Thermal physiology, foraging pattern, and worker body size interact to influence coexistence in sympatric polymorphic harvester ants (Messor spp.)

Physiological thermal limits can mediate species coexistence at local scales. However, it is challenging to untangle the role they play when coexisting species are also highly related, given that phylogeny may inform physiology. However, if species exploit similar trophic resources, there must be a degree of niche differentiation that precludes competitive exclusion. Physiological traits frequently correlate with body size. Furthermore, they often vary within and among animal populations, allowing organisms to optimize their foraging dynamics under different thermal conditions. Here, we analyzed interactions among critical thermal maxima (CTmax), foraging patterns, and forager size in three congeneric, sympatric, and polymorphic harvester ant species (Messor barbarus, M. bouvieri, and M. capitatus). We characterized CTmax for different-sized foragers sampled from co-occurring colonies of the three species and analyzed the colonies’ daily and seasonal foraging patterns. We also performed a baiting experiment using M. barbarus to explore the relationship between forager size and foraging temperature. In general, the species displayed different CTmax values. For similar-sized foragers, the less polymorphic M. bouvieri had higher CTmax values than did the highly polymorphic M. barbarus and M. capitatus. There was a strong positive relationship between worker size and CTmax within colonies, but the results of the baiting experiment found that foraging temperature did not influence forager size distributions. While interspecific differences in foraging patterns were influenced by environmental temperatures, these dynamics were not fully attributable to species physiology. Competition may be playing an important role as well, in the form of other factors. SIGNIFICANCE STATEMENT: Congeneric, sympatric, and polymorphic species that occupy similar trophic niches can display different physiological thermal limits and foraging patterns. However, physiological differences alone do not explain species coexistence. Competition might be at work as well, operating in forms other than thermal physiology. Indeed, coexistence may occur because competition acts through behavioral patterns, such as temporal segregation in foraging. Here, although larger workers had higher critical thermal maxima, Messor barbarus did not appear to send out larger foragers when temperatures were higher. The absence of such a response could potentially hobble species persistence under future conditions of climate change. To understand how global changes will affect the world’s terrestrial ecosystems, we need research that examines species physiology and biotic interactions in tandem.