To Stay Alive, Ants Dump Their Dead

Colonies that don't remove corpses have higher mortality rates, new study says.

Common red ants—shown here drinking from a water droplet—are native to Europe and Asia.

Housekeeping can be a matter of life and death—at least for social animals like ants, a new paper suggests.

According to a study published Tuesday in the journal Biology Letters, common red ants (Myrmica rubra) that were prevented from removing their nestmates' corpses died more frequently than those allowed to bring out their dead.

The tiny ants—each roughly the size of a medium-grain rice kernel—live under rocks and logs in densely packed colonies. More than a thousand worker ants can be found in a single nest.

These insects reap many benefits from group living, as they work together to gather food, care for their queen, and defend their nest.

But the situation also puts them at risk of being hit by disease epidemics: If one individual in the colony comes down with an illness, the blight can spread rapidly. This places a premium on good hygiene.

Dead Are Deadly

Many insects habitually remove dead nestmates from their colony. Scientists have long assumed that this behavior is based on a need to keep the rest of the colony healthy. But until now that idea hadn't been put to a formal test.

The new study, conducted by researchers at the Université Libre de Bruxelles and the Université de Liège in Belgium, tested the health benefits of corpse removal in common red ant colonies kept in artificial nests.

Some of the nests had wide exits. Others had narrow openings that made it difficult for the ants to transport and deposit corpses outside.

In each colony, the scientists fatally froze ten worker ants, placed their corpses back in the nest, and monitored the survival of the remaining workers.

The result: The ants in colonies that couldn't remove corpses didn't fare as well. By the end of the 50-day experiment, mortality had more than doubled in the corpse-littered colonies, from 6 percent to 13 percent.

Why the higher death rate? The researchers can't say for sure, but they speculate that "corpses artificially staying longer in the nest may have increased the occurrence of microorganisms, requiring a greater investment in the immune system for live ants and possibly resulting in a reduced lifespan."

The Benefits of Good Housekeeping

The study "shows a fundamental effect that … everybody thought we knew but nobody [had] really tested," wrote Olav Rueppell—an expert on social insects at the University of North Carolina at Greensboro who was not involved in the study—in an email to National Geographic.

The implications of corpse removal for group health may also apply to other social animals like honeybees and prairie dogs.

But for the common red ant, at least, the situation is clear: Dead ants beget more dead ants, so it's best to keep the nest nice and tidy.

Follow Katie Langin on Twitter.