Every night, the Brazilian ant Forelius pusillus takes self-sacrifice to a whole new level. At sunset, the colony protects its nest by sealing off the entrances with sand, and a few ants remain outside to complete the job. Unable to reenter, they die by the next morning--making them the first known example of a suicidal defense that is preemptive rather than a response to immediate danger.

Social insects are well-known for their willingness to die for their colonies; a number of bees, wasps, and ants succumb after their stings lodge in targets and break off. But until now, these insects were thought to engage in such suicide missions only when enemies were present.

Behavioral ecologist Adam Tofilski of the Agricultural University of Krakow, Poland, and his colleagues were studying how F. pusillus dispersed sand in a sugar cane field near São Simão in Brazil when they saw that as many as eight ants remained outside the sealed nests. These ants weren’t stragglers: They deliberately helped hide the entrances, spending up to 50 minutes carrying and kicking sand into the hole until it was indistinguishable from its surroundings.

This item requires the Flash plug-in (version 8 or higher). JavaScript must also be enabled in your browser. Please download the latest version of the free Flash plug-in. Suicide mission. A time-lapse video of the ant nest entrance closing off, with a worker ant trapped outside and helping seal up the opening with sand. Tofilski et al., American Naturalist (November 2008)

Come morning, when the nest reopened, these ants were nowhere to be seen. The researchers found out why when they plucked ants left behind into a plastic bowl: Only six of 23 survived the night. These findings, which will appear in the November issue of the journal American Naturalist, show that staying outside was suicidal. "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, an insect biologist at the University of Sussex, U.K.

The ants stuck outside might be old or sick, Tofilski conjectured. Thus, they may have essentially sacrificed themselves for the greater good, being more expendable members of the colony. Still, community ecologist Michael Kaspari of the University of Oklahoma, Norman, who did not participate in this study, noted that F. pusillus is "very, very flimsy" and that even young, healthy ants could easily die if left behind. "They might have burned through all their sugar or dehydrated outside the buffered environments of their nests," he explained.

It remains a puzzle what the ants are guarding their colonies against. Kaspari speculated that F. pusillus might be hiding from large, roaming colonies of army ants. Uncovering the pressures that drive this self-sacrifice could shed light on the evolution of altruism, Kaspari added.