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First published online August 28, 2009
Journal of Experimental Biology 212, 2960-2968 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.032763
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Endothermy of dynastine scarab beetles (Cyclocephala colasi) associated with pollination biology of a thermogenic arum lily (Philodendron solimoesense)

Roger S. Seymour1,*, Craig R. White2 and Marc Gibernau3

1 Ecology and Evolutionary Biology, School of Earth and Environmental Sciences, Darling Building, University of Adelaide, Adelaide, SA 5005, Australia
2 School of Biological Sciences, University of Queensland, St Lucia, QLD 4072, Australia
3 Laboratoire d'Evolution et Diversité Biologique UMR 5174, Université Paul Sabatier, 31062 Toulouse, Cedex 9, France

* Author for correspondence (roger.seymour{at}adelaide.edu.au)

Accepted 28 June 2009

Cyclocephala colasi beetles are facultative endotherms that spend most of their adult lives inside the inflorescences of Philodendron solimoesense, where ambient temperature (Ta) averages about 28°C due to floral thermogenesis. Measurements of respiration within a range of Ta showed that active beetles became spontaneously endothermic at Ta below 28°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 Ta 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 (Tth) increases. Stationary beetles could regulate Tth at about 33°C independently of Ta between 16 and 29°C. At Ta=20°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°C before take-off. During flight, Tth was dependent on Ta, decreasing from 37 to 28°C at Ta of 37 to 20°C, respectively. The lowest Ta at which flight could occur was about 20°C. Thermal conductance of stationary, endothermic beetles increased at higher metabolic rates, probably because of increased ventilatory heat loss.

Key words: beetle, endothermy, pollination biology, Cyclocephala, Philodendron


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