Thermoregulation in Bees

During flight, many species of bees regulate thoracic muscles at temperatures exceeding 40 °C and achieve rates of metabolism and mechanical power production approaching 800 and 120 W kg-1 body mass, respectively, and thus represent the extreme of aerobic locomotor capacity in animals. We study the patterns and mechanisms of thermoregulation in these powerful flyers and try to apply this understanding to their flight behavior in natural settings. We have shown that the bees Apis mellifera and Centris pallida thermoregulate during hovering flight in large part by varying metabolism (and concomitant heat production). The metabolic rates of flying Apis and Centris decrease significantly with increasing air temperature. It is unclear what physiological mechanisms allow these bees to decrease rates of metabolism by over 40 % during stable hovering flight as air temperature rises from 21 to 45 °C. It is possible that hovering honeybees are thermoregulating by actively decreasing mechanical power output (and metabolic power requirements) at high air temperatures by altering kinematic variables such as wingbeat frequency and stroke amplitude. Another possibility is that honeybees flying at high air tempertures are increasing the efficiency of conversion of metabolic to mechanical power. In support of the mechanical power hypothesis, we have shown that Apis and Centris flying in the laboratory and in the field decrease their wingbeat frequency as air temperature increases. A negative relationship between wingbeat frequency and air temperature has also been documented by others for orchid bees, bumblebees, dragonflies and hummingbirds, indicating that variations in wingbeat frequency and metabolic heat production may contribute to stable body temperatures in a wide variety of endothermic flying animals.