The Mechanisms and Energetics of Honeybee Swarm Temperature Regulation

Abstract

Free (active) honeybee swarms regulated their core temperature (T_c) generally within 1 °C of 35 °C. They maintained the same temperature around freshly built honeycomb, and in the brood nest of the hive, from ambient temperatures of between at least 1 and 25 °C. Captive (inactive) swarms in the laboratory often allowed T_c to decline below 35 °C. The temperature of the swarm mantle (T_m) varied with the general activity of the swarm as well as with ambient temperature (T_A), but in captive swarms (and sometimes at night in free swarms), T_m was generally held above 17 °C, even at T_A < 5 °C. Within the swarm, temperatures varied between 36 °C, an upper temperature set-point, and 17 °C, a lower temperature set-point. Before swarm take-off, all temperature gradients in the swarm were abolished and T_m equalled T_c. The regulated T_c and T_m were unrelated to size and passive cooling rates in swarms ranging from 1000 to 30000 bees. The weight-specific metabolic rate of swarms was correlated with T_A and T_c, but relatively little affected by swarm size. Bees on the mantle experiencing low temperatures pushed inward, thus contracting the mantle, diminishing the mantle porosity, and filling interior passageways. As a result, their own rate of heat loss, as well as that from the swarm core, decreased. In large tightly clumped swarms, even at T_A < 5 °C, the resting metabolic rate of the bees in the swarm core was more than sufficient to maintain T_c at 35 °C or above. The active thermoregulatory metabolism was due to the bees on the swarm mantle. There was little physical exchange of bees between core and mantle at low (< 5 °C) T_A. In addition, there was no apparent chemical or acoustic communication between the bees in the swarm mantle that are subjected to the changes of the thermal environments and the bees in the swarm interior that constantly experience 35 °C regardless of T_A. The data are summarized in a model of T_c control indicating a primary role of the mantle bees in controlling heat production and heat loss. The possible ecological significance of swarm temperature regulation is discussed.