A new study shows that ants that are about to die from illness leave the company of their nest-mates and socially isolate themselves. Since deadly infections can spread quickly among the thousands of individuals in an ant-nest, this behaviour might have evolved to reduce this risk.
Ants are highly social insects that live in colonies ranging from thousands to millions of individual ants, all daughters of the single reproductive queen. Although a social life-style gives many advantages including defence, resource utilization and in ant colonies also in the high degree of specialisation, it also results in increased risks from the easy spread of pathogens. One way to reduce this risk is to isolate sick individuals and indeed anecdotes from many different social animals tell of dying animals isolating themselves from their companions shortly before death. However, there is a lack of hard scientific evidence for this.
Jürgen Heinz and Bartosz Walter from the University of Regensburg in Germany investigated this behaviour in the worker ants of the species Temnothorax unifasciatus that died under controlled laboratory conditions. They infected worker ants with a deadly fungus and observed how more than 71% of the infected ants left the nest permanently 1-50 hours prior to their death, 9% died inside the nest while 20% were found dead outside the nests but here the biologists did not observe if they left nest actively or were carried out by nest-mates after their death. To confirm that the leaving behaviour was not controlled by the fungus, the biologists exposed other ants to 95% CO2 which caused the survivors to age prematurely and die within two weeks. More than 80% of these workers also left the nest 5 to 240 hours before their death.
This combined with similar observations from ants dying from natural causes convinced the scientists that these ants behave altruistic and leave their nestmates to die alone when severely ill in order to reduce the risk of infecting their sisters.
Heinz, J. and Walter, B. (2010) Moribund Ants Leave Their Nests to Die in Social Isolation. Current Biology 20, 249-252. DOI 10.1016/j.cub.2009.12.031
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