Linking body size clines and thermal tolerance in Colorado carrion beetles

Body size is an important trait that commonly varies across elevation and latitude and can be influenced by and affect thermoregulation in animals. Insect body sizes commonly either become larger with increasing elevation to buffer colder temperatures or become smaller with increasing elevation due to the shorter growing season. As surface area to volume ratios heavily influence heat transfer in ectotherms, we expect a link between body size clines and thermal tolerance. This project uses the 15 montane species of Colorado carrion beetles as a study system to investigate the link between body sizes, thermal tolerance, and elevation. Carrion beetles assessed for body size were collected using pitfall traps placed along four elevational gradients (1,495–3,640m) in the San Juan and Front Range Mountains in Colorado from 2010–2012. The elytral lengths of these ethanol-preserved adults were measured using digital calipers. Beetles used for thermal limit testing were collected along an elevational gradient in Boulder County, Colorado from 2020–2021. Thermal limits were measured as the air temperature when the insect lost coordination at both higher and lower temperatures. Thermal tolerances and body sizes vary among species but are primarily conserved within subfamilies. Within subfamilies, elytral length is not strongly correlated with differences in thermal tolerance. While there are no elevational trends in thermal tolerance, body size clines across elevation within- and between-species are more complex. Understanding these relationships between size and thermal tolerance will help improve predictions of species’ climate change responses and ultimately improve conservation of at-risk species.