A need for speed? It is not always necessary to be the fastest—A case study of two thermophilous ant species

Abstract Animals should adjust their trajectories and movement speeds to maximise the efficiency with which they move towards their destinations. Temperature is the most important environmental factor influencing the movement of ectothermic ants. Here, we compared how two species with different thermal adaptations, Catalyphis velox and Aphaenogaster senilis, foraged under thermally challenging conditions. More specifically, we examined the relationship of ground surface temperature with the mean speed, return time and path straightness of ants returning to the nest with prey. In general, for both species, forager return time was correlated with speed: faster foragers took less time to return to the nest. Path straightness was not related to ground surface temperature or forager speed for C . velox . In contrast, for A . senilis , path straightness was related to forager speed: faster foragers travelled along straighter paths to reach the nest. The differences in the species' behavioural responses may stem from physiological differences. Cataglyphis velox has a higher level of physiological thermotolerance and generally moves faster; here, it displayed a broader range of behaviours at higher temperatures (e.g., range of path sinuosity). Each species appears to exploit its advantageous traits: C . velox relied on its higher speed, and A . senilis relied on greater path straightness and the possibility of recruiting nestmates.