The Elytral Management of Near-Infrared Radiation Enhances Body Heat in a Dung Beetle Species without Energetic Cost

AbstractNear-infrared radiation, although invisible to the human eye, accounts for nearly half the solar energy that reaches the Earth's surface. Given this, it is reasonable to investigate whether this type of radiation influences thermoregulation in ectothermic animals, as some recent studies suggest. In this study, I examined the daily activity pattern of a beetle species from the Geotrupidae family (<i>Geotrupes stercorarius</i>), alongside measurements of the solar radiation spectrum in the environmental conditions where this species is typically found. To assess thermal responses, both elytra and specimens-either dried or freshly killed-were exposed to radiation sources of different wavelengths. These responses were compared with the reflectance, transmittance, and absorbance values obtained through spectrophotometric analyses of the elytra, spanning the 200-1,700 nm range. The results clearly show that near-infrared radiation alone can raise internal body temperature. This effect is mainly a result of the absorption of near-infrared radiation by the exoskeleton and the subsequent transmission of energy as heat. Exposure to an 850-nm radiation source increased the internal temperature of freshly killed specimens to 26.5°C, at rates comparable to those under yellow or red visible light and only 34%-50% lower than those induced by the most energetic wavelengths. The impact of near-infrared radiation on body temperature was even greater under forest conditions, where this type of radiation becomes relatively more important because of canopy filtering of other wavelengths.