Regional heterothermy in Megasoma gyas is not related to active heat dissipation by the horns

Animals rely on physiological and behavioral processes to maintain thermal balance. Some animals, however, bear structures that help dissipate excess heat when body temperatures rise. Although widespread in animals, animal weapons—exaggerated morphological structures with multiple characteristics that can make them good at dissipating heat—have rarely been studied in the context of thermoregulation. Here, we investigated whether the horns of the Rhinoceros Beetle ( Megasoma gyas ) acted as a thermal window. We heated live and dead beetles to 30ºC and allowed them to cool to 20ºC while measuring surface temperature changes in four body regions: the cephalic and thoracic horns, the scutellum, and the abdomen. If horns actively dissipated heat, they would show the lowest cooling rate among body regions. Contrary to this expectation, we found that the cephalic horn had the highest cooling rate, followed by the abdomen, thoracic horn, and scutellum, respectively. This suggests that the horns are not used for active heat dissipation in M. gyas . The low cooling rate of the scutellum can be explained by the presence of large flight muscles in the thorax, which play a role in heat generation, but could also aid in heat dissipation by pumping hemolymph across tagmata or through the low-insulated cuticle to prevent thoracic overheating. We also demonstrate that beetles show regional heterothermy even in the absence of exercise or stress. As such, we propose that regional heterothermy may result from both active (control of hemolymph flow) and passive (heat dissipation through poorly insulated structures) processes within individuals.