The History of Our Tribe Hominini Textbook Part I An Introduction to Paleoanthropology Chapter 4 Primate Social Organization

. Primate Social Organization Figure macaques . by Frans de is licensed under BY 25 . Most primates live in groups . The best explanation for why animals form groups and endure the costs of feeding competition is to minimize the risk of predation . Grouping patterns are tied to diet and the of resources . Females are out to maximize resources for themselves and their offspring , so as to their reproductive success . If a species eats grass or leaves , it does not make sense to defend those resources . However , there is safety in numbers and those species ( especially arboreal species ) will normally be found living or foraging in small groups . If a species specializes on ripe fruit , they can not defend them because of the patchy nature of fruit in geographic space and time . In the case of the few primate ripe fruit specialists , such as chimps and spider monkeys , males defend a home range that contains resources that females need , and thus females are attracted to join them . While orangutans are also preferentially , they are solitary due to their large size and strict , which limits resources to those that are accessible from supporting branches . Finally , if a species can eat a variety of things that come in sized patches , they can band together and defend those resources as they come across them in their daily ranging . In that case , females stay together in their natal group ( termed female ) and cooperate in resource defense . Social organization involves several aspects of group life , such as ( the average numbers of individuals in terms of age and sex ( whether group members remain in their natal group at maturity or leave , and hence whether individuals have relatives in the group ( whether those animals that join a group in hood stay permanently or tend to leave after a period of time ( the pattern of interactions between , whether there is a dominance hierarchy and if so , if an individual position in the hierarchy is permanent or temporary and ( the number of potential mates to which an individual has access . While we tend to categorize species by their grouping pattern or social organization , it is increasingly apparent that there is variability within primate species . Some species share our pattern of living in groups . Other categories of primate social organization are solitary , pairs , and groups . Interestingly , all of the mating systems seen in primates , monogamy , polygyny ( one male mates with multiple females ) polyandry ( one female mates with multiple males ) and ( both males and females are promiscuous ) are also seen in humans . Some men and women marry or mate for life some men have multiple wives or partners , and the same goes for some women . I

Figure . by is licensed under will discuss each type of social organization and mating pattern seen in the primates , along with example species . SOLITARY AND DISPERSED POLYGYNY Except for the orangutans , solitary are small nocturnal that forage primarily for insects and fruit . Examples of solitary are the ( see Figure ) and of Africa , most of the nocturnal lemurs of Madagascar , and the of Asia . I am going out on a limb ( too much ?

to gest that our earliest primate ancestors were as well , since we are thought to be descended from a small , insectivorous , nocturnal mammal . Prosimian solitary either avoid predation by stealth ( the slow climbers , such as and slow ) or a form of locomotion termed vertical clinging and leaping ( that allows for quick getaways . Females usually forage alone and either park their young nearby or leave them in a nest , such as a tree hole . Sleeping groups may consist of female relatives and their young females , young , and males , depending on the species and tolerance . Male home ranges often overlap multiple female home ranges , and males monitor female sexual cycles by ing the rounds and monitoring their scent , hence the use of the term dispersed polygyny , ie . one male and multiple dispersed females . One male may dominate other smaller or less dominant males in an area and may suppress them from breeding , via activity . As mentioned , orangutans are the odd man out . They are large and arboreal so they do not need to group for protection . They need a lot of resources to support them and at some sites , they suffer periodic food shortages , so that grouping would hinder foraging . Females and their dependent offspring forage together . Females maintain proximity and mate with a dominant male with developed secondary sexual , large size , a throat sac for loud calls , and facial flanges . Until there is an opportunity for males to acquire females , such as when a large male dies , males stay small and mate opportunistically . Scientists are stymied at how they can delay maturation and then facultatively develop into the larger morph . 32 The History of Our Tribe

Figure Gibbon of Southeast Asia . Gibbon Hoolock de by Programme is licensed under . TERRITORIAL PAIRS AND MONOGAMY While a few species of primates are commonly referred to as monogamous , have been observed in every one of them . The last primate to have lost the title of true monogamist was the night monkey of Central and South America . Prior to that revelation , it was always fun to ask my students who the only true monogamous primate species is and see if they answered , Monogamy begs the question , why ?

While females may benefit from a monogamous relationship , if their mate supports them or their offspring in some way , it is difficult to understand why males would tie selves to one mate when mating is not costly for them . There are several theories regarding the adaptive significance of pairing in primates . First is the idea that the female needs help defending a territory in order to obtain enough resources for herself and her offspring . Couples may actively passively defend their territories ( hence the more appropriate term territorial pair ) via threats , fighting , or , calling together to indicate that the territory is occupied by a bonded pair . In the majority of species , males help by carrying offspring . The second theory suggests that monogamy is a way for males to their offspring from infanticide . In those species that form groups ( see next section ) when a new male takes over , he may kill nursing infants . Once nursing is interrupted , a female undergoes hormonal changes and may return to ( fertile period ) It is in the new male best interest to impregnate females as soon as possible , in the hope that some of his offspring will make it to the juvenile stage before the next male comes in and wipes out the infants . Why would females mate with a homicidal maniac , you ask ?

It is not in their best interest to wait to reproduce either . That is the way natural selection works ! Those traits that maximize fitness , reproductive success , are favored . In addition , a male offspring that grows up to be infanticidal will be in a better position to reproduce , if he has what it takes to take over a group . There are territorial paired species within the ( and wooly lemurs of Madagascar and the of the Southeast Asian archipelago ) New World monkeys ( night , titi , saki , and some marmoset keys of Central South America ) and the lesser apes ( gibbons and of Southeast Figure ) We are learning that the lesser apes are much less monogamous than was previously thought . Females of some gibbon species tend toward polyandry and thus males are polygynous , making those species . We are the only great ape to have a tendency for monogamy , in that we tend to fall in love with one person at a time . Primate Social Organization 33

GROUPS AND POLYGYNY In some species , one male with one or a few females is the grouping pattern . However in other species ( baboons , geladas , drills , and some monkeys , such as keys ) units ( congregate into larger and larger groupings , in a or nested fashion , depending on their current activity . I will discuss this more complex grouping pattern after the of groups ( Except for the gorillas , all species are Old World monkeys . The majority of the form , African colobus monkeys of the genus Colobus and Asian and leaf monkeys . of the genus ( commonly known as figure ) and patas monkeys ( are also species . Figure De monkey , a type of guenon of Africa . guenon by Logan is licensed under BY . In the majority of species , females are related but as groups get larger , they split along , meaning that a group of closely related females may splinter when competition increases . In addition , females may move between groups , especially in the . Males fiercely compete for access to groups and infanticide occurs during takeovers . In those species that are seasonal breeders , it is difficult for the male to monitor and mate with all of the females and outsider males may sire some of the offspring . One guy can only do so much and females only have a small window of opportunity . While the makes sense for the and their high leaf diet , it is not as clear why the more exhibit the same pattern . Like the monkeys of Africa and Asia , it is possible that the ancestor of the extant arboreal never left the trees and thus did not evolve the tendency for a larger grouping pattern in response to terrestrial predators . In addition , if they remained arboreal in relict forests , they may have enjoyed a more stable resource base . They are to monkeys and thus can subsist on a variety of foods , primarily insects and fruit , both of which are indefensible food items , from a female perspective . Thus while a group is beneficial , it does not need to be large . It may be a bit of an oversimplification that female resources drive primate social organization , but it is a useful model with demonstrated heuristic value . For those species with a nested grouping pattern of , I will describe the system in baboons ( see Figure ) and contrast it with geladas ( see Figure ) Both species consist of that gate into three larger group levels . For some strange reason ( as if there are not enough terms in a mate course ) some use different terms for the levels in each of the species . The basic unit is the . The next level is termed the clan it consists of several , along with bachelor males , and the members tend to forage together ( see Figure ) The third level is the band , and that is the result of several clans congregating to forage over a large area . While bands are somewhat stable , 34 The History of Our Tribe

Figure baboons of East Africa and the Arabian Peninsula . harems by Brian Je Be is licensed under BY gelada bands are not . Finally the troop ( or herd ( gelada ) is a combination of multiple bands that come together to sleep on cliffs in the mainly treeless regions where both species live , primarily in Ethiopia . Troops consist of hundreds of animals , over 700 in the and slightly fewer in the geladas . This odd grouping pattern is related to their harsh environment . live in in Ethiopia and the Arabian Peninsula . They are that eat whatever they can find . They fission and fuse ( come together and separate again ) into the various grouping levels as resources allow , but predators abound and shelter is scarce , so there is safety in numbers via vigilance . The geladas is a bit different . They live in conditions in Ethiopia and eat a lot of grass and grass products , such as seeds and . Again , there is safety in numbers but resources are ubiquitous so they spread out a bit and mooch and munch ( new foraging ) along the ground . The strangest aspect of the two species ( other than their bizarre faces ! is that are male and geladas are female . While the gelada pattern makes sense , considering their relatedness to baboon species , are even more closely related to those baboons yet appear to deviate from the pattern . However , the females do not go far they transfer at the clan or band level and thus are not far from kin . Thus the real question is why do male stay ?

Figure A depiction of a gelada or baboon clan . Note circles and bachelors blue . Primate Social Organization 35 Figure Grazing geladas . as by Rae is licensed under While there are regular takeovers in gelada , as would be expected , they are not as frequent in , primarily due to the facts that males have control over their small group of females and are surrounded by male relatives . females are usually coerced away from their mothers when they are young and then herded and punished by their new male leader until they learn to obey and not stray . Female geladas have a say in male takeovers they either side with the resident male and help keep the new male out or they do not and the resident male is on his own . It is interesting that if a new male becomes established in the group , the former male may stay and help defend his offspring from becoming the victims of infanticide , but he can no longer breed . GROUPS AND POLYANDRY Figure Emperor . Emperor ZOO by Bracken is licensed under . 36 The History of Our Tribe

This type of social organization is seen only in the , the ( see Figure ) and of Central and South America . Within those groups , there is usually only one breeding female and one or two breeding males . Females gestate as many as five fetuses but on average , only two survive . Hence we talk about twinning in the . Those groups with an extra male have better offspring survival . At birth , the offspring average of the females weight and thus foraging to support them is a time job for the females . The females nurse the young and the males carry and nurture them . Females suppress cycling in their daughters and while sons become fertile , they have no mating options in the group . Mature daughters and sons also help with the care of their younger siblings . Helping behavior , while delaying their direct fitness ( genes they pass on via reproduction ) increases their inclusive fitness ( genes they share with relatives ) Full siblings share half of their genes ( the same as between parents and offspring ) and half siblings share , on average . While the proximate tion ( current stimulus or condition favoring the behavior , versus ultimate causation , the behavior was favored by natural selection due to its fitness benefits ) for older siblings to stay is unclear , it is likely tive in some situations to delay reproduction . For example , it may be difficult for young animals to compete for territory or mates , and they are small and inexperienced and thus easy prey . GROUPS AND There are two types of groups ( The first is the more common . They are medium to large groups of related females ( female ) with a sex ratio skewed in favor of females . Outsider males may congregate in bands . Females and males are promiscuous , the mating pattern known as . Many New World monkey species and most of the Old World ( Figures and ) exhibit this type of social organization . Females cooperate in resource defense and males may have a more peripheral position within the group , except during the mating season in seasonal breeders . Many species exhibit this type of social organization , baboons ( are the exception ) and macaques . Terrestriality is associated with larger body and group size , likely for predation avoidance . With more females , come more males and with more males , females can benefit from seasonal breeding . There are enough males to go around and the glut of offspring that are then born reduces the probability that any one of them is eaten . the dilution effect . In addition , related females help keep watch over the young that then have playmates . Seasonal breeding is tied to mental conditions , so that females benefit by timing events to coordinate with resource availability . The second type of is commonly called a community social organization . Species that exhibit this type of social organization are male ripe fruit specialists . As mentioned , females can not defend fruit , so they do not band together into . Related males defend a territory that contains enough resources to attract females . Females and their offspring forage independently but group members come together periodically into larger , hence the other term for this type of social organization , New World spider and monkeys and the chimps and bonobos of Africa ( see figure ) are all categorized as community species . Primate Social Organization 37

Figure ( also known as ) macaque . monkey by Miami is licensed under . Figure Bonobo group hug . group hug by is licensed under . 38 The History of Our Tribe