Thermoregulation in Bumblebee Colonies. I. Thermoregulatory versus Brood-Maintenance Behaviors during Acute Changes in Ambient Temperature

Colonies with 10–42 workers were subjected to ambient temperatures (Ta) of 3–38 C. Brood temperature (Tbr), the rate of oxygen consumption, the proportion of workers engaged in thermoregulatory behaviors (brood incubation and wing fanning), and the proportion of workers engaged in general brood maintenance were monitored. At Ta < 32 C Tbr was usually maintained at 28–32 C, and the percentage occurrence (incidence) of brood incubation varied inversely with Ta. At Ta > 32 C, wing fanning associated with a rise in Tbr and metabolism was prevalent. The incidence of brood maintenance was highest (40%–80%) at Ta = 25–32 C, when the combined incidence of brood incubation and wing fanning was relatively low. In contrast, the incidence of brood maintenance was <30% at near-freezing Ta and was 0%–10% at Ta > 34 C. These data suggest that the colonies attempt to regulate Tbr at a set-point temperature between 28 and 32 C and that the increased employment of workers for thermoregulation occurred at the expense of brood maintenance.