Do animals commit suicide?

By Brian Palmer in Slate.

A pod of 61 whales beached themselves at Farewell Spit in New Zealand on Monday. Officials decided to euthanize the 18 that were still alive Wednesday. It’s not clear why whales beach themselves, but one theory holds that when a sick individual heads to shore to die, the others follow. Is suicide a thing in the animal kingdom?

Sort of. There is plenty of evidence that animals engage in self-destructive behavior. In addition to the beached whales, ducks and dogs have been observed drowning themselves, cows have walked off cliffs, and naked mole rats (like some insects) leave the colony to die when infected with a communicable disease. It’s not clear that any of these behaviors are comparable to human suicide, though, because suicide involves a set of higher-order cognitive abilities. It requires an awareness of one’s own existence, an ability to speculate about the future, and the knowledge that an act will result in death. There are indications that certain animals have some of these capacities. Dolphins, many primates, magpies, and elephants can recognize themselves in a mirror, suggesting self-awareness. Some animals know how to pretend during play activities (PDF), which indicates an ability to imagine counterfactual worlds. Still, no one really knows which animals, if any, can combine these capacities to perform an act similar to human suicide.

Victorian scientists were particularly interested in this question, according to historian Edmund Ramsden in a 2010 article. Humane societies were eager to prove that animals experienced humanlike emotions, and animal suicides offered proof. A series of such stories began to appear in periodicals in 1845. One involved a depressed Newfoundland dog that repeatedly leapt into the water, kept its limbs still, and held his “head determinedly under water for a few minutes.” Other dogs drowned or starved themselves after losing their owners. A deer jumped from a precipice to avoid capture by hunting dogs. A duck drowned itself after the death of its mate. Scorpions were thought to sting themselves when surrounded by fire. Researchers engaged in a fierce and ultimately inconclusive debate over whether any of these behaviors should be considered suicide. (Except for the scorpions, which clearly were not attempting suicide—they’re immune to their own venom.)

Even when scientists can explain the neurobiological basis of an animal’s self-destructive behavior, it’s still not always clear whether it’s fair to call the act suicide. The parasite Toxoplasma gondii affects the brains of rodents and causes them to be attracted to their mortal enemy—the cat. It would be easy to dismiss this kind of rat suicide as irrelevant to our own behavior if not for some hints that infection can play a role in human suicides as well. In a 2009 study of patients with recurrent mood disorders, University of Maryland researchers found that those with high levels of Toxoplasma gondii antibodies were more likely to have attempted suicide. This study is preliminary, though, and there’s no sign of a causal connection.

No matter the motivation, self-destruction appears to be something that exists in even the simplest life forms. Single-celled marine algae engage in programmed cell death when exposed to stresses that they’re fully capable of overcoming. Researchers recently discovered that the “suicide” of some cells promoted growth in the survivors. Like infected mole rats or bees that abandon the colony to prevent an epidemic, algae die for the good of the community.