Discrimination of Methyl-Branched Hydrocarbons by Tetramorium tsushimae Ants: a Focus on Branch Position and Chain Length.

In nature, organisms are exposed to scents and tastes composed of multiple rather than single chemicals. The ability to sense and correctly identify different chemicals within these complex mixtures is essential for optimized behavior. However, when minor variations in chemical structure do not significantly impact the organisms, a generalized response to similar chemicals without discrimination might be more adaptive. In this study, we investigated the ability of ants to discriminate among methyl-branched alkanes. Ants recognize each other using cuticular hydrocarbons (CHCs), typically composed of a mixture of n-alkanes, n-alkenes, and methyl-branched alkanes. Tetramorium tsushimae ants have been shown to use the methylalkane fraction of CHCs to identify their mutualistic partners. We measured the behavioral responses of ant workers to dummies coated with various hydrocarbons, after presenting them with dummies treated with methylalkanes and a sucrose solution as a reward. The results showed that ants previously exposed to 2-methyltetracosane (2-MeC24) decreased their aggression not only toward 2-MeC24 but also toward 2-MeC26, despite the difference in the chain lengths. Conversely, ants exposed to 13-MeC27 maintained high levels of aggression toward 5-MeC27, which has the methyl branch in a different position. These findings suggest that T. tsushimae ants can differentiate between methylalkanes with different methyl branch positions, but are less able to discriminate between those with the same methyl branch position but different chain lengths.