Does metabolic rate and evaporative water loss reflect differences in migratory strategy in sexually dimorphic hoverflies?

A typical explanation for ecologically stable strategies that apply to only a proportion of a population, is bet hedging, where increased reproductive success offsets reduced reproductive rate. One such is partial migration, where only a proportion of a population moves seasonally to avoid inclement climatic conditions. Bet hedging may overlook unseen costs to maintain broad physiological resilience, implied by encountering a breadth of environmental conditions. We investigated the physiological correlates of partial migration by measuring standard metabolic rates, and rates of evaporative water loss, and then estimating upper and lower thermal tolerance in males and females of two hoverfly species, Episyrphus balteatus and Eristalis tenax. In central Europe, females of these species may either migrate or overwinter, whereas males may migrate south to the Mediterranean, but have not been found overwintering. Both species were sexually dimorphic; female Ep. balteatus were lighter than males, but female Er. tenax were heavier than males. While allometrically- corrected metabolic rate in both species increased with temperature, the most parsimonious models included no sex-specific differences in metabolic rate for either species. Evaporative water loss of both species also increased with temperature, but was higher for females of both species than males. Assuming that resting metabolism is congruent with the activity requirements of migration, highly consistent thermal tolerance and metabolic rate suggests that any given fly could migrate, although water loss patterns suggest that females may be less well-adapted to Mediterranean climates. We infer that partial migration probably results from the imperatives of their reproductive strategies.