on the newborn's similarity in appearance to him. Success in promoting that belief should increase his willingness to invest resources in that child.

    To examine this possibility, Daly and Wilson collected videotapes of 111 American births that ranged in duration from 5 to 45 minutes. Verbal utterances were recorded verbatim. Of the 111 tapes, 68 contained explicit references to the baby's appearance.

    By chance alone, you would expect any random baby to resemble the mother about 50% of the time and the father about 50% of the time. However, on the tapes, mother's remarks about the resemblance of the baby to the father were four times more frequent (80%) than remarks about resemblance to herself (20%).

    In another study, questionnaires were sent out to 526 new parents in Canada. 25% responded. Those who responded were asked "Who do you think the baby is most similar to?" Of the mothers who commented, 81% indicated the baby was more similar to the father (19% said similar to themselves). The mother's relatives (who were contacted by the mother as part of the survey) also indicated a bias, in that 66% indicated the baby was most similar to the father. (71% of fathers' relatives indicated the baby looked like the father).

    Interestingly, it is possible that babies really do resemble their fathers more than their mothers early in life. In one study of one-year-olds, people were asked to match babies' pictures with those of their parents (three sets of possible parent pictures were provided). Chance would predict that the correct mother or father would be selected 33.3% of the time. Subjects were able to pick out the biological father of baby boys 50% of the time and of baby girls 48% of the time. They were not able to match one-year-olds with their mothers above chance selection. It is possible that young babies, by whatever mechanism, might actually resemble their fathers more than their mothers.

    Monetary investment by Fathers:

    Under the current conditions that humans in the Western world live in, one of the best markers for parental investment is how much money parents are willing to spend on their kids. In one study of 615 men in Albuquerque, NM, men were asked how much money they were investing or had invested in their children's college education. The researchers found that:



    Thus, genetic relatedness (plus mating opportunity with the current partner) predicted how much money men invested in children.

    B. Quality vs. Quantity:

    The second fundamental tradeoff is between parental investment (the resources and care expended on each offspring) and fertility (the number of offspring born and reared) (Trivers 1972).

    High levels of parental investment in existing offspring necessarily require lower fertility, while low levels of parental investment in offspring allow for higher fertility.

    MacArthur and Wilson (1967) defined two general strategies organisms can take in negotiating this tradeoff.

    1. An r-selected strategy occurs in unpredictable environments with high mortality rates, where it is important to reproduce quickly or risk not being able to reproduce at all; r-selected species have early ages at maturation, high fertility, and low levels of parental investment.

    2. In more stable populations that are nearer the carrying capacity (K) of an environment, however, within-species competition for resources increases. In these circumstances, natural selection favors organisms that have later ages at maturation and fewer offspring, but invest more heavily in each.


    Primates as an order are K-selected organisms, known for their high levels of parental investment. Most species of Old World monkeys and apes (our closest relatives) bear one offspring at a time, engage in a lengthy period of lactation, and have relatively long interbirth intervals lasting from one to eight years.

    Humans fit much of this pattern. We usually only give birth to a single offspring at a time, periods of breastfeeding last from 2-4 years in traditional societies, and children are not self-sufficient or productive foragers until well into their teens or even later.

    While the terms r-selection and K-selection are usually used to designate differences between species, they are sometimes applied to differences within species. For example, Wilson and Daly (1997) found that women in Chicago neighborhoods with high mortality rates had both higher fertility rates and earlier ages at first birth (and thus relatively more r-selective reproductive strategies) than women in neighborhoods with lower mortality rates.

    Interbirth Intervals:

    In humans, as in other species, the amount of time between the birth of one offspring and the next is one measure of parental investment. Longer interbirth intervals are generally associated with higher levels of parental investment or a more quality-oriented strategy, while shorter birth intervals are associated with a low parental investment and high fertility strategy.

    Humans are unusual, however, in that we customarily have several dependent offspring at one time, meaning that short interbirth intervals may affect not only the most recent child but older children as well.

    In two large comparative studies of 26 and 39 developing nations, Hobcraft, McDonald, and Rutstein (1985) found that birth intervals shorter than 2 years were associated with significantly higher risks of infant and child mortality than were longer birth intervals. Similar effects are also found in the developed world. Among women from the United Arab Emirates, short inter-pregnancy intervals were associated with preterm births, a significant risk factor for child mortality and developmental complications.

    Sons vs. Daughters: Sex Biases in Parental Investment:

    Interest in the evolutionary relationship between parental investment and offspring sex began with Fisher (1930), who argued that, since sons and daughters receive equal genetic contributions from both parents, investing in one sex will, on average, yield the same effect on parental fitness as investing in the other.

    However, further interest in sex-biased investment began when Hamilton (1967) suggested that when siblings of one sex compete with each other for mates, parents may benefit by producing more of the opposite sex.

    Perhaps the best-known perspective on sex bias in parental investment comes from the 1973 Science article by Trivers and Willard (The Trivers-Willard Effect), who argued that parents should bias investment towards the sex of offspring with the greatest potential for reproduction. In situations where the variance in reproductive success is higher for males than for females (which is usually the case in mammals), their model predicts that parents in good condition should invest more heavily in sons to take advantage of their higher potential reproductive success, while parents in poor condition should invest more heavily in daughters because they are more assured of reproducing (low quality sons may not be able to compete through mate selection, combat or sperm competition and thus, may never mate).

    In humans, there are many cultures in which parents have been shown to systematically bias investment towards one sex or the other based on parental characteristics such as health, wealth, or social status. Sex-based investment can take many forms, from alterations of the sex ratio itself through infanticide or abortion, to mild or extreme forms of neglect, discrimination or favoritism, to investing different types of resources or employing different strategies in raising and marrying sons vs. daughters.

    Evidence of son-biased investment is typically found among high status families or social groups. For example, daughters in elite families in traditional North India and China were often subject to infanticide because their marriage prospects were limited by rules of hypergyny dictating that women marry men of the same or a higher social rank. Daughters of high-ranking families faced a circumscribed marriage market because there were few places for them to marry upwardly, whereas sons of elite families had good marriage prospects among lower-status women and in some cases were able to take multiple wives and/or concubines.

    Several studies have found that high-status fathers have more sons than average in modern environments, that sons of high-status fathers achieve more education than their sisters (although daughters of low-status fathers achieve more education than their brothers).

    The argument is that the high proportion of sons born to male billionaires compared to the general public is an adaptive strategy because the same population also leaves more grandchildren through sons than daughters.

    Conclusions:

    The study of parental investment has been one of the most active areas of enquiry among evolutionary researchers during the last twenty years, since reproduction is the most fundamental of evolutionary behaviors and human parents face especially complex tradeoffs when deciding how many children to have and how much to invest in each. The wide variety of ecologies and economic systems in which humans live, coupled with the complexity of human social/cultural systems, results in a multitude of behavioral options for individuals.

    A Broader Perspective on the Evolution of the Family:

    What is a family? Various disciplines have different definitions, with sociologists often emphasizing the child-rearing function of the family, and defining families as groups of adults living together bearing the responsibility for producing and raising children. Anthropologists tend to stress kinship, defining families as groups of parents, unmarried children and extended kin through which lines of descent can be traced.

    Evolutionary biologist Stephen Emlen defines a family as those cases where offspring continue to interact regularly, into adulthood, with their parents (1995). He distinguishes two types of families:

    1. Simple families with a single parent or a conjugal pair in which only one female reproduces and

    2. Extended families; groups in which two or more relatives of the same sex may reproduce.


    Note that in his definition, the presence of a breeding male is not essential to the definition. When the male is present, the family is called biparental because both mother and father are sharing responsibility for parenting.

    When the male is absent, the family is called matrilineal, because the female or the female and her relatives are responsible for parenting.

    One defining feature of all families is that offspring continue to live with their parents past the age at which they are capable of reproducing on their own.

    Families are so much a fact of life for humans that we take the concept of family for granted. However, only 3% of all birds and mammals form families. Why are families so rare? Why do offspring usually leave the home or nest as soon as they are capable of doing so and so few remain with their parents past the age of reproduction? The most likely reason is that remaining in the parental home or delaying departure carries a tremendous reproductive cost, since offspring delay reproduction until they leave home. Families thus inflict two costs on offspring:

    1. Reproduction is delayed and sometimes directly suppressed and

    2. Competition for resources such as food is concentrated rather than dispersed, making life more challenging for both parents and offspring.


    The only way a family can evolve is when the reproductive benefits of remaining in the family are so great that they outweigh the costs of forgoing early reproduction.

    Two major theories have been suggested to explain the evolution of families:

    1. The ecological constraints model: in which families emerge when there is a scarcity of reproductive vacancies that might be available to the sexually mature offspring. Under these conditions, both the cost of staying in the family and benefits of leaving are low. The typical heavy cost of delaying reproduction by staying in the family vanishes because early reproduction is not possible due to lack of reproductive vacancies.

    2. The familial benefits model: according to this theory, families form because of the bounty of benefits they provide for offspring. These benefits include: enhanced survival as a result of aid and protection within from family members; an enhanced ability to compete subsequently perhaps by acquiring skills or greater size and maturity as a result of staying home; the possibility of inheriting or sharing the family territory or resources as a result of staying home; and inclusive fitness benefits gained by being in a position to help and be helped by genetic relatives while staying home.


    Emlen has synthesized these two theories into one unified theory of the origin of families. His theory of family has 3 premises:

    1. Families form when more offspring are produced than there are available reproductive vacancies to fill (ecological constraints);

    2. Families will form when offspring must wait for available reproductive vacancies until they are in a good position to compete for them; and

    3. Families will form when the benefits of staying home are large in the form of increased survival, increased ability to develop skills, increased access to family resources and increased inclusive fitness (familial benefits model).


    Several predictions follow from this synthesis:

    1. Families will form when there is a shortage of reproductive vacancies but will break up when the vacancies become available: This idea has been tested in several species of birds in which new breeding vacancies were artificially created where there had previously been none. When that happened, mature offspring left the nest to fill the vacancies, thus splitting apart the family.

    2. Families that control many resources will be more stable and enduring than families that lack resources: there is evidence to suggest that in humans, high income families are more likely to maintain social ties than are low income families. In birds, a number of studies have shown that birds inherit nesting sites or territories or even mates by staying with the family.

    3. Help with rearing young will be more prevalent among families than among comparable groups lacking kin relatives: Thus, a sister or brother might assist in rearing younger siblings, providing inclusive fitness to the older sibling. This is found in a variety of birds who show 'helping at the nest' (also primates, wolves, geese and ground squirrels)

    4. Sexual aggression will tend to be low in families compared with groups of non-relatives because relatives will evolve to avoid the risks associated with inbreeding: among birds and mammals, incestuous matings are rare. Among 18 of 19 bird species studied, mating was invariably exogamous (outside the family). In human studies, incest among genetic relatives is rare, but is more common among stepfathers and stepdaughters.


    Davis and Daly have criticized Emlen's theory as it applies to humans and have offered some modifications:

    1. Human families may remain together because of competition from other groups; remaining in a large kin-based coalition may be advantageous in such group-on-group competition;

    2. Humans engage in extensive social exchange based on reciprocal altruism with non-kin (this idea is also related to group competition theories); and

    3. Non-reproductive helpers, such as postmenopausal women, have little incentive to encourage their offspring to disperse, which may help stabilize families.