Thermoregulation and flight performance in honeybee drones (Apis mellifera)

Honeybee drones (Apis mellifera) are twice the size of workers, exhibit higher body temperatures (T{dollar}sb{lcub}rm b{rcub}{dollar}), and fly almost exclusively in order to mate. Hence, drones represent an excellent model for the study of thermoregulation and flight adaptation. Drones retain heat in the thorax by preventing the shunting of blood to the abdomen, and are unable to lower T{dollar}sb{lcub}rm b{rcub}{dollar} through regurgitation. Because they lack cooling mechanisms and are not especially tolerant of high T{dollar}sb{lcub}rm b{rcub}{dollar}, they experience very high, nearly lethal T{dollar}sb{lcub}rm b{rcub}{dollar} at high ambient temperatures (T{dollar}sb{lcub}rm a{rcub}{dollar}). Heat production during hovering increases with body size, while heat loss decreases with body mass (M{dollar}sb{lcub}rm b{rcub}{dollar}) and increases with flight speed. A heat budget model demonstrated that the higher T{dollar}sb{lcub}rm b{rcub}{dollar} of drones is primarily a result of large M{dollar}sb{lcub}rm b{rcub}{dollar} and high ratio of thorax to body mass, which result in lower heat loss and higher heat production. Drones do not seem to be designed for carrying internal cargo, and increases in T{dollar}sb{lcub}rm b{rcub}{dollar} result in decreases in the time and range available from energy stores during a mating flight. Increases in T{dollar}sb{lcub}rm b{rcub}{dollar} have a positive effect on flight speed and force production up to a limit, then they have a negative effect, which decreases drone flight performance at high T{dollar}sb{lcub}rm a{rcub}{dollar}. The extremely high T{dollar}sb{lcub}rm b{rcub}{dollar} of drones may be a result of selection for the large M{dollar}sb{lcub}rm b{rcub}{dollar} and may not be adaptive for flight performance. Drones cannot fly faster than workers, but still generate more kinetic energy due to the drones' greater mass. Drones have not evolved for maximal speed, but may have evolved for the usurpation of rivals during aerial mating, although few observations support this notion. Drones produce the same mass-specific force during flight as workers ({dollar}sim{dollar}20 N/kg), but more total force (3.95 vs 1.73 mN); therefore, it appears that they have evolved not for maneuverability, but for carrying large external loads. Observations indicate that the drone must carry the queen during mating. Neither workers nor queens produce sufficient force to carry a queen. Thus, the large size of drones may be a result of selection for queen-carrying ability.