Chemical mimicry in an incipient leaf-cutting ant social parasite

Abstract

Some social parasites of insect societies are known to use brute force when usurping a host colony, but most use more subtle forms of chemical cheating either by expressing as few recognition cues as possible to avoid being recognized or by producing similar recognition cues to the host to achieve positive discrimination. The former "chemical insignificance" strategy represents a more general adaptive syndrome than the latter "chemical mimicry" strategy and is expected to be characteristic of early evolutionary stages of social parasitism. We tested this hypothesis by experimentally analyzing the efficiency by which Acromyrmex echinatior leaf-cutting ants recognize intruding workers of the incipient social parasite Acromyrmex insinuator. The results were consistent with the parasite being "chemically insignificant" and not with the "chemical mimicry" hypothesis. Gas chromatography-mass spectrometry analysis of cuticular hydrocarbon profiles showed that social parasite workers produce significantly fewer hydrocarbons overall and that their typical profiles have very low amounts of hydrocarbons in the "normal" C29-C35 range but large quantities of unusually heavy C43-C45 hydrocarbons. This suggests that the C29-C35 hydrocarbons are instrumental in normal host nestmate recognition and that the C43-C45 compounds, all of which are dienes and thus more fluid than the corresponding saturated compounds, may reinforce "chemical insignificance" by blurring any remaining variation in recognition cues.