Examination of Annual Variation in the Adult Sex Ratio of Pronghorn (Antilocapra americana)

Abstract

The adult sex ratio (ASR) is an important component of a population's demographics and can be used as an indicator of a population's status. However, the causes of annual variation in ASRs are unknown for many species. Fluctuations in ASR can arise through demographic stochasticity and intense selective harvesting. In this study we investigate the long-term patterns of variation in the ASRs (bucks: 100 does) for four populations of pronghorn (Antilocapra americana) in western Nebraska. We used multiple variables in a model selection process to predict annual fluctuation of pronghorn ASRs. We found that the number of bucks: 100 does significantly varied over the four populations. The best predictors of annual variation in pronghorn ASRs were variable across all populations. The number of pronghorn bucks harvested in the previous year's hunting season and the previous year's density of pronghorn were the most common predictors of ASR fluctuation. Buck harvest was an important predictor variable in only two of the four populations. Variation of harvest strategies within the populations could account for the lack of importance of buck harvest in half of the populations. The relationship between density and ASR is novel but difficult to interpret due to lack of data on birth sex ratios and fawn survival. More data on pronghorn demographics are needed in order to better explain the connection between density and ASR.