Microclimate and body size affect thermal tolerance among Neotropical army ants (Ecitoninae)

As small-bodied, ecologically dominant ectotherms, ants are important models for understanding animal responses to temperature variation and climate change. We took advantage of extreme variation in body size and species habitat use among Neotropical army ants (Ecitoninae) to explore how physiology and ecology affect thermal tolerance. The relationship between body size and thermal tolerance has been shown by previous studies to vary substantially among ant taxa, though this relationship was previously unknown for army ants. We predicted larger-bodied army ant workers within each species would survive more extreme temperatures, as body size increased with desiccation resistance and high-heat running speeds in other ants. We also predicted the thermal buffering effects of soil would reduce selective pressure for ability to cope with high heat stress in below-ground dwelling ants. We measured surface and 10 cm depth soil temperatures, as well as maximum and minimum thermal tolerance in several army ant species that vary in body size and above- vs. below-ground activity. We found a “weak link” effect, where the smallest workers within each species were the least heat tolerant. This effect of size on thermal tolerance was less pronounced in above-ground species. Even 10 cm of soil provided a significantly less extreme foraging environment for the below-ground army ants. We show sympatric, similar-sized species that use thermally distinct microhabitats can have different thermal physiologies. These patterns should be accounted for in models of species and community responses to thermal variation and climate change.