Beetling the heat - the diurnal Namib Desert beetle Onymacris plana cools by running.

Abstract

Onymacris plana (Coleoptera: Tenebrionidae) is a black beetle that runs at high speed for a pedestrian insect in direct solar radiation in the Namib Desert, a behaviour expected to impose potentially lethal body temperature within minutes. We measured the body temperature of beetles active in their natural habitat using fine thermocouples inserted into the prothorax. The measurements revealed that when beetles sprinted in conditions of low wind, high radiation and moderate ambient temperature, their body temperature dropped rather than rose. The effect depended on convective cooling and efficient locomotion, i.e., sprinting with low energy expenditure. We confirmed the convection effect in the laboratory by exposing beetles to combinations of radiation, air temperature and wind speed comparable to those found in the Namib Desert and simulating the forced convection of running in a headwind. Under these simulated conditions, peak radiation caused the temperature of stationary male beetles to rise at about 6°C min-1 and females at almost 4°C min-1. However, in wind-calm conditions at peak radiation, the convection of simulated running dropped the equilibrium body temperature of live beetles by about 13°C. We believe that ours is the first report of exercise-induced cooling in a pedestrian animal and that O. plana's diurnal lifestyle depends on that exercise-induced cooling.