Beyond thermal limits: comprehensive metrics of performance identify key axes of thermal adaptation in ants

How species respond to temperature change depends in large part on their physiology. Physiological traits, such as critical thermal limits (CT_max and CT_min), provide estimates of thermal performance but may not capture the full impacts of temperature on fitness. Rather, thermal performance likely depends on a combination of traits—including thermal limits—that vary among species. Here, we examine how thermal limits correlate with the main components that influence fitness in ants. First, we compare how temperature affected colony survival and growth in two ant species that differ in their responses to warming in the field—Aphaenogaster rudis (heat-intolerant) and Temnothorax curvispinosus (heat-tolerant). We then extended our study to compare CT_max, thermal requirements of brood and yearly activity season among a broader set of ant species. While thermal limits were higher for workers of T. curvispinosus than A. rudis, T. curvispinosus colonies also required higher temperatures for survival and colony growth. This pattern generalized across 17 ant species, such that species whose foragers had a high CT_max also required higher temperatures for brood development. Finally, species whose foragers had a high CT_max had relatively short activity seasons compared with less heat-tolerant species. The relationships between CT_max, thermal requirements of brood and seasonal activity suggest two main strategies for growth and development in changing thermal environments: one where ants forage at higher temperatures over a short activity season and another where ants forage at lower temperatures for an extended activity season. Where species fall on this spectrum may influence a broad range of life-history characteristics and aid in explaining the current distributions of ants as well as their responses to future climate change. A lay summary is available for this article.