A full-grown male chimpanzee carries a stick at the
Sweetwaters Chimpanzee Sanctuary in Kenya. The sanctuary is the work of
primatologist Jane Goodall. Jean-Marc Bouju/AP
For years, there have been two main theories about why
chimpanzees, our closest evolutionary cousins, sometimes kill each
other. One theory blames human encroachment on the chimpanzees' native
habit in Africa. Another says that (male) chimps kill in the normal
course of competition with rival groups.
A new study published in Nature
appears to support the second theory. In short, it found that the
numerical makeup of chimpanzee communities is roughly proportional to
the "chimp murder rate."
"Variation in killing rates was
unrelated to measures of human impacts," the authors, Michael L. Wilson,
Christophe Boesch, et al., write in the abstract. "Our results are
compatible with previously proposed adaptive explanations for killing by
chimpanzees, whereas the human impact hypothesis is not supported."
be sure, the knowledge that chimps will occasionally carry out
organized killings on groups of rivals is nothing new. As early as the
mid-1970s, researchers in Tanzania's Gombe National Park observed gangs of a half dozen or more male chimpanzees conducting lethal raids in neighboring territories.
As The New York Times
wrote in 1988: "For some time after the pioneering studies of Jane
Goodall and others, it was thought that chimps were generally peaceful,
playful, sophisticated and easygoing. ... Then, from Ms. Goodall's own
work, and in particular from her associate Richard Wrangham, it became
evident that chimpanzee males engaged in active killing of other chimps
and other primates."
Still, the question of how common the behavior was and why exactly it occurred remained open to debate.
In an article in 2011 published in Psychology Today,
University of Notre Dame professor Darcia Narvaez summed up the
argument for human impact. She noted that in the first 14 years that
Goodall and Wrangham observed chimps at Gombe, "aggression patterns were
no different from other primates (peaceful and unaggressive)."
the behavior suddenly changed: "With hindsight, it turned out that
human feeding of the chimpanzees, with its restrictions and control,
deeply affected the behavior and culture of the chimpanzees, such as
keeping large groups of animals near the feeding site, which promoted
increased fighting among the males," Narvaez wrote in Psychology Today, citing The Egalitarians: Human and Chimpanzee, a 1991 book by Margaret Power.
In a rebuttal to Narvaez published soon after in Psychology Today,
Kevin D. Hunt, an anthropology professor at Indiana University who had
Goodall colleague Wrangham as his doctoral co-supervisor, concludes:
is irrefutable evidence that the threat of lethal violence has exerted a
strong evolutionary force on chimpanzee nature, and its effects are
visible on a minute-to-minute basis in chimpanzee society. It is the
origin of the very unusual social bonding among male chimpanzees — they
must hang together to protect against extra-group murderers."
As bleak as this sounds, Wrangham — although he adheres to the chimps-as-natural-born-killers theory in the book Demonic Males — finds cause for optimism when it comes to the ability of humans to change their own violent tendencies.
In observing bonobos
(the closely related but less-violent cousins of chimpanzees), Wrangham
observed peaceful communities based on a power-sharing arrangement
between males and females. Chimps, by contrast, live in patriarchal
groups where dominant males run roughshod over compliant females.
reason for the difference, he concludes, is sex selection. Female
chimps select aggressive males as mates; female bonobos don't.
example of the bonobos reminds us that females and males can be equally
important players in a society," Wrangham is quoted in Harvard Magazine
as saying. "And by giving us a model in which female action works in
suppressing the excesses of male aggression, the bonobos show us that in
democracies like our own, women's voices should be heard more than they
The progress of social justice is slow and measured. Its growth depends on an increasing number of us becoming aware of the truth and consequences of our actions.
I believe that it is morally wrong to allow our wanton desires to interfere with the basic needs and interests of other sentient beings.
I believe the physical and psychological abuse – confinement, social deprivation, mutilation, genetic and reproductive manipulation, and profit exploitation – imposed by us on other animals is morally wrong.
I believe the suggestion that the exploitation of other sentient beings by humans can be achieved without cruelty, violence, or injustice is false and misleading.
As an advocate for all life, committed to compassion and justice, I refuse to take part in the exploitation of other sentient beings or to collaborate with those caught up in such injustice.
I pledge to do my best to live a life that conveys a clear, sincere and uncompromised message that is free of resentment, fear, exploitation, anger, cynicism, and manipulation.
Furthermore, I pledge to continue to support a broad range of nonviolent initiatives and programs that will hopefully one day eliminate the needless pain and suffering we inflict upon all the many wonderful creatures with whom we share this planet.