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6.1.3: Reproductive Strategies

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    It is important to recognize that primate reproductive strategies have evolved to maximize individual reproductive success. These strategies are divided into those dealing with offspring production and care (parental investment) and those that maximize mating success (sexual selection). Because the reproductive physiology of male and female primates differs (males produce sperm and cannot gestate or lactate; females produce eggs and gestate and lactate), males and females differ with regard to parental investment and sexual selection strategies. Female strategies, on the one hand, focus on obtaining the food necessary to sustain a pregnancy and choosing the best male(s) to father offspring. Male strategies, on the other hand, focus on obtaining access to receptive females.

    Parental Investment

    Biologically speaking, parental investment is any time or energy a parent devotes to the current offspring that enhances its survival (and eventual reproductive success) at the expense of the parent’s ability to invest in the next offspring (Trivers 1972). Female primates invest more heavily in offspring than males. Even before conception, females produce energy-laden eggs, and will be responsible for sustaining a fertilized egg until it implants in the uterus. After that, they invest in pregnancy and lactation (Figure 6.1a). Because all of this investment is energetically expensive, female primates can only produce one offspring (or litter) at a time. A species’ interbirth interval is determined by the length of time necessary to maximize each offspring’s survival without jeopardizing the female’s ability to produce the greatest number of offspring possible. If a female invests too little (i.e., weans an offspring too early), she may give birth to many offspring, but very few (if any) of them will survive. If she invests too much (i.e., nurses an offspring even though it could be weaned), she ensures the survival of that individual offspring but will not be able to produce very many during her lifetime. To maximize her reproductive success, a female must invest just long enough to ensure the greatest number of offspring survive to reproduce.

    We often think of maternal care as a natural, instinctive behavior. Yet this is not the case. Zoos, for example, almost always have nurseries where infants are cared for by zookeepers if their mothers will not care for them. These exhibits are among the most popular because the babies are so cute and so much fun to watch. And the caretaking positions in zoo nurseries are often among the most coveted by zoo personnel for the same reasons. But if maternal behavior is instinctive, why do zoo nurseries even exist? The answer is that in many species, including primates, maternal behavior is not purely instinctual; it is dependent on social learning (behavior learned by observing and imitating others), as well. Captive female primates, including gorillas and chimpanzees, who have not had the opportunity to observe their mother or other females care for infants do not know how to care for their own offspring. Although it is preferred that the mother care for her infant, in cases when she will not, humans must step in to ensure the offspring survives. When hand-rearing by humans is necessary, the infant is returned to the group as soon as possible in the hopes that it will learn species-typical behavior from its mother and other conspecifics. Observations such as these indicate that maternal behavior is learned, not innate, and that maternal care is critically important to the social and psychological development of young primates.

    Although females invest more in offspring than males, there are some conditions under which males will invest. Male investment takes many forms, ranging from carrying or grooming infants to sharing food with them, to protecting them from infanticide (killing of infants of one’s own species) or predators, to simply tolerating their presence. A male who has some degree of paternity certainty, or confidence that he is the father, is more likely to invest in an offspring than a male who does not because any investment in the offspring may increase his own reproductive success. Males appear to use a very simple rule, “Have I recently mated with this infant’s mother?”, to determine their paternity certainty. For example, male mantled howler monkeys only care for infants they may have fathered while Hanuman langur males protect and never attack infants who might be their own (Borries et al. 1999; Clarke et al. 1998). During my fieldwork in Kenya, I observed the first suspected case of infanticide in patas monkeys (Enstam et al. 2002), committed by the only resident male over a 10-year study period who took over the group too late in the breeding season to have fathered any of the offspring. It is certainly not a perfect rule, and males may sometimes invest in an offspring they did not father. However, this is less costly than killing your own infant.

    Sexual Selection

    Sexual selection, or selection for traits that maximize mating success, comes in two forms. Intrasexual selection is selection for traits that enhance the ability of members of one sex to compete amongst themselves (“intrasexual” = within one sex). Intersexual selection is selection for traits that enhance the ability of one sex to attract the other (“intersexual” = between the sexes). Intrasexual selection most often operates on males. In the wild, adult females are either pregnant or lactating for most of their adult lives. So the ratio of sexually available males to sexually receptive females (the operational sex ratio) usually includes more males than females. The result? Receptive females are a scarce resource over which males compete. Intrasexual selection favors traits that make a male a better competitor (i.e., a winner). Competition between males (hereafter referred to as male-male competition) can take many forms but comes in two main categories: direct competition and indirect competition (just like competition between females for food). Intersexual selection also tends to operate on males, selecting traits that make a male more attractive to females. Females, in turn, choose among potential fathers. Because female primates invest more in offspring production and care than males (see the “Parental Investment” section), they pay a significantly higher cost if the offspring dies before maturity or reaches maturity but does not reproduce. Thus, it benefits a female primate to be choosy, and this requires males to display traits that tell a female why she should choose him, and not another male, as her mate.

    Intrasexual Selection: Competition for Mates

    image46-1.jpgFigure \(\PageIndex{1}\): Hamadryas baboons are sexually dimorphic. The male (left) is much bigger than the female (center) and also has different colored fur.
    image31-3.jpgFigure \(\PageIndex{2}\): In the Simien Mountains of Ethiopia, an adult male gelada baboon displays his large canines.

    If females live together in groups, a male (or males) may be able to monopolize access to them. Under such circumstances, intrasexual selection favors traits like large body size (Figure 6.19a) and large canines, which increase a male’s competitive ability in fights with other males. Because females don’t possess these same traits, males and females of some species look different, which is called sexual dimorphism (Figure 6.19a). We will discuss sexual dimorphism in greater detail in the next section. In some species, a single, highly competitive male is able to defend a group of females from other males. Males may use vocalizations, displays, or physical combat to defend their group of females from extra-group males. In other species, it is impossible for a single male to monopolize a group of females. In these species, groups contain multiple females and multiple males. In combat between two males, the stronger, larger male is more likely to win, all else being equal. However, when groups contain multiple males, males have the opportunity to form coalitions, or temporary alliances to cooperate in an effort to enhance their competitive ability.

    image57.jpgFigure \(\PageIndex{3}\): An adult male orangutan displaying secondary sexual characteristics including cheek phalanges and a throat sac.
    image56-1.jpgFigure \(\PageIndex{4}\): An adult male orangutan in a state of arrested development who has not developed secondary sexual characteristics associated with adult males.

    If one male cannot keep another from mating with a female, indirect competition occurs. Indirect competition can take many forms, but in all cases, the males do not interact; they may, in fact, never even see each other. Sperm competition occurs when multiple males mate with the same female in relatively close succession. Under such circumstances, the male that produces the greatest quantity of long-lived sperm should have a better chance of fertilizing the female’s egg. Evidence for sperm competition comes from correlations between mating system and testes weight, which is used as a proxy for sperm production (Harcourt et al. 1981). Take chimpanzees and gorillas as an example. Chimpanzees live in groups with multiple adult males and females while gorilla troops contain one adult male (the silverback) and multiple females (for more information on social and mating systems, see the next section). Because male chimpanzees cannot keep others from mating with females, producing greater quantities of sperm is perhaps their best way to ensure paternity. Male gorillas who are able to monopolize a group of females (through direct competition with other males) do not need to compete with sperm, and so they do not need to produce it in large amounts. Therefore, although male gorillas are much larger in body size, male chimpanzees have larger testicles to produce more sperm. In other species, males engage in alternative mating strategies. Orangutans are socially solitary, but a single large adult male’s territory overlaps the territories of multiple females. The male actively keeps other males out and away from the females. A non-territorial male may compete directly with a territorial male, but this is dangerous and can result in serious injury. Some males avoid this by delaying the development of secondary sexual characteristics, or traits associated with sexual maturity. In orangutans, these traits include large cheek phalanges, a throat sac, and large body size (Figure 6.20a). Males that do not develop these traits look like juveniles (Figure 6.20b) and seem to use their non-threatening appearance to sneak into the territories of fully developed males to mate with females. The mechanism that results in the two male morphologies is not well understood, but males lacking secondary sexual characteristics have lower testosterone levels (Marty et al. 2015). Lastly, males may compete indirectly by committing infanticide. Infanticide occurs in many primate species, including red howler monkeys, chacma baboons, crab-eating macaques, diademed sifakas, ring-tailed lemurs, Hanuman langurs, and mountain gorillas. If a male kills a competitor’s infant, the mother will resume ovulation more quickly, providing the infanticidal male with an opportunity to father her next infant. Thus, under the right circumstances, an infanticidal male benefits by removing his competitor’s genes from the gene pool while adding his own to it.

    image28-4.jpgFigure \(\PageIndex{5}\): An emperor tamarin carrying twins
    image48-2.jpgFigure \(\PageIndex{6}\): A common marmoset family with twins.

    Although more rare than male-male competition, sometimes females compete for mates. The callitrichids, the primate family that includes marmosets and tamarins, are unusual in their reproductive pattern. Breeding females often give birth to twins (Figure 6.21a–b), sometimes producing litters twice a year. Another interesting characteristic of callitrichid reproductive behavior is the fact that often only one female reproduces, a phenomenon that is achieved through reproductive suppression (Digby et al. 2011). The mechanisms differ across species but generally involve the prevention of reproduction by subordinate females through physiological and/or behavioral means. These subordinate females are often the older daughters of the breeding female. In some species, the dominant female emits chemicals that delay ovulation in subordinates. In others, she physically breaks up matings between males and subordinate females. Regardless of the exact mechanism, the goal is the same: to limit the opportunities for subordinate females to become pregnant. But why? Although a breeding female can give birth to triplets or quadruplets, it is rare for more than two offspring from each litter to survive. Even ensuring the survival of twins is more than the mother can manage by herself. To maximize her offsprings’ survival, she needs all group members, including other (reproductive-age) females, to care for her offspring instead of focusing on their own. It is clear that this strategy helps the breeding female’s reproductive success. But why would her reproductive-age daughters “agree” to stay in their natal group and help mom raise their siblings instead of dispersing to another group and breeding themselves? If a subordinate female cannot find a group to transfer into as the breeding female, she has two options: stay in her natal group and raise younger siblings, or transfer to another group as a subordinate and raise the offspring of a female to whom she is not related. Because she shares genes with her siblings (50% if they are full siblings, 25% if they are half siblings), some of the subordinate female’s genes get passed down if her siblings survive and reproduce. On the one hand, fewer of the female’s genes get passed down through siblings (called indirect fitness) than if she had produced her own offspring (called direct fitness). But, on the other hand, she passes on more genes by raising her siblings than if she helped to raise the offspring of a female to whom she was not related. Not surprisingly, subordinate females rarely leave their natal group unless a breeding position opens in another group.

    Intersexual Selection: Mate Choice

    As we discussed at the beginning of this section, female primates are choosy because it is more costly for them (in terms of reproductive success) if they produce an offspring that either does not survive or that survives but cannot or does not reproduce. But what is it that they are choosing in males? Like all other examples of primate behavior and ecology, there is both species-level and individual-level variation in female choice. In many animals, including humans, females choose a male who can provide important resources, such as food, paternal care, or protection. Examples of such direct benefits are rare in primates, since most females do not require males to supply them these resources. Female mountain gorillas and chacma baboons, however, may choose males based on who can protect them from infanticidal males (Henzi and Barrett 2003; van Schaik and Kappeler 1997). More commonly, female primates obtain indirect (i.e., genetic) benefits from choosing one male over another. Often the specific criteria by which females select mates is unknown. However, if a female chooses a healthy (as indicated by traits like a plush coat, bright coloration, or large body size) or older male, she may obtain genes for her offspring that code for health or longevity. If a male’s rank is determined by competitive ability that has a genetic component, females who choose such males may acquire these genes (and qualities) for their offspring. Females in some species appear to prefer new immigrants, sometimes even “sneaking” copulations with males who are not established members of their groups. Such a preference may provide their offspring with novel genes and increase genetic variation (for more about the importance of genetic variation, see Chapter 4). Lastly, female choice does not necessarily imply that females are choosing only one male with whom to mate. In many species, females actively choose to mate with multiple males. The most likely explanation for this phenomenon is an attempt to avoid infanticide by ensuring that multiple males think they are possibly the father. This is called paternity confusion. In such cases, females may not be choosing mates based on direct (resource-based) or indirect (genetic) benefits but, rather, ensuring that any male who might be in close proximity to her infant after birth will not kill it.

    Female choice is often more subtle than male-male competition, so it can be more difficult to study. However, as more research is conducted, we better understand the ways that female primates exert their choice. In many species, females actively solicit sexual interactions with some males and not others. In other cases, females reject advances by some males and not others. Grey-cheeked mangabeys in Kibale National Park, Uganda, exert female choice in multiple ways (Arlet et al. 2007; Smith 1994). They present their hindquarters (which signals interest in mating) significantly more to high-ranking and immigrant males; they refuse to mate with some males; and most mate with multiple males when they are receptive. These results indicate that rather than being passive actors who accept matings with eager males, female primates actively participate in choosing amongst suitors.

    Social and Mating Systems

    Sometimes the terms social system and mating system are used interchangeably, but there are important differences between the two terms. A social system describes the typical number of males and females of all age classes that live together. A mating system describes which male(s) and female(s) mate. Two species can have the same social system but a different mating system and vice versa. For example, the mating system of both orangutans and mountain gorillas is polygyny—that is, one male mating with multiple females—but the social systems of these two great apes is very different. The home range of one large adult male orangutan overlaps the home ranges of many females, with whom he mates, but they do not travel together as a cohesive group (Figure 6.22a). Mountain gorillas travel in cohesive one-male, multi-female groups consisting of a silverback male, multiple females, and their dependent young, and the silverback male mates with the females of his group (Figure 6.22b). So how is it that two species can have the same mating system and different social systems or, alternately, the same social system and different mating systems? It all depends on how food, females, and males are distributed.

    We can understand primate social and mating systems by thinking of layers of a map. The first layer is food distribution. Because female reproductive success is limited by access to food, females “map onto food” and form the second layer of the map. If food exists in large clumps that can feed multiple individuals (like fruit), females can also exist in “clumps” (i.e., groups) and will benefit from doing so because living in groups helps with defense of food sources. Finally, because male reproductive success is limited by access to females, males “map onto females” forming the third layer of the map. If females live in cohesive groups, one or a few males have the opportunity to monopolize them. If females are widely distributed, it is more difficult (sometimes impossible) for males to monopolize multiple females.

    Key: alt= adult male; alt= adult female; open black circle represents the outline of the male’s home range (solitary species) or group’s home range; open red circle represents individual female home ranges (solitary species). Illustrations by Karin Enstam Jaffe.

    Table 6.3.1: When Females Are Solitary
    alt alt alt

    Figure 6.22a Polygyny in a solitary species, like orangutans.

    Figure 6.22b Polygyny in a group-living species with a single-male, multi-female social group, like mountain gorillas.

    Figure 6.22c Monogamy in species that form family groups, like gibbons.

    alt alt

    Figure 6.22d Polyandry in species like tamarins and marmosets.

    Figure 6.22e Polygamy in species that live in multi-male, multi-female groups, like vervets.

    When food is distributed in such a way that females are unable to live in close proximity to each other, they must spread out to avoid too much competition. A male may choose to guard one female, try to monopolize multiple females by himself, cooperate with other males to monopolize multiple females, or cooperate with other males to help raise the offspring of an individual female. The difference in these male strategies is illustrated by the gibbon, orangutan, chimpanzee, and tamarin.

    Both gibbon and orangutan females eat fruit found in relatively small patches that does not support groups, so females of both species are solitary. However, the way in which males map onto the distribution of females is quite different. A male gibbon guards a single female, resulting in a monogamous mating system (Figure 6.22c). A pair of gibbons form a long-term bond that includes defending a territory and relatively high paternity certainty that results in male care of offspring. Mated pairs defend their territory by calling together in a patterned vocalization called a duet. These coordinated vocalizations tell other gibbons that the territory is occupied and to stay away. Because most males get a mate, male-male competition is relaxed, and there is little pressure for males to develop large body size or weaponry to use in competition with other males. Thus, it is not surprising that male and female gibbons exhibit sexual monomorphism, meaning that males and females are similar in body size and often look alike. Because males and females both exclude same-sex competitors, the social group consists of an adult male, an adult female, and their dependent offspring, sometimes referred to as a family group.

    Like gibbons, orangutan females are also solitary. But unlike gibbon males, who cannot monopolize access to multiple females, a male orangutan has a very large home range that overlaps the home ranges of two or more females (Figure 6.22a). The females do not regularly travel with each other or the male, but he mates with them, resulting in a polygynous mating system but a solitary social system. Because some males monopolize multiple females, many male orangutans do not have access to females. Male-male competition is intense, and males benefit from large body size, weaponry, and other traits that increase their competitiveness. The result is significant sexual dimorphism. Male orangutans are twice the size of females and have large canines, cheek phalanges, and throat sacs (Figure 6.20a) that help them defend their home range (and females) through direct (fighting) and indirect (territorial vocalizations) competition with other males. As we discussed in the previous section, the competition is so intense that some males remain in a state of arrested development (Figure 6.20b).

    Chimpanzees have a fluid social system referred to as fission-fusion. When food is plentiful, female chimpanzees of the same community travel together within their community territory. When food is scarce, the group “fissions” and females travel independently, with their dependent offspring, in their own range but still within the community territory. Because male chimpanzees are philopatric and related to one another, they share more genes in common than males in other primate species who are unrelated. The high degree of relatedness results in high levels of cooperation (see the discussion of chimpanzee cooperative hunting in the “Why Do Primates Live in Groups?” section) and reduced sexual competition between males. Even males who do not father their own offspring have some genes passed on by male relatives who do (this is another example of indirect fitness). Male chimpanzees do compete to be at the top of the dominance hierarchy so as to obtain priority of access to females. However, no male in the community is excluded from mating with community females, so chimpanzees practice polygamy as a mating system (in which multiple males mate with multiple females), even though females are solitary for some of the year. Competition between males is relaxed because they are related and all get to mate. This results in reduced sexual dimorphism. Unlike orangutan males, male chimpanzees are only about 25% heavier than females. But like orangutans, male chimpanzees compete indirectly, particularly through sperm competition.

    Although there are many examples of multiple males living in groups with multiple females (we’ll discuss some examples below), it is rare for multiple males to live with a single breeding female, a mating system referred to as polyandry (Figure 6.22d). Yet this is the pattern we often see in the callitrichids: tamarins and marmosets. As we discussed in the “Parental Investment” section, due to their rapid reproductive rate and propensity for twinning (Figure 6.21a–b), breeding females need help from all group members to raise their offspring, and they suppress reproduction in other females in their group, effectively making breeding females solitary. In some callitrichid species, the dominant male fathers most or all of the offspring, but the males in the group are relatives so they benefit genetically, similar to chimpanzee males (Baker et al. 1993). In other species, males are not related, but the breeding female mates with all the males in the group, so every male has a chance of being the father of the offspring (Díaz-Muñoz 2011). In both cases, males help rear offspring because they cannot afford not to do so. Although social systems differ across tamarin and marmoset species, and even across populations of the same species, polyandry is common among callitrichids but extremely rare in other primates.

    SPECIAL TOPIC: WOMEN IN PRIMATOLOGY: MEET “THE TRIMATES”

    While many STEM (Science, Technology, Engineering, and Math) fields have traditionally been, and continue to be, dominated by men, primatology has a long history of significant research conducted by women. This is due, in part, to the fact that three of the most well-known primatologists are women, making it clear that this is a field in which women can excel. In the early 1960s, British paleoanthropologist Louis Leakey (see Chapters 9 and 10 for more about Louis Leakey’s work) was looking for students to study the great apes in hopes of shedding light on the behavior of our early ancestors. He chose Jane Goodall (Figure 6.23a) to study chimpanzees, Dian Fossey (Figure 6.23b) to study mountain gorillas, and Birute Galdikas (Figure 6.23c) to study orangutans. These three women, sometimes referred to as Leakey’s “Trimates,” have transformed our understanding of ape (and primate) behavior through their work.

    image59.jpgFigure \(\PageIndex{7}\): Jane Goodall is a primatologist, anthropologist, conservationist and activist. Her research on the Gombe chimpanzees spans over half a century.

    Arriving at the Gombe Stream Reserve in Tanzania in 1960, Jane Goodall (Figure 6.23a) was the first scientist to conduct a long-term study of wild nonhuman primates. Until then, most field studies lasted less than a year. By 1961, she had made two astounding observations that forced us to reconsider what differentiates humans from the rest of the primate order: She observed chimpanzees eating a colobus monkey, the first reported evidence of meat eating in our closest relatives (she later observed them hunting and killing other mammals and sharing the meat) and also discovered that chimpanzees make and use tools by stripping leaves off twigs to “fish” for termites. After several decades of study, her work has produced long-term data on chimpanzee mating strategies, mother-infant bonds, and aggression within and between communities. When her study group, the Kasakela community, fissioned in the mid-1970s, she observed males of the larger community attack and kill those of the smaller one. This behavior, which Goodall compared to human warfare, is now known to be typical of wild chimpanzees and is another behavior we share with our closest relatives. In the mid-1980s, Goodall transitioned from field researcher to conservationist and activist, advocating for the humane use of nonhuman animals (Stanford 2017).

    image18-2.jpgFigure \(\PageIndex{8}\): Dian Fossey was a primatologist and conservationist. She studied mountain gorillas in Rwanda for almost 20 years, until her murder in 1985.

    In 1967, Dian Fossey (Figure 6.23b) began her long-term study of mountain gorillas and founded the Karisoke Research Center in Rwanda. Through patience and hard work, Fossey habituated several groups of gorillas to the presence of human observers, and their research over several decades has formed the foundation of our understanding of gorilla social behavior, ecology, and life history. Gaining the gorillas’ trust was difficult as they were fearful of humans they had known only as poachers. Censuses of the Virunga gorilla population in the 1970s by Fossey and her colleagues estimated a population of fewer than 300. This represented a decline of 40% from the previous decade. The primary causes of this decline were habitat loss and illegal hunting. Fossey’s advocacy for mountain gorilla conservation kicked into high gear when, at the end of 1977, poachers killed her favorite gorilla, Digit, as he protected his group. A year later, poachers attacked one of her main study groups and killed several gorillas as they tried to kidnap an infant to sell to a zoo. Her efforts to publicize the killings led to the development of conservation programs that ultimately saved the mountain gorilla. By the end of the 1980s, the population had begun to recover and continues to increase. Tragically, Dian Fossey was murdered in her research cabin at Karisoke in December 1985; the case remains unsolved (Stewart 2017).

    image20-3.jpgFigure \(\PageIndex{9}\): Birute Galdikas is an anthropologist, primatologist, and conservationist. Her research and rescue work on behalf of orangutans spans 40 years.

    Birute Galdikas (Figure 6.23c) began her study of orangutans in Kalimantan, Borneo, in 1971 and set up a field station called Camp Leakey. Hers was the first long-term study conducted on the Bornean orangutan. Her research still continues, and over 150,000 hours of observational data have been collected by Galdikas and her colleagues, focusing on the life histories of individual orangutans. While conducting her behavioral research, Galdikas discovered that the pet trade and habitat loss were adversely affecting the orangutan population. She began working with the Indonesian government to confiscate orangutans that had been removed from the wild illegally, many of whom ended up as pets. Taking these orphaned orangutans back to Camp Leakey, Galdikas’s conservation efforts began to extend beyond advocacy and into rehabilitation and forest preservation (Bell 2017). If you would like to learn more about primate conservation, please see Appendix B.

    When Females Live in Groups

    When females live together, either because their food is abundant (in the case of folivores) or because their food is distributed in large patches that are worth defending (in the case of frugivores), males have the opportunity to monopolize multiple females. Sometimes a single male is able to monopolize a group of females. Other times, a male may not be able to exclude other males from the group.

    Generally speaking, when female groups are small and cohesive, it tends to be easier for a single male to monopolize a group of females. Mountain gorillas, hanuman langurs, red howler monkeys, and patas monkeys are examples of single-male, multi-female groups, which consist of one adult resident male, multiple adult females, and their dependent offspring. The mating system for single-male, multi-female groups is polygyny (Figure 6.22b). Because a relatively small number of males monopolize all the breeding females, there are many adult males who do not have mates. As with orangutans, this results in strong competition between males, resulting in sexual dimorphism where males are much larger than females. In mountain gorillas, fights between silverbacks can be intense. Males can use their large canines to cause serious wounds that may even result in death (Fossey 1983).

    When a single male cannot monopolize a group of females, often because the group consists of many females that may be spread out over a wide area, the result is a multi-male, multi-female group consisting of multiple adult males, multiple adult females, and their dependent offspring (Figure 6.22e). Olive baboons, ring-tailed lemurs, and squirrel monkeys are examples of primate species with this type of social system. Because a single male cannot exclude others in the group from mating, the mating system in multi-male, multi-female groups is polygamy, but that does not mean that all males have equal reproductive success. When multiple males live in a group, they often form a dominance hierarchy that determines their priority of access to females in the group. This is similar to the way a female dominance hierarchy determines a female’s priority of access to food. Because their place in the hierarchy can affect their reproductive success, males in multi-male groups engage in male-male competition, but because it is rare for males to be excluded from mating altogether, the level of competition, and degree of sexual dimorphism, is less extreme than what we see in polygynous species.


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