Not Dead Yet

Not Dead Yet

Chemical signals save Argentinian ants from being wrongly carted off for dead.


How ants prove they're alive. I'm Bob Hirshon and this is Science Update.

Like many social insects, Argentinian ants operate a sort of morgue: they pile their dead outside the nest to reduce the spread of disease. Now, scientists have figured out how they tell truly dead nest-mates from merely sluggish ones. Dong-Hwan Choe and his colleagues at the University of California, Riverside, found that when ants that had died just a few minutes ago were dropped in a nest, the other ants left them alone.

But if I put one-hour-dead ants into the nest, they promptly carried them away.

Further tests showed that the ants identified their dead by chemicals called triglycerides. They're actually found on both living and dead ants, but living ones produce other chemicals that mask them. Choe's team showed that those masking chemicals dissipate within an hour postmortem. I'm Bob Hirshon for AAAS, the Science Society. 

Making Sense of the Research

Social insects, like ants, bees, and termites, live in large colonies and perform specific tasks that benefit the entire group. One of these tasks is carting away their dead nestmates and tossing them on a heap outside the nest. After all, you wouldn't want to live in an apartment building littered with the corpses of dead residents: besides being disgusting, it would also be unsanitary. Ants live in even closer quarters than apartment-dwelling people, so it's to their advantage to get rid of dead bodies as soon as possible.

But how can ants tell a living ant from a dead one? They generally don't toss live ants on the trash heap, so they must know. But simply not moving isn't enough of a clue. So Choe's team set out to learn how the ants were spotting the deceased.

In the first experiment, the researchers dropped either freshly killed or one-hour-dead ants into the nest. The other ants ignored the fresh corpses, but immediately carted away ants that had been dead for just one hour. This showed that something about an ant changes within the first hour after death that distinguishes it from a living ant.

Choe's team suspected that this change was chemical, since insects commonly rely on chemical signals to sense their environment or communicate with one another. They found that it was the absence of a chemical, and not the presence of one, that mattered: when an ant dies, it stops producing chemicals that mask the scent of other chemicals called triglycerides. When other ants detect those triglycerides, they assume that the ant in question has died. (The researchers also showed that ants exposed to canned triglycerides started behaving as they would around a dead ant.)

"Unmasking" underlies many natural phenomena. For example, red, orange, and yellow pigments are always present in a tree's leaves, but normally, the green pigment chlorophyll overwhelms them so the leaves appear green. Only in the fall, when the chlorophyll in the leaf breaks down, do those warmer colors become visible. In the same way, the triglycerides coming from the ants only become noticeable when other chemicals disappear. This allows the ants to keep a clean nest, with little risk of being buried alive.

Now try and answer these questions:

  1. What are social insects?
  2. Why do ants need to identify their dead?
  3. How does chemical unmasking make this possible?
  4. What are some advantages of chemical signaling, compared to other ways of sensing the environment? 

Going Further

For Educators

In the podcast James McLurkin Explores Robot Communities, from the Lemelson Center for the Study of Invention & Innovation at the Smithsonian's National Museum of American History, a young MIT researcher explains how his robots work like a group of ants.

National Geographic Xpeditions offers news articles on chemical signaling, including:

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