Ant colonies: Behavioral variability wins

The evolution and behavior of ants, in particular the relationship between socially parasitic ants and their hosts, is the research topic of a work group headed by Professor Dr Susanne Foitzik at the Institute of Zoology at Johannes Gutenberg University Mainz (JGU), Germany. Evolutionary biologists at Mainz University found that ant colonies are more productive and raise more offspring when the workers in the colony display considerable variation in their levels of aggression. This variation in aggression is possibly part of their division of labor, which is regarded as the basis of the success of social insect societies.

There are more than 15,000 ant species worldwide. About a third of the 150 of the Central European species are parasitic, i.e., they live at the expense of other ant species. This includes "slave-making ants," which are being studied with particular interest at Johannes Gutenberg University Mainz. Temnothorax longispinosus is not one of these slave-making species but can become a victim itself: enslaved T. longispinosus worker ants search for food and care for the brood of the slavemaker. T. longispinosus lives in mixed oak forests in the northeastern United States of America (USA), where it builds nests in acorns, hickory nuts, and little twigs. They form colonies averaging 35 workers and feed mainly on dead insects. The workers are very small, measuring only between 2 and 3 millimeters in length.

"Temnothorax is particularly suitable for our experiments, as their colonies are easy to keep in the laboratory, and this makes it possible to use large sample sizes," explains Andreas Modlmeier, who is investigating the 'personality' of ants for his PhD thesis. The concept of 'personality' has gained popularity among behavioral researchers in recent years. "We now assume that ants have a colony character, but that there are also many individual personality characteristics within an ant colony," explains Susanne Foitzik. One such characteristic is aggression. Aggressive colonies, for example, flee much more rarely than others do.

For the purpose of his experiments, Modlmeier brought individual ants together with a dead worker of another colony and observed how often aggressive interactions took place. He registered actions such as the opening of the mandibles (threat display), biting, pulling, and stinging. Ten worker ants were selected from each of 39 different colonies to be classified by their size, level of aggression, and exploratory behavior. It was shown that the productivity of ant colonies -- measured by the total biomass in new workers and sexuals produced per worker ant -- increased with the variation in the level of aggression within the colony; in other words, this correlated with the differences displayed in level of aggression within each set of ten ant workers.

"Colonies might be more productive when tasks such as nest defense and brood care are distributed between specialized workers with different aggression levels," is Modlmeier's assumption. Animals with high aggression levels could participate in competition and fights with other colonies, while less aggressive social workers care for the offspring. A remarkable finding was that not one of the 39 colonies was highly aggressive. "There are no fully aggressive colonies. It seems that this is not beneficial in the natural world and could rather be a disadvantage," assumes Modlmeier.

It had previously been suggested that there is a connection between the character or behavior of worker ants and the division of labor in a colony, and that this might possibly be the basis for the ecological success of social insects, but this hypothesis had not been proven yet. Modlmeier has now provided the first empirical proof that variability in behavior patterns, which might be the basis of the division of labor in ant colonies, enhances the productivity and thus the fitness of social insect colonies.

The research on the behavior and personality characteristics of ants has been funded as part of the project "The evolutionary significance of within- and between-colony variation in behavior, morphology, genetic composition and immuno-competence in ants," funded by the German Research Foundation (DFG) since November 2010.