Energetic costs of raising brood in honey bee colonies are high, but heater bees are cheap

Little is known about the energetic costs to insects of raising young. Honey bees collectively raise young, or brood, through a series of complex behaviors that appear to accelerate and synchronize the timing of brood maturation. These include maintaining the brood nest at warmer and consistent temperatures (33–36 °C) and the exceptional activity of heater bees. Heater bees are a part of the larger group of nurse bees that care for brood by rapidly contracting thoracic muscles to generate high body temperatures, from 42 to 47 °C. Heater bees move among brood cells and display this behavior to regulate the temperatures of individual larvae and pupae. We constructed three sets of experimental hives to explore the energy costs of raising brood in general and the cost of heater bees specifically. One set was designed to estimate the numerical allocation of individuals to the heater bee task. The second set was designed to contain only brood, which eliminated foraging and allowed us to quantify stored honey use when rearing juveniles at 10 and 30 °C. The final set was used to measure the respiration rates and energy expenditure of individual bees displaying resting, walking, heating, and agitated behavior. By integrating honey used by brood-only experimental colonies with whole-colony measurements of honey storage in the literature, we estimated that raising brood costs colonies half of their annual energy budgets stored as honey, or approximately 43.7 ± 0.9 kg·yr⁻¹. We estimated that roughly 2 % of individuals in a colony perform as heater bees. Respiration rates of heater bees (19 mW) were more than those of resting bees (8 mW) but similar to those of walking bees (20 mW) and about half of those that were agitated (46 mW). The energetic cost of heating was more than an order of magnitude lower than the reported values for the energetic cost of flying. By integrating data from our three experimental hives, we estimate that the annual cost of raising brood is relatively high. However, heater bee behavior and physiology may require only about 7 % of the annual honey stored by a colony.