Killer chimpanzees and human warfare

5 minute read

This article from World Science Net popped into my inbox today. It's basically a short news story on an upcoming research presentation by David Watts (Yale University) on the incidence of intergroup chimpanzee violence at Kibale. The article has Watts and Richard Wrangham (Harvard University) in support of the idea that chimpanzee violence is widespread and homologous with human violence, and Brian Ferguson (Rutgers University) speaking against the idea.

The basic observation has been well understood: under certain circumstances, chimpanzee males will kill members of neighboring groups. This occurs most commonly when the victims are caught alone, and repeated instances have in at least one case basically wiped out the males in a targeted group (at Gombe). The new observations at Kibale just add to this record, with Wrangham noting that 49 killings have been documented to date, with two-thirds "either directly seen...or inferred from clear evidence such as chimps prancing around a brutalized corpse." Wrangham offers this as a counterargument to the idea that the chimpanzees have merely disappeared without researchers really knowing if they have been killed, although that argument itself is weak on its face to anyone who has lived on a farm and wondered why the dog suddenly stopped coming for dinner. Maybe sometimes chimps, like dogs, just wander off; but with the dog it is a lot more likely that it has been hit by a car, and I have to imagine that the idea that chimpanzees are just "disappearing" is about as likely as the dog finding a new home that somehow isn't with one of the neighbors.

Really, the central question that most people think is interesting is whether the behavior of chimpanzees has any predictive value for the behavior of ancient humans. Here, I think the data is very weak. The chimpanzee observations clearly show that deliberate killing is within the cognitive range of chimpanzees. If killing is cognitively possible for chimpanzees, we may infer that it would have been so for early hominids also. But this really does nothing to demonstrate that killing would have been in any way adaptive for early hominids. For chimpanzees, the adaptive story has to do with the sizes of groups and types of competition that individuals--especially males--face. And as Wrangham points out, chimpanzees not only have motive, but also opportunity stemming from their fission-fusion community structure, which leaves chimpanzees occasionally isolated as easy prey for packs of marauding males.

Did early hominids similarly have motive and opportunity for killing? If early hominids had more cohesive groups than chimpanzees, they might have been relatively immune from violence even if the motive were present. The relatively cohesive groups of gorillas are suggested as effective defenses against the risk of violence or infanticide from rogue males, so it would not be unusual if early hominids pursued such a strategy. And the advantages of killing or other modes of violence are not assured for early humans. For chimpanzees and other primates like langurs, male coalitions give their members an opportunity to gain mating access through coordinated action, including violence. But the freedom of action of human coalitions is more limited. Cultural inhibitions, driven by the long memories of victims and their kin, present the substantial risks of vengeance or other costs for human killers. It remains an open question whether these inhibitions against violence are more or less effective in small-scale societies like those of ancient humans. Arguably, they are ineffective in almost every society, but they nevertheless present potent reasons for most humans to avoid killing and to coordinate actions to punish killers. So we may question whether the motive to kill was likely to characterize ancient humans.

But on the other hand, biologists like Richard Alexander (University of Michigan) suggest that interactions among early human groups were likely highly competitive. Alexander suggests that this competition was one of the major factors behind the evolution of human cognitive abilities. This idea dovetails with the notion that early human groups may have been significant units of selection, with coordinated actions and altruistic behaviors characterizing within-group interactions, and violent competitions characterizing between-group interactions.

For me, a more interesting issue is the effect of violent behaviors on the pattern of natural selection within ancient human and primate populations. Unlike many modes of competition, killing has a direct selective effect, as competitors are immediately removed from the population. Selective coefficients of 0.01 or less are nonetheless immensely powerful over the course of hundreds of generations. With 49 observed cases of killing among chimpanzees, panicide seems to account for over one percent of chimpanzee deaths, and in the areas where it has been most noted (Gombe, Kibale) its incidence is much higher. The benefits to a coalition of males from killing members of adjacent communities plausibly are very high. At the very least, since males are philopatric, the reduction in size of neighboring groups will provide an assurance of the survival of the males' community, which will contain their own male progeny. Moreover, the weakening of neighboring groups may enable their female offspring to have an easier time dispersing or establishing status in the weakened neighboring groups. And of course there is the possibility that all males in the neighboring group will be exterminated, allowing some or all of the male coalition to extend their mating access to those females. Each of these advantages is potentially powerful, and together they may imply that the slight incidence of killing could nevertheless create a strong selective advantage.

One criticism of the chimpanzee observations is that the chimpanzees may be suffering resource shortages or other extraordinary effects of human presence in the study areas. This might well be true, and Wrangham's argument that the chimp population is growing hardly mitigates the possible effects of overcrowding in a small forest preserve. But even if the behavior is influenced by anthropogenic processes today, this does not mean that the behavior did not occur in the past. Environmental fluctuations and ecological crises have probably recurred many times during the evolution of chimpanzees. Each crisis may not have been as serious as today's human-induced habitat loss, but each might well have influenced population sizes and community structures in ways that made killing a viable behavioral adaptation. An occurrence in populations under stress might well have a high selective effect even if it were normally neutral or maladaptive. And of course it has not been demonstrated that killing is maladaptive in normal circumstances in stable populations.