Brazilian Ants Sacrifice a Few Relatives Each Day for the Greater Good

By Eliza Strickland | September 29, 2008 1:03 pm

antsIn a striking example of the evolutionary benefits of altruism, researchers have found a species of ants that sends a few workers out each evening on a suicide mission to ensure the continued survival of the colony. The tiny ant Forelius pusillus, which makes its home in sugar cane fields in Brazil, makes a nightly ritual of covering the entrance to its nest with sand. To be sure that the entrance is sealed shut tightly, a few ants remain outside each evening to finish kicking sand over the hole. Those ants, stuck outside in the cold and the wind, die during the night.

“In a colony with many thousands of workers, losing a few workers each evening to improve nest defense would be favored by natural selection,” said co-author Francis Ratnieks…. The ants stuck outside might be old or sick, [co-author Adam] Tofilski conjectured. Thus, they may have essentially sacrificed themselves for the greater good, being more expendable members of the colony [ScienceNOW Daily News].

The new study, which will be published in the November issue of The American Naturalist [subscription required], adds a new twist to the phenomenon of altruism and self-sacrifice among insects. There are many examples in nature of insects sacrificing themselves when a colony or nest is under attack, such as when bees use their stings to defend the hive and die in the process. But the door-sealing activities of these 2-millimetre ants … represent the first recorded case of insects sacrificing themselves through a premeditated and pre-emptive procedure [New Scientist].

It remains a puzzle what the ants are guarding their colonies against. [One entomologist] speculated that F. pusillus might be hiding from large, roaming colonies of army ants [ScienceNOW Daily News].

Delve deeper into the puzzle posed by cooperative insects in the DISCOVER interview with the scientist who first promoted the ideas of “kin selection” and “group selection,” E. O. Wilson.

Image: Wikimedia Commons

MORE ABOUT: altruism, evolution, insects

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