Roles of individual honeybee workers and drones in colonial thermogenesis

The individual roles of honeybee workers and drones in heat regulation were investigated using single combs of bees and brood (about 1000 individuals) placed in boxes at 1S°C. After 1h and before cluster formation, I measured the elevation of bee thoracic surface temperature (T_ths) above local ambient temperature (T_a). Bees were then left overnight at 15 °C.

During the preclustering period, the density of bees over the brood slowly increased. In the clusters left overnight, bees in the innermost layer were significantly younger than bees in the outermost layer. One-day-old bees and drones were always located in the innermost cluster layer.

89% of all workers measured had T_ths−T_a 3 ⩾ 2°C, indicating that most workers contribute to colonial thermogenesis. Average T_ths−T_a was 4·1 °C. Drones measured had the same average T_ths−T_a as unmarked workers. T_ths−T_a did not differ among bees 2 days of age and older. Location on or off the brood did not affect T_ths−T_a.

Cooling constants of dead bees placed near the comb in the box averaged 1·036 min⁻¹ and were independent of location on the comb. Calculated average thoracic conductance was 0·829 calg⁻¹⁰C⁻¹ min⁻¹.

Average calculated heat production per worker was 0·095 cal min⁻¹, less than 15 % of the maximal oxygen consumption of 4-day-old bees. Calculations indicate that the larger drones contribute more heat per bee than do the workers.

Brood warming prior to clustering is due to relatively uniform heat production by individual bees and the attraction of bees to the brood area. This results in a higher heat production per unit volume of air over the brood than away from the brood.