Roles of endothermy and size in inter- and intraspecific competition for elephant dung in an African dung beetle, Scarabaeus laevistriatus

At night during the rainy season in Tsavo East National Park, Kenya, dung beetles were active in enormous numbers. A half liter of fresh elephant dung attracted as many as 4,000 beetles within 15 min. Deposits of 10 or more liters of elephant dung were usually rendered useless for ball making in less than 30 min. The activity time of the large, nocturnal, ball-rolling dung beetle, Scarabaeus laevistriatus, largely coincided with that of the small endocoprids that were mainly responsible for the rapid dung depletion. Scarabaeus laevistriatus sometimes maintained endothermically elevated thoracic temperature (38-42 C) during terrestrial activity (air temperatures 23-26 C) for as long as 25 min. The rate of ball construction by S. laevistriatus was related to thoracic temperature. Moreover, when the S. laevistriatus first arrived at a dung pile they routinely attempted to steal balls already constructed by beetles of the same and other species. Spirited fights invariably ensued, and the winners of these fights were primarily those with a higher thoracic temperature, and secondarily those of greater body size. The evolution of the capacity for facultative endothermy during terrestrial activity in S. laevistriatus appears to be related to the intense competition for dung that they experience. For them to be successful in this competition requires that they sustain near maximal rates of activity, which in turn requires that they maintain body temperature near the level obligatorily produced during flight.