Sometimes, zoo animals behave unnaturally. Most animals on display at zoos are not really designed for captive living. If you've been to a zoo, no doubt you've noticed evidence of this: a tiger who paces back and forth, or a monkey that does nothing but circle the enclosure. Life in captivity can even result in various forms of self-harm: a bird that plucks out its feathers, or a horse that bites at her own body, occasionally drawing blood.

Sometimes, zoo animals behave naturally. They mate. Or refuse to mate. They groom eachother. They get sick. They get better. They care for their young. They sleep - a lot.

They fight.

It must have been extremely unsettling for a handful of zoogoers to watch a male chimpanzee kill a three month old infant female chimpanzee at the LA Zoo on Tuesday. She was the first chimpanzee to be born at the LA Zoo in thirteen years and was therefore, in a sense, symbolic. It's a serious setback for conservation efforts, since there are fewer than 300,000 chimpanzees living in the wild, according to the International Union for Conservation of Nature. The chimpanzee colony at the LA Zoo, now numbering fifteen members, is one of the largest in the country, and is considered a model for the Association of Zoos and Aquariums' Species Survival Plan Program.

It is sad whenever an animal at a zoo dies, especially at so young an age. I am certain that members of the LA Zoo's staff - especially those on the Great Ape Team who worked directly with the unnamed infant - will be grieving along with Gracie, the mother.

In a statement, the zoo expressed its surprise and regret:

Chimpanzee behavior can sometimes be aggressive and violent and the Zoo is sorry that visitors had to be exposed to this. Gracie is being allowed to keep the infant overnight to allow her the opportunity to grieve. This is a heartbreaking and tragic loss for the Zoo and especially for the Great Ape Team who have worked diligently to care for the infant and its mother since its birth.

Watch a chimpanzee troop at a zoo for a few minutes, and you can't help but be struck by the similarity between humans and our evolutionary cousins. Familiar expressions of emotion dance across their faces. They playfully chase eachother around. They cool off in the shade, and warm up in the sunlight. They eat and play, fight and make up.

On one visit to a zoo, a juvenile chimpanzee approached me from the other side of the glass. As I looked into the eyes of my new chimpanzee friend, I experienced the overwhelming sense that there was a someone in there who was looking right back at me. It feels like looking into a mirror and seeing a part of yourself in the reflection.

I suspect that this sense of familiarity is what prompted one woman, in an interview with the LA Times, to express her anger at the zoo's staff:

Zoo patron Victoria Pipkin-Lane said she was "furious" at zoo staff because she said she saw two tussles between chimps just days before the baby was killed. Pipkin-Lane, the executive director of Los Angeles County's Quality and Productivity Commission, said she witnessed a 10-minute fight between two chimps Saturday. One of the chimps appeared to be protecting Gracie and her baby chimp. "It was scary," Pipkin-Lane said. "Those of us that were near the compound were like, 'My God, why aren't they going to do something about this?'" In a letter to Mayor Antonio Villaraigosa, she asked that he demand a full report and action plan from the zoo.

One reason that the keepers were unable to intervene in the chimpanzee fight was that chimpanzees are much stronger than humans, despite their lower average weight. If the keepers had intervened, it is possible that journalists would be reporting the death of the keepers in addition to the young infant. In a 2009 article in Slate, anthropologist John Hawks commented on the strength of chimpanzees. While not five-to-eight times stronger than humans, as the common myth goes, chimpanzees remain quite impressive. "A chimpanzee had, pound for pound, as much as twice the strength of a human when it came to pulling weights," Hawks wrote. "The apes beat us in leg strength, too, despite our reliance on our legs for locomotion."

A few fights here and there among chimpanzees would not have been cause for concern. Chimpanzees fight all the time, both in captivity and in the wild. For the chimps as for the members of the zoo's primate team, Saturday was very likely just another day, because as far as species go, chimpanzees are fairly aggressive. They are unlikely to voluntarily share food - even between mother and infant! In the wild, neighboring groups often go to war, competing over territory and resources. Within social groups, fights commonly occur over access to females and to food.

And chimpanzees don't only pick on those their own size.

Infanticide is a common feature of chimpanzee life, both in captivity and in the wild. In a 1999 paper in the journal Primates, primatologists Adam Clark Arcadi and Richard Wrangham reported that at least twenty cases of infanticide had been recorded in three chimpanzee study areas in Africa. By 2002, according to a paper published that year in the same journal, chimpanzee infanticides had been observed in four study areas. Given how little of wild chimpanzee behavior can actually observed by human researchers, this statistic suggests that infanticide is a fairly common occurrence.

The details, however, vary quite a bit. The killers can be males or females. They kill alone or with partners. Occasionally the killer is from a different social group, but usually the killer and infant belong to the same troop. Since female chimpanzees mate promiscuously, males may even kill infants that they fathered themselves. The mothers of the victims may disassociate entirely from the killers, or they may continue or increase their association with the killers. Sometimes, they even go on to mate with the killer. Often, but not always, the victim is cannibalized.

Scientists have not yet reached consensus as to the reason for infanticide in chimpanzees. Whatever the explanation, it is clear that infanticide is a recurring feature of their complex social lives.

What was natural for the chimpanzees, however, was clearly unnatural for the humans. Indeed, the zoo is an unnatural environment for the animals on both sides of the glass.

In nature, it is impossible for a human to come face to face with a polar bear and a penguin on the same day. A visitor to a typical zoo can get up close and personal with big cats like lions and leopards without fearing for their own safety. A group of social primates can intimately observe the lives of a second group of social primates. And they can do it without trekking through the jungles of Uganda or the Congo. At times, it isn't pretty. It can be downright scary.

Rather than castigate the staff of the LA Zoo (which is owned, operated, and maintained by the City of Los Angeles), Mayor Villaraigosa should instead remind the public that the chimpanzees in the zoo are real, wild animals, who lead complex, highly nuanced social lives that we do not yet completely understand. Humans look at chimpanzees and see a partial reflection of themselves, and still become surprised when things turn violent. Tragedy befalls humans all the time; why should chimpanzees be any different?

Zoogoers need to remember that nature is not all rainbows and unicorns. We do not live in a mythical Edenic garden or Elysian field. That isn't to say that nature does not feature cooperation in abundance. One of the most impressive examples of cooperation in chimpanzees is the way they work together in intricate choreography as they hunt colobus monkeys. While examples of cooperation are omnipresent in the natural world, even among chimpanzees, at times nature is indeed "red in tooth and claw," as Tennyson famously wrote.

Zoos can be fun, educational, awe-inspiring places. Curated and presented correctly, they can inspire children and adults to appreciate and learn more about the natural world. Nature documentaries of even the highest quality are unlikely to match the visceral experience of observing living animals. But we cannot expect animals to act anything other than naturally, and until we more completely understand their behavior, there will always be a certain amount of unpredictability associated with rearing animals in captivity.

More predictable critters can be found at Disneyland, where they're made of felt and foam.

Adam Clark Arcadi, & Richard Wrangham (1999). Infanticide in Chimpanzees: Review of Cases and a New Within-group Observation from the Kanyawara Study Group in Kibale National Park Primates, 40 (2), 337-351 DOI: 10.1007/BF02557557

David P. Watts, & John C. Mitani (2000). Infanticide and Cannibalismby Male Chimpanzees at Ngogo, Kibale National Park, Uganda Primates, 41 (4), 357-365 DOI: 10.1007/BF02557646

Watts DP, Mitani JC, & Sherrow HM (2002). New cases of inter-community infanticide by male chimpanzees at Ngogo, Kibale National Park, Uganda. Primates; journal of primatology, 43 (4), 263-70 PMID: 12426461

de Waal FB (2005). A century of getting to know the chimpanzee. Nature, 437 (7055), 56-9 PMID: 16136128

Photo of Gracie with her infant by Tad Motoyama, courtesy of the LA Zoo, used with permission. Header image by derekkeats/Fotopedia.

Related:

L.A. Zoo chimp killing: A Q&A with primatologist Craig Stanford - LA Times ScienceNOW blog

Why Chimpanzees Kill - by Kate Wong at the Scientific American Observations blog

From the archives:

Chimpanzees Help, But Only When Asked

Chimpanzees Should Not Be Used in TV or Movies