What does infrared thermography tell us about the evolutionary potential of heat tolerance in endotherms?

Abstract

Anthropogenic climate change affects wild animal populations through increasing average temperatures and more frequent extreme climatic events. Endotherms have evolved the capacity to regulate their body temperature but little is still known about how they can physiologically adapt to the pace of global warming. Adaptive responses would require that heat-tolerance mechanisms, such as the capacity to withstand high body temperatures and regulate evaporative water loss, exhibit sufficient heritable genetic variation for selection to act upon. Unfortunately, the quantitative genetics of these traits in endotherms remains poorly understood. In a recent study using infrared thermography (IRT) on semi-captive ostriches, Svensson et al., (Heritable variation in thermal profiles is associated with reproductive success in the world's largest bird. Evolution Letters, 8(2), 200-211.) sought to address this knowledge gap by measuring relative heat exchange from the head and neck and assessing the link between among-individual variation in heat dissipation and reproductive fitness. We discuss how IRT serves as a valuable tool for non-invasive data collecting, highlighting its potential for field studies of the evolutionary potential of thermal tolerance. Nevertheless, interpreting IRT data is not as straightforward as it may seem and thus must be conducted carefully. For instance, body parts from which surface temperatures are measured need to be unequivocally identified as sources of dry heat exchange in order to inform on thermoregulation-something lacking in the mentioned study. Furthermore, there is still no conclusive evidence that surface temperatures reflect core body temperatures in endotherms. Critical underlying mechanisms of the heat response, such as evaporative cooling, must also be considered. Assumptions stemming from uncertain proxies of thermoregulation can obscure our understanding of the endothermic adaptation of heat-tolerance traits to rapid global warming. These considerations emphasize that, while IRT can be a valuable tool for developing quantitative genetic approaches to estimate the evolutionary potential of heat tolerance in endotherms-particularly for species most vulnerable to warming, its application warrants careful planning.

Keywords: adaptation; climate change; evolutionary physiology.

Figure 1.

Examples of infrared thermal images of a pigeon (Columba livia) inside an indoor cage (A) and of a Canada goose (Branta canadensis) in an outdoor aviary during winter (B), captured with a thermographic camera (model FLIR T530). Thermal images can be used to identify featherless areas of the body where heat loss and retention occur, such as the eye region, the beak, and the tarsus. Both pictures show the temperature scale in degrees Celsius on the right side. Photos kindly provided by Andrea Ferretti^(c).