Back in the heyday of The Far Side, Gary Larson drew a Sunday panel showing a middle-aged couple slumped on their living room floor and another couple exiting. The caption: “The Arnolds feign death until the Wagners, sensing awkwardness, are compelled to leave.”
What the Arnolds had done is known technically as thanatosis. The practice is widespread among arthropods—a Google Scholar search turned up references to carrion beetles, ladybeetles, weevils, stick insects, crickets, and spiders—but its best known exemplar is a mammal, the Virginia opossum, as in “playing ‘possum.” (I’m using the “o” and the apostrophe for clarity’s sake. Australia has a bunch of vaguely similar mammals called possums, but they’re no more closely related to the American opossums than are kangaroos, koalas, or Tasmanian devils).
I’ve known my share of opossums in Berkeley, including the one that passed away in my garage a few years ago, but I’ve never seen one feign death. They’ve tended to stand their ground, giving me an insolent toothy leer. But death-feigning behavior is well documented in the wild, and there have been two confirming laboratory studies.
It appears that an opossum has to feel seriously threatened before it keels over, and that physical violence is a necessary trigger. This can be intramural—John McManus at Cornell, who worked with a captive population, saw a small male opossum feign death after being bitten by a larger cagemate—but more often involves a predator. A study in the mid-‘60s at Children’s Hospital of Los Angeles used what was described as an “artificial dog jaw,” resembling a large pair of pliers, to grab the opossum by the neck and shake it, to the accompaniment of recorded barking. A later project at the University of New Hampshire started out using real dogs. They proved unreliable, though, and the researcher, Edward Francq, wound up picking up the opossums and shaking them manually.
What happens when you shake an opossum follows a predictable course. The animal falls over on its side and lies still with its body flexed and its feet grasping whatever substrate it’s on. It drools and may discharge a noxious substance from its anal glands. Its eyes remain open and its ears twitch in response to squeaks or other sharp sounds, but it doesn’t react if prodded or pricked. Recovery time varies from a couple of minutes to half an hour or more.
But is the creature really out for the duration? It appears not, based on the Children’s Hospital study. Electroencephalograph readings before, during, and after what the authors called the “opossum state” showed no significant changes. They concluded that the EEG during thanatosis “is that of a normal, waking, highly alert behavioural state.” The opossum does not lose consciousness, whatever consciousness might be in an opossum. Francq later obtained normal electrocardiogram results during the opossum state.
If you were wondering, it also appears that Tennessee fainting goats do not actually faint when startled. According to the International Fainting Goat Association’s website, the goats suffer from myotonia; their muscles stiffen up, and they fall over. But they remain fully conscious and aware of their surroundings. Experienced goats learn to lean against something.
The goats’ condition is caused by a combination of recessive genes, but playing ‘possum is part of a normal opossum’s standard behavioral repertoire. Feigning death may frustrate a predator’s tendency to chase things that move. And the opossum may also render itself unpalatable. Something similar happens with another group of death artists, the North American hog-nosed snakes, but in a more elaborate way. First they bluff; then they play dead. Here’s how the late great herpetologist Archie Carr described it:
He will coil in a purposeful way, rear back and spread the whole first third of his body as thin as your belt, and lunge out at you repeatedly, each time hissing with almost intolerable menace. If instead of recoiling you steel yourself and reach over and pat the snake on the back, his menace will wilt before your eyes, and he will proceed to prove that you have killed him. He will turn over onto his back, open his mouth,... and then, after writhing about until his moist parts are all coated with debris, lie there belly-up as clearly defunct as any snake could be.
But don’t feel badly about him. Give him two minutes, say, and the catalepsy will wane. He will draw his tongue back in and ever so slowly turn and raise his head to see whether you are still there. Move your hand quickly before him, and he will flip back over into his supine seizure. Reach down and turn him right side up, and he will instantly twist over onto his back again. But then get up and move off a little way and wait patiently behind a tree, and you can watch him slowly come back to life, turn right side up, and quietly ease away.
The opossum’s behavior and the snake’s may have evolved convergently, like the wings of bats and birds; or they may both have inherited the genes that code for death-feigning from some remote common ancestor. We may never know. The South American short-tailed opossum is having its genome sequenced, but neither our local opossum nor any of the hog-nosed snakes is on the short list of candidates.
Opossums and hog-nosed snakes play dead as a defense (as, we presume, did the Arnolds). But there are at least two instances of thanatosis as an offensive strategy, both involving cichlid fish from the Great Lakes of Central Africa: Nimbochromis livingstonii in Lake Malawi and Lamprologus lemairii in Lake Tanganyika. Both have unhealthy-looking blotched and mottled color patterns, and both lie on their sides on the lake bottom, doing convincing impressions of dead fish. But when smaller fish swim by to investigate the corpses, the cichlids snap them up. Cichlid specialist George Barlow says the two are not close relatives and appear to have developed their appearance and behavior independently. ›