Wednesday, January 28, 2015

Are the famous, sex-loving bonobos really more peaceful than their chimpanzee relatives?

Juvenile bonobo eating
Bonobos (Pan paniscus) are often referred to as the peaceful apes or the "make love, not war" apes. Previously known as the pygmy chimpanzee, they're thought of as more gentle and peaceful than their closely related cousins, the chimpanzee (Pan troglodytes). These two species are very closely related: 99.6% of their genome is the same.  Many people are aware that chimpanzees hunt other primates. You may even have heard that they will go to war against other chimps, brutally killing members of their own species, wiping out another group of chimpanzees.  With 99.6% of their DNA identical to chimpanzees, do bonobos really deserve this wholesome image?

Bonobos are only found in the Democratic Republic of Congo. They're smaller in body statue than chimpanzees but otherwise they look very much alike. In chimpanzee social systems, the males are dominant over the females. In bonobo groups, its the females who are dominant over males. Females remain in their natal unit and develop strong bonds with other females. Bonobos also play more as adults, engage in sexual activity more, and display less severe aggression than chimpanzees. So far, it looks like bonobos are living up to their reputation as the more peaceful primate.

As we learn more and more about bonobos, at least one of our original ideas about them has been proven wrong. Originally, only chimps were thought to hunt and eat other primates, but this isn't true. Surbeck and Hohmann (2008) published the first account of wild bonobos hunting other monkeys. Both males and females were active in the hunts and red colobus monkeys, black mangabeys, and other primate species were on the dinner menu for bonobos.  Seems like we've busted that myth.

Bonobos sharing food
However, a study compiling over fifty years worth of data on chimpanzees and bonobos seems to suggest that there is quite a difference. Studying 18 communities of chimps versus 4 communities of bonobos, scientists reported 152 killings by chimpanzees compared to just one killing by bonobos (Wilson et al., 2014). It is thought that differences in the brain between these two great apes may be the underlying cause behind the increased violence and aggression seen in chimpanzees. The brain of a bonobo seems to be better adapted to impulse control, perceiving stress in others. Rilling and colleagues (2012)  state that their neurology "...allows them to more strongly represent distress of both self and others compared with chimpanzees. This translates into greater empathy, as well as reduced aggression." The Rilling article is a great read, as it includes not only their findings but also a thorough overview of the behavioral differences between chimpanzees and bonobos.

To summarize, while there may not be much separating these two species in terms of their DNA, how their DNA is expressed does appear quite different. At first, scientists wondered if we simply had less data available to us on bonobos, and that lack of data meant we were missing behaviors in bonobos that are seen in chimps. (The chimpanzee population is far larger than the bonobo population.) As we study them more and more, chimpanzees come out on top in terms of aggressive behaviors.  It's important to remember through that all of these aggressive behaviors don't make up the majority of a chimpanzee's day. It's quite the opposite. Social behaviors make up a small amount of any primate's day. Feeding and resting take top priority. That said, I think the conclusion that bonobos are more peaceful than chimpanzees stands for now.

Food for thought: I think it's most surprising that bonobos will share first with strangers before they share with friends or associate bonobos (Tan and Hare, 2013). This isn't something we see with chimpanzees (and arguably something we don't see in all humans). Why would a bonobo share with a stranger first? How is this an adaptive behavior?

Check out this incredible BBC video explaining the hunting tactics of chimpanzees. (Yes, they really do have tactics.)

Read NOVA's interview with the well-known primatologist, Frans de Waal, about studying bonobos.

Journal articles referenced:
Surbeck, M., & Hohmann, G. (2008). Primate hunting by bonobos at LuiKotale, salonga national park. Current Biology, 18(19), R906-R907. doi:10.1016/j.cub.2008.08.040
Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, I. C., Hashimoto, C., . . . Wrangham, R. W. (2014). Lethal aggression in pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414. 
Rilling, J. K., Scholz, J., Preuss, T. M., Glasser, M. F., Errangi, B. K., & Behrens, T. E. (2012). Differences between chimpanzees and bonobos in neural systems supporting social cognition. Social Cognitive and Affective Neuroscience, 7(4), 369-379. doi:10.1093/scan/nsr017
Tan J, Hare B. Bonobos Share with Strangers. PLOS ONE, 2013; 8 (1): e51922 DOI: 10.1371/journal.pone.0051922

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