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Of Genes and Apes: Chimpanzee Social Organisation and Reproduction

Anne E. Pusey in 'Tree of Origin: What Primate Behaviour Can Tell Us about Human Social Evolution'

Notes by Georgina Holmes




Tree of Origin: What Primate Behaviour Can Tell Us about Human Social Evolution


Of Genes and Apes: Chimpanzee Social Organisation and Reproduction

by Anne E. Pusey


Social structure: Patterns of kinship, residence, philopatry and mating

Influence: Patterns of cooperation and social behaviour

Observing Chimpanzees:

  • Typically difficult to observe wild

  • Provisioning use to be used to habituate chimpanzees

  • 1960 Jane Goodall, Gombe National Park: banana provisioning

  • 1965: Toshidada Nishida, Mahle Mountains: sugar cane provisioning

    • Adv: make close observations, habituate shy chimpanzees, follow individuals easily

    • Dis Adv: effect behaviour unnaturally, less valid results

  • 1970-1980: non-provisioned


  • Jane Goodall: chimpanzees move alone/small temporary parties, any gender/age, only sometimes gather in large communities, only mother and offspring stable group

  • Toshidada Nishida: chimpanzees rarely all together, live in large "unit groups" (communities), several adult males and more/same females and offspring

  • Now we know: chimpanzees live in communities of 20-120 individuals, males remain in the group they were born into, females tend to disperse and they isolate with offspring

Fission-fusion society:

  • Male philopatry (remaining in birth group) and female dispersal

  • Individuals of a community spend time alone and time in temporary sub-groups

  • Females 50% time alone, males 18% time alone

  • Females localised but do visit all ranges, males large 8-15km range

  • Unusual among primates

Due to female receptivity

  • Sexual receptivity cycles by swellings lasting 13 days in a 36-day cycle

  • 5.2 year Gombe/7 year Kibale interbirth intervals

  • Swellings: females over large range/outgoing

  • No swellings: feed alone accompanied by dependent offspring over small range

  • Females distributed SO Males range over wide area in groups to protect access to receptive females

Due to feeding behaviour

  • Richard Wrangham: diet consists of seasonal/sparse fruits which cannot suit large groups, females minimise competition and maximise efficiency and reproductive success by feeding alone


  • 1960s Gombe: no evidence of lethal aggression

  • 1970s Gombe: habituated communities at feeding stations split into sub-groups, these groups became extreme separate communities, Kaesekela North and Kahamana South and they stopped travelling into the other range, patrolling boundaries and violent group attacks observed

  • Male chimpanzees participate in intergroup hostility


  • Feeding territory protection

  • Female access protection

  • Female access increase by expanding territory

Philopatry and dispersal:

  • Nishida: males and females belong to set particular group

  • Wrangham: male-only groups with female crossing both groups so male groups compete for territory with more females

  • BUT males attack both opposition/foreign community males and females

  • Goodall: 20 different strange females at Gombe attacked, 15/20 had infants

  • Females sensitive to community borders and expand core range with groups range

  • SO female group spatial allegiance means males don't gain more females by expanding range

  • So true dynamic: Male territoriality repel males and females of other communities

  • Males remain with kin and females disperse

Due to feeding

  • Goodall, Williams, Pusey: Males defend community feeding territory

  • Small range: smaller core area of females, chimpanzee density higher, lower body weight, longer interbirth interval

  • Expanding range: increase reproductive success and rates

Due to avoiding interbreeding

  • Male chimpanzees return to birth group from mothers stay within kin community range

  • Females mate with natal community and then disperse to avoid interbreeding with kin

  • Females show less sexual receptivity to male relatives, and scream to avoid them

  • This behaviour and social structure prevents interbreeding, reduced viability and fertility of offspring

  • Philopatry-dispersal of opposite sexes observed in many animals as interbreeding avoidance mechanism

  • Common pattern is for female philopatry and male dispersal

  • Chimpanzees have male philopatry and female dispersal

Due to kin selection

  • Male philopatry: cooperate with kin, genetics are passed on via relatives

  • Female dispersal: male kin remain for kin selection, so females must disperse to avoid interbreeding

  • Evidence: Gombe individuals shared mitochondrial DNA with individuals several km away which proves extensive female migration

Group dynamics and behaviour:

  • Friendlier behaviour towards relatives

  • Bonobos: females (which show philopatry in bonobos) have strict dominance hierarchy, with relatives and dominant females showing most support for one another and males have hierarchies with tense relationships and do not groom

  • Chimpanzees: male hierarchies depend on alliances with other males, many aggressive interaction but also more friendly male relationships, rival males groom and cooperate against external male threats, female rank has effect on reproductive success, offspring mature younger as they gain better access to food

But is it due to relatedness/kinship?

  • Mitochondrial DNA shows the most genetically related chimpanzees did not have close relationships

  • Chimpanzees with close behaviour relationships, cooperation and grooming were not maternally related

  • Relationships are opportunistic and not kinship related, for advantage of power

  • Philopatry-dispersal may have initially evolved for kinship

  • Levels of relatedness reduced by individual strategies and mating choice

Mating patterns and reproduction:

  • Female swell to advertise readiness to mate

  • Swollen females are centre of attention and subject to attack, wounding, less feeding time, infection

    • IMO: Though not advantages to the female. This feature is retained perhaps as it is those who have it who are more likely to reproduce and pass it on

      • Females can mate multiple times per day when receptive

Due to infanticide prevention

  • Infanticide is frequent but mating with many males confuses relation

  • Males will not want to kill their own offspring

  • By confusing who the father is, any male who has mated with the female will not kill the offspring, thus mating with more males offers greater protection from infanticide

  • Sarah Hrdy: Male primates taking over another group kill group infants so females return to receptivity

  • Mating with many males, even when pregnant, helps confuse paternity

  • e.g. Mahle: males kill boy offspring from newly immigrated females, though these males had mated with the females the fact that the females' ranging pattern was close to the border of their old group proved that infants were of dubious paternity, eventually females spend more time with the infanticidal males

Due to female gain

  • Females may mate with many males for personal gain

  • Males are more likely to protect and share food with females they have mated with

Due to genetic gain

  • Females mating with many males increase chance of high quality genes

  • More likely chance of high-quality male fertilisation

  • Increases male competition so more high-quality males mate with females

Male behaviour

  • Generally competition for female is low

  • Towards end of ovulation when fertilisation more likely high-ranking males become possessive of females

  • In large groups, high-ranking males work together to guard female and share copulation

  • Male may take female to edge of community range for long period, but the single pair at risk of neighbouring attacks

    • IMO: this last one sounds similar to human partnership behaviour

      • Males may also force females by attack to mate, or they may go willingly

Sexual dimorphism

  • Males much larger than female chimpanzees

  • Darwin: males compete aggressively over females

  • Male chimpanzees have large testes compared to other apes

  • As females mate with many males and house multiple male gametes, males who deposit more sperm are more likely to fertilise the female gamete

Paternity - who is the father?

  • Consortships high in chimpanzees (conceptions range from 50% Tutin and McGinnis, to 25% Goodall and Wallis)

  • Goodall: unlikely rank relates to paternity, even though alpha males may be possessive, females still mate with many males when likelihood of fertilisation high, so chance of paternity is varies

  • Other studies show rank and paternity correlation is strong but not absolute

  • Due to these mating patterns relatedness is rare, and few offspring are actually full siblings from the same parents

Human and Chimpanzee social structures


  • Hunter-gatherer bands consist of 150 members similar to chimpanzees

  • Female-biased dispersal and male philopatry (in agricultural, 2/3 hunter-gatherers)

  • Intergroup hostility

  • Patrolling

  • Kinship importance

  • Male philopatry- female dispersal so rare but occurs in chimps, bonobos and humans suggests it was also seen in our common ancestor

  • Sexual dimorphism with larger males


  • Human hostility is organised warfare and pitched battles with many opponents unlike chimpanzees

  • Kin bonds elaborated beyond group boundaries to extent of marriage in humans

  • Human copulations private unlike primates

  • Humans form conjugal marriage bonds, monogamy or man with many female partners unlike chimpanzees where females not bound and have multiple mates

  • Humans mate over more of total life span

  • Human mating occurs at lower frequencies, but at any time

  • Lower sexual dimorphism in humans (15%) than chimpanzees (25%)

  • Humans do not show cyclic changes to advertise ovulation

Why do humans form pair bonds and forgo high promiscuity?

  • Labour division: females gather vegetables and males hunt so the sexes share, infant dependency increased as brain size increased so males help protect and care for infant

  • Female benefits: Smuts suggests females gain protection by male partners and a male has increased mating opportunity, Wrangham proposes that after invention of cooking females needed males to guard their food


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