Every night the Brazilian ant Forelius pusillus takes selflessness to a whole new level. At dusk, as the ants defend their homes by sealing off the entrances with sand, up to eight workers remain outside to finish the job. Left behind, they die by the next day—the first known example of a suicidal mission that is preemptive rather than a response to immediate danger.

Behavioral ecologist Adam Tofilski of the Agricultural University of Kraków in Poland and his colleagues found that these ants were not just stragglers trapped outside. They were deliberately helping to cover the entrances, spending up to 50 minutes kicking sand into the holes until the entrances were indistinguishable from the surroundings. In experiments, the scientists found that only six of 23 ants left behind were alive the next morning, showing that their act was a sacrifice. Just why the ants died is unclear—the species is fragile, but the researchers also suspect that the outside individuals could be old or sick. The findings, in the November American Naturalist, could elucidate the evolution of altruism.