Is foraging in the desert ant, Messor pergandei (Hymenoptera: Formiceidae), limited by water?

The seed‐harvesting ant Messor (Veromessor) prrgandei (Mayr) is a common inhabitant of southwestern deserts of the U.S.A. Foragers vary in size from less than 1 mg to more than 10 mg in body mass and may travel over 80 m on a single foraging trip. Their small size, long foraging range, and hot, arid habitat suggest that water stress may limit foraging activity. We examined intercolony and interindividual variation in water loss of M.pergandei foragers under several different situations in the field. Colonies differed significantly in minimum critical water content ( W c ) of individual foragers (water content below which foragers are incapable of normal locomotion). In one colony small workers had disproportionately higher W c than larger workers; in the other colony W c was isometric with body size. Groups of workers confined in the field approached W c only after normal foraging stopped and substrate temperatures exceeded 45°C, while water content of individual foragers did not approach the W c during normal foraging periods. Moreover, seed load and distance travelled did not negatively affect forager water content, as measured on return to the nest: indeed, our results suggest that forager hydration level may influence load selection and/or foraging distance. We conclude that, under normal circumstances, foraging in M.pergandei is not water‐limited.