Some like it hot: body and weapon size affect thermoregulation in horned beetles

Body size and shape affect thermoregulatory properties of organisms, and in turn are believed to have shaped macroevolutionary patterns of morphological diversity across many taxa. However, it is less clear whether thermoregulation plays a role in shaping intraspecific morphological diversity such as sexual dimorphisms or the conditional expression of exaggerated secondary sexual traits. Here, we investigate individual thermoregulatory properties in two species of horned beetles that share similar ecologies and body size ranges, but differ substantially in degree of sexual and male dimorphism. We find that intraspecific variation in body size had an unexpectedly large effect on thermal preference behavior and the ability to passively regulate body temperature. Furthermore, we find that the presence or absence of exaggerated secondary sexual traits dramatically altered thermal preference behavior, consistent with a thermoregulatory cost of horn possession. Lastly, we show that the increase in surface area associated with the expression of enlarged horns is, by itself, insufficient to account for the radically altered thermoregulatory behavior observed in horn-bearing males, and discuss possible alternative, physiological explanations. These findings are among the first to link intra-and interspecific variation in body- and weapon size to thermal preferences within and between insect species.