Endothermy of dynastine scarab beetles (Cyclocephala colasi) associated with pollination biology of a thermogenic arum lily (Philodendron solimoesense)

Abstract

Cyclocephala colasi beetles are facultative endotherms that spend most of their adult lives inside the inflorescences of Philodendron solimoesense, where ambient temperature (T(a)) averages about 28 degrees C due to floral thermogenesis. Measurements of respiration within a range of T(a) showed that active beetles became spontaneously endothermic at T(a) below 28 degrees C but were rarely endothermic above it. There was no evidence of endothermy within the inflorescences, indicating that activities in the floral chamber can occur without the high energy expense of endothermy. Bouts of endothermy occurred at lower T(a) in respirometer chambers mainly in the evening, when the insects normally fly from one inflorescence to another, and during the night, when they normally eat and mate within the inflorescence. Patterns of endothermy in individual episodes were studied in non-flying beetles with respirometry and infrared thermal imaging. Heat was generated in the thorax by oscillatory waves of respiration that were coupled with thoracic temperature (T(th)) increases. Stationary beetles could regulate T(th) at about 33 degrees C independently of T(a) between 16 and 29 degrees C. At T(a)=20 degrees C, this represents a 116-fold increase in metabolic rate over resting, ectothermic values. Endothermy was clearly a requirement for flight, and beetles departing inflorescences warmed to about 30 degrees C before take-off. During flight, T(th) was dependent on T(a), decreasing from 37 to 28 degrees C at T(a) of 37 to 20 degrees C, respectively. The lowest T(a) at which flight could occur was about 20 degrees C. Thermal conductance of stationary, endothermic beetles increased at higher metabolic rates, probably because of increased ventilatory heat loss.