High surface temperatures select for individual foraging in ants

Natural selection favors signals, receptors, and signaling hehavior that maximize the received signal relative to background noise and that minimize signal degradation. The physical properties of the environment affect rates of attenuation and degradation of the signal, and thus temperature may influence the evolution and maintenance of volatile chemical signals. We tested this hypothesis in ants, where nest mate recruitment to a food source by laying trail pheromones on a surface is a common phenomenon. We collected data on maximal soil surface temperatures during the ants' activity and mode of foraging (recruitment or solitary). By using two different comparative methodologies, we demonstrated a relationship between maximal soil temperature at which species are active and recruitment behavior (which is hypothesized to be related to the presence or absence of chemical signals). The species that were active at lower temperatures proved to be those that used chemical signals to recruit nest mates during foraging. This is also the case when comparing sympatric species and thereby controlling for other environmental factors. Moreover, all seven nonrecruiter species developed from recruiter ancestries, which is consistent with our hypothesis because ample evidence suggests a forest and tropical origin for ants. Thus, contrary to previous hypotheses, species that forage individually cannot be categorically considered primitive, but rather appear to be derived from recruiter species. Therefore. we conclude that temperature influences the evolution and/or stability of chemical signals in ants by determining the recruitment of nest mates.