Are bonobos retarded chimpanzees?

A group of researchers led by Harvard’s Richard Wrangham has sought to prove a theory that because the unusual food sharing behavior of bonobos is found in juvenile but not adult chimpanzees, we can think of bonobos as developmentally delayed chimpanzees. “Developmental delayed” is the politically correct replacement for “retarded.” Human behaviors more closely resemble those of chimpanzees than bonobos, so some may infer that the researchers began with the presumption that chimpanzee behaviors are more advanced. After all, aren’t humans the most advanced species on the planet, and isn’t capitalism – based on selfishness, not sharing – the most advanced exchange system? Humans, except when they attend Burning Man, don’t rely on a gift economy.

“Grumpiness is a sign of advanced civilisation” screamed one headline over a story about the research. Does Wrangham assume that selfish and conflict prone patriarchal chimpanzees are more advanced than matriarchal bonobos, peacemakers and sharers? If so, that offends both fans of peace and admirers of women. Putting aside the gender issue, Darwin himself recognized that cooperation is highly evolved way of helping one’s species survive. Before criticizing further, it’s worth examining how Wrangham’s gang came to their theory and their attempt to prove it.

They would argue they are simply trying to find a Darwinian mechanism to explain one of the key behavioral differences between the species, not to make a value judgment about them. The two species split from a common ancestor relatively recently, possibly less than a million years ago, but live in different habitats. Bonobos’ available food resources generally are far more abundant than chimpanzees’. The researchers presume that chimpanzee selfishness most likely was characteristic of the common ancestor of the two species in part because the species closest to them is us, and human children start out wired to behave selfishly and come to share only after considerable socialization. (Chimpanzee youngsters default to sharing, but then seem wired to learn by experience to be selfish to compete for scarce food.) Thus, in trying to explain the two species’ divergence, the scientists sought to explain the evolution of sharing in bonobos rather than selfishness in chimpanzees.

In formulating their developmental delay theory, the researchers observed that chimpanzees’ brain structures change more from childhood to adulthood than do bonobos’. For additional support, they looked to a famous long term experiment involving Siberian silver foxes bred for greater tameness. Those foxes retain more juvenile physical characteristics into adulthood that did wild foxes, leading to the conclusion that natural selection for peacefulness also tends to select for individuals with more juvenile characteristics. Also, sexual dimorphism – size differences between the genders – is less in these tame foxes, as it is in bonobos as compared to chimpanzees. Perhaps the researcher also were inspired by the fact that bonobos appear less muscular than chimpanzees and their calls are higher pitched.

All of these phenomena support the theory that a mechanism for making a species less aggressive is for natural selection to favor individuals with more juvenile characteristics. As the authors of the study say, “This idea does not imply that bonobos are juvenilized globally; instead, it suggests that juvenilization has occurred in a set of traits that are genetically linked.”

The thirty chimps and thirty bonobos in the study were in “semi-captivity” on Congo reserves for apes orphaned by the bushmeat trade. Assuming it would even be possible to replicate the experiments on wild animals, the same results might not be obtained in a wild population. In one part of the experiment , the researchers observed pairs of either chimpanzees or bonobos in an enclosure with food. Younger chimpanzees shared food with each other at about the as much as either young or adult bonobos. Adult chimpanzees shared much less than juveniles, while bonobos’ tendency did not change with age.

Testing young apes’ skills of social discrimination, three experimenters stood side-by-side, arms outstretched toward the each subject, who were either infants, age two to four, or adolescents, age five to seven. After a few trials is which all three experimenters held food and then just two adjacent ones did, in the experimental trial only the two on the outside reached into a food container in view of the subjects and only those two then had food concealed in their hands. The chimpanzees soon only begged from outsiders with food and not from the one without in the middle. There was no difference in skill between infant chimpanzees and adolescent chimpanzees. Infant bonobos performed much less well, but adolescents performed as well as the chimpanzees. The authors of the study saw this as evidence that bonobos are developmentally delayed compared to chimpanzees.

The second social experiment tested adolescent and adult apes, and for each trial employed two experimenters. One had food and the other did not. Halfway through the trial, their roles changed. Again, chimpanzees of both ages did well recognizing the switch in their begging, while adolescent bonobos did not and only adult bonobos eventually did as well as the chimpanzees.

The begging experiments using the experimenters as objects of begging are fraught with design biases and value judgments. The authors describe these trials as simulating what the apes would experience in feeding with intolerant individuals. But what evidence is there that adult apes switch their generosity on and off toward particular individuals in their group several times day? Even if one stretches things enormously regarding what chimpanzees experience in the wild, this most certainly is not what bonobos deal with. Thus, the experiment was designed to test skills that chimps have, but bonobos don’t, and as such is a self-fulfilling prophesy: chimpanzees are better at chimpanzee skills than bonobos are.

The authors argue that if characteristics don’t change with age, that is a sign of retention of juvenile characteristics. In the social discrimination experiments, the chimpanzees did not change as they got older, but the bonobos did. Which species most retains its juvenile characteristics? To be complete, they’d have to design some tests at which bonobos excel, and see if chimpanzees fall behind bonobos of the same age. If they did could we then say that chimps are retarded?

Maybe sharing, which has to be taught to human children, is the more advanced adult trait. Maybe it is chimpanzees that lack self-control, much as we often label a selfish adult human as immature. Perhaps the bonobos’ advanced brains, despite their relative lack of structural change in adulthood, can see forward in time and better recognize the concept of future reciprocity of reward for present sharing, which the chimps cannot do as well. Maybe bonobos, known for their greater communication skills, assume that begging to humans with food, as well as those without, maximizes their chances of success. By soliciting to the foodless humans, perhaps the bonobos believe they are encouraging those humans to beg from those with food, thereby increasing the odds that will get something in the exchange instead of nothing. This might make sense in the context of bonobo culture, even if in the context of their cultures human experimenters and chimpanzees don’t get it.

Putting aside any value judgments about the apes’ cultures and just talking about the mechanism of natural selection for peaceableness, one prominent behavioral biologist criticized the experiment saying, “Unfortunately, their results just don't support their conclusions” But he did concede, “There seems to be something there, and they're probably right about what it is.”