http://www.digitaljournal.com/article/260552

Ants commit suicide to protect their colony

Posted Oct 1, 2008 by Chris V. Thangham
Scientists have observed that some ants stay outside the nest to work and seal the hole to protect the colony inside. They say these ants commit suicide in order to save the colony.
Polish scientists observed a rare phenomenon in Brazilian ant colonies. At sunset, some Brazilian ants (Forelius pusillus) remain outside and seal the hole to protect the colony inside. Since they cannot enter after it is sealed, they remain outside and die by morning as a preemptive measure.
Researchers have observed this suicide phenomenon in species such as bees, wasps and ants only when they are attacked. This colony of ants is the first time they've seen an ant die preemptively.
Behavioral ecologist Adam Tofilski of the Agricultural University in Krakow, Poland, and his colleagues, saw this behavior close to a sugar cane field near São Simão in Brazil. A small group of ants remained outside the nest for about 50 minutes, carrying and putting sand grains into the entrance until it was sealed and hidden from its surroundings -- a sort of camouflage.
The next morning, ants that remained outside were nowhere near to be seen. Tofilski and his colleagues captured these ants and left them in a plastic bowl for observation. They found most of the ants died by the next morning.
Tofilski and co-author Francis Ratnieks, an insect biologist at the University of Sussex in the U.K., wrote in their in the journal “American Naturalist”:
"In a colony with many thousands of workers, losing a few workers each evening to improve nest defense would be favored by natural selection."
Tofilski believes the ants that chose to die might be either old or sick, and might be willing to die for the greater good for their group. But Michael Kaspari from the University of Oklahoma, Norman believes that even young ants could easily die if they are left behind. He told Science Now:
"They might have burned through all their sugar or dehydrated outside the buffered environments of their nests."
They may die from exhaustion after doing their work outside the colony without any food. Kaspari speculates the Forelius pusillus ants might be doing this to protect their colonies from other large colonies of army ants.