Species differ in worker body size effects on critical thermal limits in seed-harvesting desert ants (Messor ebeninus and M. arenarius)

Abstract

Extreme temperatures can constrain foraging behavior, and individual differences in thermal tolerances may affect foraging performance within and among species. Ambient temperatures may thus mediate competitive interactions among species that share resources. Different species of desert seed-harvesting ants (genus Messor) forage for similar food resources, and colonies can overlap in foraging areas. Because Messor species differ in body size distributions, and thermal tolerance is often size related in ants, we hypothesized that body size differences within and between Messor species would predict individual variation in worker thermal tolerances. Body size effects on thermal physiology could have implications for interspecific competition. We measured tolerances of extreme high (maximum critical temperature or CTmax) and low temperatures (CTmin) in two Messor species simultaneously at the same study site, smaller bodied M. ebeninus and larger bodied M. arenarius. Although the species did not differ significantly in CTmin or CTmax, tolerance of high temperatures was significantly size dependent for M. ebeninus: worker tolerances of high temperatures decreased with body size in this species. The patterns suggest the foraging activity of the smallest workers of smaller bodied species could be more constrained by high ambient temperatures, which could impact the division of labor within colonies as well as interspecific interactions.