Fluctuations in oxygen influences facultative endothermy in bumblebees

Abstract

Bumblebees are facultative endotherms, having the ability to elevate thorax temperature above ambient temperature by elevating metabolism. Here we investigated the influence of hypoxia on metabolic demands and thermoregulatory capabilities of the bumblebee Bombus terrestris. We measured thorax temperature, oxygen consumption, carbon dioxide production, and ventilation rates of bees exposed to oxygen levels of 21, 15, 10, and 5 kPa at 26°C. Under normoxia, bumblebees maintained an elevated thorax temperature of 35.5 °C. There was no significant change in thorax temperature at 15 kPa oxygen (33.4°C). Mean thorax temperature decreased significantly at 10 kPa (31.6 °C) and 5 kPa (27.3 °C). Bees were able to maintain an elevated metabolic rate at 15 kPa and 10 kPa. In normoxia, thermoregulating bees exhibited periods of rapid abdominal pumping (327 s−1) interspaced by periods of no abdominal pumping. At 10 kPa oxygen, abdominal pumping rate deceased (255 s−1) but became continuous. Upon exposure to 5 kPa, metabolic rate and abdominal pumping rate (152 s−1) dropped rapidly although the animals continued abdominal pumping at the reduced rate throughout the exposure period. Bumblebees are able to meet the energetic demands of endothermy at 15 kPa oxygen, but begin to become compromised at levels of 10 kPa oxygen.