New data on ant adaptability within their environment

By examining the worker castes in colonies of the ant, Pheidole morrisi, researchers have found new evidence that ants alter the organization of their colonies in different environments. Researchers Andrew Yang and colleagues from Duke University compared populations of P. morrisi in Florida, North Carolina and New York, and uncovered evidence supporting the idea that some insects adapt to their environment by adjusting body size and the relative proportion of different castes within their population.

Over the past 30 years, theoretical work on social organization has generated a number of predictions about how the colonies of eusocial insects, like ants and bees, could adapt to different environments. Many ants have morphologically distinct “castes,” such as workers and soldiers, that specialize in different tasks within the colony. One long-standing prediction is that the optimal proportion of these castes in a colony should vary as conditions in nature vary. To date, the only evidence that ant colonies can exhibit any flexibility in the relative numbers of workers and soldiers has been from studies that measure the response to short-term environmental change that occurs within the normal life cycle of a colony.

In the new work, Yang and colleagues headed to the field to explore the question of whether caste structure can undergo similar changes on much larger spatial and temporal scale. The researchers found that the proportion of castes, as well as the ants’ body sizes, differed between populations of P. morrisi in three geographically distinct sites in the eastern United States. The differing population characteristics persisted when the ants were brought into the laboratory environment, suggesting that the differences between the populations reflects evolutionary divergence. Moreover, behavioral experiments – so-called “laboratory contests” between P. morrisi and a competitor species, the fire ant Solenopsis invicta – demonstrated that members of the soldier caste play an important role in colony defense. This role for soldiers, in combination with geographic differences in competitors, may explain the variation in the proportions of castes found between populations.

Andrew S. Yang, Christopher H. Martin, and H. Frederik Nijhout: “Geographic Variation of Caste Structure among Ant Populations”

Published in Current Biology, Volume 14, Number 6, 23 March 2003.