Interspecific Comparisons of Endothermy in Honey-Bees (APIS): Deviations from the Expected Size-Related Patterns

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Abstract
We report the first measurements of thoracic flight temperature (Tth) in foragers of the three Asian honey-bee species (genus Apis), which, together with the European species A. mellifera, span a five-fold range in body mass from the smallest species to the largest. Over a 15°C range in ambient temperature (Ta), we found that Tth in each species is strongly dependent upon Ta, as previously shown for A. mellifera. However, the temperature gradients (Tth-Ta) at a given Ta do not appear to increase with body size in the four species, as expected from many previous studies of endothermy in insects. The smallest species, A.florea, shows the smallest Tth-Ta, but the intermediate-sized A. cerana and A. mellifera both show a consistently higher Tth-Ta than the largest species, A. dorsata. We found that the rate of passive convective heat loss from the thorax scales linearly and inversely with body size in the four species, as in most insects, and that there is no striking anatomical evidence for differences in efficiency with which heat flow from the thorax to the abdomen is restricted. However, two important correlates of heat production - wing-loading and flight speed - are disproportionately high in A. cerana and A. mellifera relative to A. dorsata and A. florea, suggesting that an elevated mass-specific metabolic rate in flight may account for their unexpectedly high Tth-Ta. Furthermore, compared on a mass-specific basis, A. dorsata and A. florea are more similar to each other than either is to the other two species. This physiological dichotomy among the four species parallels a dichotomy in nesting behaviour and colony demography. Hence our results, in addition to raising many questions about physiological mechanisms in the energetics of honey-bees, suggest that there may be functional links between the energetic constraints on individuals and on colonies.