Assemblage-level analysis of sex-ratios in Coloradan bats in relation to climate variables: A model for future expectations

Bats act as important bioindicators of environmental changes, including climate disruptions. In the Rocky Mountain West, that includes western Colorado, the pace of climate warming outpaces other areas of North America. Using data on bats captured between 1996 and 2015 we found significant trends towards male-bias (R2 = 0.38) over the 20-year period for both adults and juveniles. Capture data for juveniles showed markedly less females captured than males after 2000, hitting all-time lows in 2000, 2002, and 2007 (R2 = 0.12). When juvenile captures were plotted against an index of annual monthly temperatures for the June, July, and August, divided by annual precipitation for the same months (T/P index) we found that sex ratios favored males at the extreme ends of the spectrum (cooler/wetter and hotter/drier), however, male-biased outcomes were much more prevalent in hotter/drier years. We found that nearly 1:1 sex ratios occurred within an optimal range of temperature and precipitation that peaked at 28.7° C with 7.5 cm precipitation. To explore the possible impacts of future climate conditions on sex ratios, we used juvenile capture data to build a predictive logistic regression model and projected it through year 2090 using four carbon emission scenarios (RCP2.6, RCP4.5, RCP6.0, and RCP8.5). All scenarios showed overall declining proportions of females in future years. Thus, mitigation of climate change on bat populations must take into account not only reproductive declines in some species, but also shifting sex ratio outcomes towards increasing male abundances in populations.