Endothermy in African Dung Beetles During Flight, Ball Making, and Ball Rolling

Abstract

Body temperatures of diurnal and nocturnal ball-rolling and non ballrolling dung beetles (in particular the genera Scarabaeus, Kheper, Gymnopleurus and Heliocopris) were studied in the field in Kenya. The beetles were conspicuously endothermic during flight, ball making, and ball rolling. When at rest, their body temperatures did not differ significantly from that of their surroundings. They did not always generate high body temperatures while walking. However, they sometimes warmed up prior to walking and subsequently maintained a large difference between body and ambient temperatures. The nocturnal species, S. laevistriatus, was conspicuously endothermic during ball making and ball rolling, often maintaining thoracic temperatures of 40 °C or more in an ambient temperature of 25–26 °C. In this species, the velocity of ball rolling increased linearly with body temperature from 5 cm/sec at 28 °C to 18 cm/sec at 40 °C. The take-off temperatures of dung beetles increased with body mass up to 2.5g. In beetles larger than 2.5g metathoracic temperatures at take-off ranged from 40 to 44 °CC and were independent of mass. Flight temperatures showed essentially the same pattern as take-off temperatures, increasing with mass up to about 42 °C at 2.5 g and being independent of mass thereafter. Both flight and take-off temperatures increased with wing-loading up to about 35 N/m², but were independent of wing loading from 35 to 65 N/m². In each of the species for which we had an adequate sample, wing loading increased linearly with body mass. Partial correlation analysis suggests that metathoracic temperature is more dependent on body mass than on wing-loading. During periods of endothermy, metathoracic temperature exceeded prothoracic temperature, and abdominal temperature was lower than that of either the prothorax or metathorax. Cooling rate was inversely related to body mass. The removal of elytra and wings caused the cooling rate to increase by about one-quarter. Elevated body temperatures in dung beetles during terrestrial activity is probably selectively advantageous in competition for food. A high metathoracic temperature is a necessary condition for take-off and flight in the larger species. Mechanisms of heat production and the evidence for thermoregulation are discussed.