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Killer Chimps Reveal Why Violence Persists

Chimps and humans share violent tendencies, with a new study showing that the evolutionary roots of warfare exist in our primate ancestry.

Chimpanzees and humans share much in common, including cooperating to kill perceived rivals, and now a new study finds that this kind of lethal aggression -- at least among chimps -- is "normal" and sadly all too common.

"Normal," in this case, means that the behavior results from natural and evolved tendencies and does not, as some other researchers have suggested, emerge in response to human pressures, such as habitat loss.

The study, published in the journal Nature, sheds light on the evolutionary roots of lethal conflict among certain primates, including humans. An accompanying "News & Views" article in the same journal, for example, points out that in 2013 alone, there were 33 armed state-level conflicts around the world. Many of them have persisted for decades.

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Chimps are just as violent.

"Most killings involve gang attacks," Michael Wilson, who led the new study, told Discovery News. "When attacking adults, many attackers pile onto the victim. They pin the victim to the ground and hit, kick and bite the victim."

"Attackers may cause massive trauma to internal organs, break bones, inflict numerous puncture wound from canine teeth, and bite or tear off fingernails genitalia, and even the throats of victims," added Wilson, who is an associate professor of anthropology, ecology, evolution and behavior at the University of Minnesota.

Male chimpanzees, he continued, also sometimes kill infants after attacking their mothers and snatching their babies away. They will then kill the infant by biting it to death or hitting it against the ground or trees.

Photos: How Santino the Chimp Attacks Visitors

Such traumatic moments became evident after the researchers analyzed 426 combined years of research at 18 chimpanzee study sites and 92 combined years of research at 4 bonobo sites. Coalitional killings were documented at 15 of the 18 chimp study sites, but there was only one suspected killing among bonobos.

The muriqui, a South American monkey, is also known for peaceful behavior, based on earlier studies.

Chimps and humans, however, have similar rates of killing among rival groups.

"Males kill more often than females in both species, and humans and chimpanzees share an unusual pattern of cooperating to kill," Reynolds said.

This method of killing appears to evolve in "fission-fusion" social systems, where males from one group split off into more temporary subgroups. Both chimps and humans have such "gangs" that may attack rival individuals or gangs, particularly when they outnumber or outmuscle the victim(s).

Reynolds believes that the behavior is driven by "both genetic and non-genetic components."

Does that then mean some chimps and humans are inherently predisposed to kill others of their own kind?

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"The data tell us that there are some ecological and demographic circumstances in which the benefits of lethal aggression exceed the costs for chimpanzees, nothing more," Joan Silk, author of the "News & Views" piece, wrote.

"Humans are not destined to be warlike because chimpanzees sometimes kill their neighbors," added Silk, a professor in Arizona State University's School of Human Evolution & Social Change.

Reynolds agrees. He believes that trade is a powerful tool that humans have to curb warfare.

"Political scientists have found that countries that trade a lot with each other are less likely to go to war with one another," he explained. "Another important tool is democracy. Countries with mature democracies almost never go to war with other democracies."

Portrait of Titan an adult male from the Kasekela community.

Chimps are smart, sassy and funny, especially when dressed up like little people. But did you know that they’re much more like us than we might realize? For one, they’re cold-blooded killers, carrying out brutal raids on other chimp groups to expand their territory. The attacks are most often done by patrolling packs of male chimps that are "quiet and move with stealth," according to the study’s lead author, John Mitani of the University of Michigan. And it works for them: they get land, extra food and resources, and even better access to females.

Chimps Engage in 'War' for Turf

Male chimpanzees have spines on their penises that likely increase stimulation during mating, according to a study in Nature. Human males once had them too, but they dropped this trait. Lucky for the ladies –- or unlucky, as the case may be -- because the penis spines, while improving stimulation, can also be pret-ty painful for females during intercourse.

Chimps Have Better Sex Than Humans

Chimps looove green monkey oranges and will go to great lengths to open the stubborn fruit. In fact, three chimpanzee groups opened the fruit in different ways, showing that chimps can innovate. The chimps, all living in Zambia, invented eight different ways to get inside the hard-shelled fruit: bang it against a tree or a rock; throw it; nibble a hole; go at it with your teeth; smack the fruits together; stomp on it; and peel it.

Chimps Invent 8 Ways to Open Hard Fruits

Tickle chimps and what do you think happens? That’s right, they giggle like children. Their laughter comes in the same sorts of situations as humans, sounds like a human and they laugh more than we do, since they can do it while inhaling AND exhaling. Other animals giggle too when tickled, including rats and puppies.

Apes Giggle Like Humans

When you’re working with someone on a project, you follow along with what they’re doing and offer them help when you sense they need it, right? Chimps do the same thing, sharing a tool or physically chipping in on team projects. And once they learn to help a friend out one time, they’ll do it up to 97 percent of the time.

Chimps Trade Tools To Help Out Pals

Videos of chimp colonies show our closest cousins treating each other in dying and death much like we do. In one video, two chimps kept a vigil over a dying community member, touching and grooming her. In another, a mother shoos flies away from her recently deceased infant. Mother chimps have also been seen carrying dead babies around for weeks after they pass, some until they’re mummified.

Chimps Face Death Like Humans Do