Session 10: The Chimpanzees

1.  Key Concepts:

 The Chimpanzees are, phylogenetically, the closest relatives to humans with at least five million years passing since the shared human-chimp ancestor.  It is comprised of two species.

2.  Terms & Definitions:

Border Patrol

Groups of males, often unrelated,young adults "patrol" between home groups and neighboring groups.  Patrols are often the source of intergroup aggression.

Sociosexual behavior

Sex in a social context.  Often associated with aggression resolution and maintenance of status.

3.  Chimp Taxonomy:

Two species of chimpanzee: Pan troglodytes schweinfurthii limited to eastern Tanzania troglodytes limited central Tanzania vellerosus limited to central Tanzania verus limited to western Tanzania Pan paniscus found in central DRC

Photo by Melanie and John Kotsopoulos. Some rights reserved.

More commonalities exist between trogoldytes and paniscus than not.  It is rare to see all individuals at one time due to the high levels of fission/fusion within groups.  Pansicus is more likely to exist together. Generalities between the two species include:

• Live in multi-female/multi-male communities.
• Primarily female dispersal and male philopatry (but not 100%).
• Community size ranges between 20 and over 120 individuals.
  
• P. troglodytes community members seldom all in the same place.
• P. paniscus community members sometimes in same place.
• P. troglodytes higher sexual dimorphism than in P. paniscus.

Chimpanzee communities:

Chimpanzees are male philopatric with females dispersing in both species.  Mothers will continue to interact with adult offspring, even in context of young offspring.  Mixed sex groupings are unusual, except in context of mother offspring groups.  Groups of non-related individuals is common, especially around mating and hunting behaviors.  Solitary individuals tend to be older individuals.  Membership in subgroups is fluid.

Subgroup types: mother-offspring females and offspring all male cluster of related individuals heterosexual group consort pair solitary individual

Photo by Chi King. Some rights reserved.

4.  Dominance & Aggression:

Pan troglodytes:

Top rank males form coalitions.  Overthrows come from lower ranking coalitions and frequently involve aggression and pan-tooting, the call.  Physical contact and bonding is common in males – includes hugging, genital manipulation with hands and mouth.  Displays and fighting are common with a high frequency of serious wounds.  Coalitions are core to the group, and aggressive interactions occur with high frequency.

• Males on average dominant to females.

• Males compete for hierarchical ranks.
• Females also occupy ranks but less marked.
• Males sometimes use coalitions to manipulate other individuals.
• Much social negotiation via display and fighting.

Pan paniscus:

Females are dominant, and hierarchical ranking is based on coalitions.  Females have been documented participating in mobbing behaviors directed towards males.  Although the popular literature highlights the high levels of sociosexual behavior in paniscus, the behavior and social context is more complex than often recognized.  Sociosexual behaviors include: copulatory behavior, oral and physical contact, brief, 10-15s social sexual contact is common, especially in the context of resolution.  Confrontations can occur in context of resource acquisition.  Aggression does occur with  potentially serious ramifications.  However, the frequency of aggressive behaviors is significantly lower than in trogolodytes.  Coalitions are core, here too, but are maintained through social sexual contact.

• Females dominant and compete for hierarchical ranking.
• Males have hierarchy but are seldom dominant to females.
• Female-female coalitions and mother-son coalitions important.
• Much social negotiation through social sex.
  

P. troglodytes: Aggression:

Aggression tends to be intercommunity.  However, this aggression has resulted in severe wounding and death.  At Gombe, this has resulted in at least one population being decimated.  This level of aggression has not been observed in pansicus.  Border patrols have been documented – small subgroups of mid age males travel in morning, often single file – travel to/between other groups and join/avoid other groups depending on how many individuals they hear.  Border patrols have moved in and killed neighboring community individuals.  These represent the “imbalance of power hypothesis” – if there is a difference in strength/number between group and patrol, patrol will move to obtain large resources.

• Some inter-community violence, especially from the Gombe.
• “Border patrols” reported for most, but not all, communities.
• Coalitionary attacks have resulted in deaths.

P. troglodytes: Hunting:

Nutritionally, meat intake is inconsequential in the diet of chimpanzees.  Social fervor, though, is important.  Chimps will preferentially hunt red colobus, including avoiding freely available alternatives.  Around Gombe National Park, chimps take ~32% of local red colobus monkeys.  High ranking individuals take from low ranking individuals.  Coalition partners and female kin are often allowed to take meat through partitioning.  No evidence of meat for sex, but strong evidence of meat sharing for coalition bonding. 

• Animal matter can be up to 5-6% of diet.
• Large subgroups hunt Red Colobus monkeys.
• Meat is shared to the extent that sometimes the actual captor does not receive any.
  

5.  Chimpanzee Culture:  Tools & Sex

P. troglodytes:  Tool Use: 

Tool use has been documented repeatedly by troglodytes.  Tools include unmodified stones, wood, and bone.  Uses include:

• Ant dipping
• Termite fishing
• Nut cracking
• Moss dipping
• Leaf cupping.
  

P. paniscus: Social sex:

Homosexual activity between male-male and female-female, but also heterosexual activity.  Fruit masting or large fruit sources, such as jackfruits or durian, results in social coalitionary building context for sociosexual behavior.   Sociosexual activity is social bonding. 

• Conflicts frequently resolved via sexual contact between females (and males as well).
• Mega-fruit food sharing amongst females mediated by sociosexual behavior.
• Overall rates of aggression low.

Do Chimpanzees have culture?

Have highly variable, cross-community, non-functional behavior.  Ex: hand holding, wrist linking, far away high 5 (of sorts).  Some primatologists have argued this is culture.  This includes stylized differences between communities.

Chimpanzees: the basics:

• Generally male philopatric, but not exclusively.
• Communities are “territorial.”
  
• “Border patrols” are common.
  
• Frugivores, but do hunt meat (esp. red colobus).
• Some lethal violence.
• Some infant killings.
• Some meat eating (~5% of diet), but meat always a prized resource and not all obtain equal, or any, amounts.

6.  Violence & War:

Chimpanzee Violence Hypothesis:

The chimpanzee violence hypothesis posits that individual and community fitness increases through group violence.  Groups form coalitions to intimidate and perform violent acts, to further the imbalance of power, to ultimately gain access to resources.  Hypothesized as the root of human warfare, sexual coercion.  Male-male bonds are the nexus of aggression, expressed as border patrols, warfare, etc. 

• Coalitionary killing, strategic assessment, and intimidation.
• Imbalance of power hypothesis: intercommunity dominance pays benefits to the winners.
• Hypothesized roots of warfare.
• Role of females unclear.
  
• Role of male-male bonds.

“Demonic” Chimpanzees: the basic assumptions

Demonic male hypothesis:  in chimps, humans, & gorillas, male philopatry has led to male-male bonds stronger than male-female bonds.  Some evidence that infanticide is intimidation for sex; weak evidence for meat for show-off for access to sex hypothesis. 

• Male philopatry-female dispersal.
• Male homosexual bonds stronger than heterosexual bonds.
• Coalitionary violence as both territorial control tool and sexual coercion.
• Infanticide as a male strategy.
• Sexual coercion and male control.
• These elements characterize all chimpanzee communities.

Coalitionary killing: the root of human/chimp aggression in males?

• In eight studies of common chimpanzees spanning 38 years (170 study years) there have been 10 adult deaths directly related to lethal violence and as many as 20 deaths assumed to be the result of such violence.
• All of the observed and assumed instances of lethal aggression come from studies of the subspecies Pan troglodytes schweinfurthii at four sites in the Eastern/central portion of the chimpanzee’s range.

Documented instances of Pan troglodytes schweinfurthii violence:

• Gombe: 38 years, seven observed and three inferred instances of lethal aggression.
• Mahale: 33 years, two observed and six inferred instances of lethal aggression.
  
• Kibale: 11 years, three instances of lethal aggression observed instances of lethal aggression.
  
• Budongo: 8 years, two observed and one inferred instances of lethal aggression.
  

Documented instances at Western Species/Subspecies sites:

• Tai (P.troglodytes verus): 24 years, one instance of lethal aggression.
  
• Bossou (P.troglodytes verus): 22 years, none BUT SEMI-ISOLATED.
• Wamba (P. paniscus): 24 years, no lethal aggression.
  
• Lomako (P. paniscus): 15 years, no lethal aggression.
  

Testing the assumptions: Additional Information:

P. troglodytes

• Meat for sex?  One observed exchange in all studies, which includes hundreds of hunts, and lack of clear correlation between hunting and female reproductive status, BUT meat is shared amongst coalition partners.
  
• Male-male bonds only?  Not exclusively.
• Females usually disperse (at least after one birth), and sometimes males disperse as well.
• Complex mating patterns, but evidence for some benefits to high rank for males and mate guarding by male coalitions, male aggression in some cases?  Sexual coercion?
  
• Role of biological kinship for males? Unclear.
• Coalitionary violence as a strategy?  Unclear.
• There are 12 observed and 10 inferred instances of adults killing infants, however, the majority of these attacks were carried out by adult females, not males.

What is the current thought… Do chimpanzees display coalitionary violence?

• Richard Wrangham says yes, selected for violence and warfare is our common heritage.
• Jane Goodall says yes, sort of…the evidence is unclear.
• John Mitani says yes, but warfare not fully supported by evidence.
• Bill McGrew says yes, but warfare not fully supported by evidence.
• General scientific consensus: lack of evidence for big picture, but chimpanzees to engage in intercommunity aggression with lethal results.  Not clear that males are demonic or use coalitions as exclusive female dominating mechanisms.
• Prof Fuentes says there is not enough evidence to argue evolutionary similarity between chimp inter-community interactions and male aggression, and human warfare, rape and murder.  Cultural complexity, agriculture and different scales make comparison extremely difficult, but the hypothesis has not been refuted, just not yet supported to significant extent.

7.  Additional Material:

Primates in Perspective.  2007.  C.J. Campbell. A. Fuentes.  K.C. MacKinnon.  M. Panger. S.K. Bearder.  Oxford University Press.

Chapter 19: Chimpanzees and Bonobos: Diversity within and between species - Stumpf.