Implications of fetal sex ratio hypotheses in endangered populations: simulated dynamics of Florida Key deer, Florida, USA

Fetal sex ratios (FSRs) have important implications for managing small isolated populations. Mean male-biased FSRs ranging from 2.67:1 to 1:1 have been reported for the endangered Florida Key deer (Odocoileus virginianus clavium). Several general hypotheses have been proposed that describe the manner in which FSR may vary within a deer population over time: (1) tendency to equalize the sex ratio by producing the minority sex, (2) poor body condition results in more males, and (3) excellent body condition results in more males. Our objective was to evaluate implications of FSR for Key deer as suggested by these hypotheses. Because of their small numbers and geographic isolation, the issue of FSR is important for predicting response of Key deer to management actions. We developed nine alternative deterministic models representing combinations of three mean or median FSRs that have been hypothesized for Key deer and the three hypothesized mechanisms of FSR variation. With each model, we simulated Key deer demographics and compared predictions with actual survey data (1971–2000). The model with the best prediction was based on a median FSR of 1.45:1, and the FSR variation within the survey data best supported the hypothesis that poor body condition would result in more males. Our results indicate the most commonly cited FSR (2.67:1) for Florida Key deer is inaccurate, probably due to small sample size. We conclude that FSR variation has the potential to shape the response of endangered populations to disturbance.