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The Zombie Ants
When ants are accidentally marked as dead, they find a way to rejoin the living.
By
Dr. Wilson is an emeritus professor at Harvard and the winner of two Pulitzer Prizes.
Every corpse is an ecosystem. Each fallen bird, landed fish, beached whale, decomposing log, plucked flower is destined to change from a conglomerate of giant molecules, the most complex system in the universe known, into clouds and drifts of much smaller organic molecules. The process of decay is driven by scavengers, in nature beginning with vultures and blowflies and ending with fungi and bacteria.
What do ants do with their dead? In many species, if a colony member is badly injured in the field it is carried home and eaten. If injured only moderately, it may be allowed to live and heal. Most ant warriors that die in battle outside the nest never return. They instead fill the jaws and beaks of predators.
An ant that dies from old age or disease inside the nest simply comes to a standstill or else falls to the side with her legs crumpled up. In most cases, she is allowed to stay in place. After, at most, a few days, a nest mate picks her up and carries her out of the nest or to a refuse pile in one of the chambers within the nest. In this cemetery chamber is also dumped miscellaneous refuse, including the inedible remains of prey. There is no ceremony.
It occurred to me early in my studies of chemical communication in ants that the bodies of the dead are likely recognized by the odor of their decomposition. Of all the substances uniquely present in dead insects, one or more must be the signal that triggers corpse disposal by ants. If live ants demonstrably use such molecules to release other instinctive social behavior in the service of the colony, why not in death also?
It was my good fortune at the time to find a published account, understandably obscure, that identified substances found in dead cockroaches. Using this work as a guide, I set out to learn what chemicals stimulate necrophoric (corpse removal) behavior in ants.
As a first step, I made extracts of decomposing ants. I put droplets of this material on “dummies” of dead ants made of flecks of balsam about the size of workers. When these were dropped into nests of laboratory colonies of harvesting ants, each was picked up and taken speedily to the refuse pile.
So now I had a working bioassay, the essential step in biological experimentation. At the same time, I acquired synthetic, chemically pure samples of the decomposed cockroaches. For a while the laboratory smelled faintly of a mixture of charnel house and sewer. (Two of the substances, for example, were the terpenoids indole and skatole, elements of mammalian feces.)
Most of the substances tested caused excitement and aggressive circling by the ants, but did not result in immediate removal. Where bits of balsam treated with odorous substances were attacked or simply ignored, those carrying indole or skatole were picked up and carried to the cemetery.
There is no procedure more pleasing to a biologist than an experiment that works. This one was successful, at least for Florida harvester ants, and I repeated it for visitors to witness until I grew bored. So I asked a new question: What would happen if I daubed a live, healthy worker with one of the funereal substances?
The result was gratifying. Worker ants that met their daubed nest mates picked them up, carried them alive and kicking to the cemetery, dropped them there, and left. The behavior of the undertaker was relatively calm, even casual. The dead belong with the dead. The daubed ants did what you and I would do if we were turned into zombies: We would take a bath.
It should be no surprise that this solution is also used by ants that suffer unwanted material on their bodies. They pull the flexible outer segments of their antennae, the funiculae, through comblike structures on their forelegs. They lick as much as possible of their body and legs with their pad-shaped tongues. They curl the gaster, the rearmost part of the body, as far forward as possible and wipe and wash it. They take a typically ant bath.
Then they return to the main living quarters of the nest. If enough of the necrophoric substance on their bodies has been removed, they are accepted back into the nest. If not, they are returned to the cemetery by their nest mates. There, they continue to clean themselves, and perhaps others assist them. They wait. In time, if the contaminants are removed or sufficiently dissipated, they rejoin the living in full.
Edward O. Wilson is a professor emeritus at Harvard University, the winner of two Pulitzer Prizes and the author of more than 30 books. His most recent book is “Tales From the Ant World,” from which this essay is adapted.
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