Increasing thermal stress with flight distance in stingless bees (Melipona subnitida) in the Brazilian tropical dry forest: Implications for constraint on foraging range.

The thoracic temperature (T) of foraging bees usually exceeds ambient air temperatures (T) by several degrees. In hot tropical climate zones, therefore, individuals may reach body temperatures close to their critical thermal maxima, which might constrain their activity. In the present study, we tested the hypothesis that thermal stress increases with flight distance in nectar foragers of M. subnitida, a stingless bee species native to the hottest regions of the Brazilian tropical dry forest. Using infrared thermography, we recorded the body surface temperature of individuals foraging at distances of 15, 50, and 100 m. Closest to the nests, foragers stabilized T at 40 °C when collecting sugar solution at T > 30 °C. The simultaneous decrease of the temperature excess ratio of head and abdomen suggests evaporative cooling at these body parts. With increasing foraging distance, foragers increased heat dissipation to the head and abdomen. Thus, despite more intensive heating of the thorax due to faster and longer flights, the bees maintained similar T as foragers at close feeding sites. However, at T > 30 °C, bees could no longer compensate the elevated heat gain at the head (50 m) and abdomen (50, 100 m), which caused an increasing temperature excess in these body parts. Thus, foragers of M. subnitida suffer overheating of the head and abdomen instead of the thorax when foraging in high temperatures at far feeding sites. Consequently, to avoid heat stress in the Brazilian tropical dry forest, the bees should forage close to the nest.