Sexual dichromatism in the fur of a bat: An exploration of color differences and potential signaling functions

Sex differences in body color (i.e., sexual dichromatism) are rare in bats and, more broadly, in mammals. The eastern red bat (Lasiurus borealis) is a common tree-roosting bat that occupies much of North America and has long been described as sexually dichromatic. However, previous research on this species found that absolute body size and collection year were better predictors of fur color in preserved specimens than sex. We revisited this issue and photographed 82 live eastern red bats under standardized conditions, then used image analysis to quantify pelage hue, saturation, and value. We used an information theoretic approach to evaluate four competing hypotheses about the principal drivers of color differences in the fur of eastern red bats. Our analyses demonstrated that sex was a better predictor of pelage color than body size; males had redder, more saturated, and lighter pelages than females. Additionally, the fur color of juvenile versus adult bats differed somewhat, as juveniles were darker than adults. In general, absolute body size (i.e., forearm length in bats) was a poor predictor of color in live eastern red bats. In an exploratory post-hoc analysis, we confirm that fur color is related to body mass (i.e., a proxy for body condition in bats), suggesting color might serve as a sexually selected signal of mate quality in this partially diurnal species. Future work should investigate the functional role of sexual dichromatism in this species, which could be related to signaling or possibly thermoregulation.