Small bees overheat in sunlit flowers: do they make cooling flights?

Abstract

1. Although thermoregulation by large bees in cool climates has been well studied, less is known about the very different thermoregulatory strategies of small bees, especially those subjected to heat stress.2. Studies were carried out on small (< 20 mg fresh weight), dark‐coloured, solitary bees (mostly halictids and hylaeine colletids) experiencing an extreme radiative heat load, enhanced by the high‐altitude location and by reflection of incident radiation by the high‐albedo petals of the flowers of Potentilla lancinata.3. When foraging in the flowers, such bees experienced peak operative temperatures exceeding 44 °C. In these conditions, males largely stopped foraging but females continued, usually limiting their flower visits to a few seconds and making frequent short flights. These flights would cool the bees down, because bees suspended in air were cooler than bees in sunlit flowers, and convective cooling during flight would further enhance the cooling effect of departure from the flower.4. As far as is known, cooling flights in small bees have not been proposed before, providing a new avenue for exploration of bee thermoregulatory strategies.