This is interesting, its an article that describes how ants display a preemptive response to danger. In a Brazilian ant colony some ants choose to sacrifice themselves for the good of the colony. At sunset, the colony will protect itself by sealing off the entrances with sand. The problem with this is that a few ants must choose to remain outside to complete the job. These ants will be unable to reenter once the nest is sealed and so will die when the temperature drops overnight. A behavioural ecologist found that as many as eight ants would remain outside the nests. The remaining ants, however, were not stragglers they stayed outside the nest to deliberately help to hide the entrances and would spend up to 50 minutes carrying sand into the hole until it was sealed. By the time the morning came the ants that had stayed outside were dead. It is thought that the ants who remained outside were old or sick ants that chose to sacrifice themselves for the greater good of the colony.