Anne E. Pusey in 'Tree of Origin: What Primate Behaviour Can Tell Us about Human Social Evolution'
Notes by Georgina Holmes
BOOK CHAPTER SUMMARY
BOOK:
Tree of Origin: What Primate Behaviour Can Tell Us about Human Social Evolution
CHAPTER 1:
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
Communities:
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
Violence:
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
Why?
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
Similarities
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
Differences
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