Identifying Offspring Sex Ratio Skews in Zoological Facilities Using Large Historical Datasets

Abstract

The Fisher principle states that species should produce offspring at a ratio of 1:1 unless there are sex-specific differences in rearing costs. Research conducted across taxa has found that animals will vary the sex ratio of their offspring so as to maximize personal fitness in response to various ecological and biological variables. This phenomenon has been especially well studied within the Avian class. Professionally managed populations provide a useful framework within which to study the impacts of numerous variables on sex allocation strategies. Zoological facilities may be especially motivated to investigate this phenomenon due to corresponding conservation and welfare implications. We analyzed a large dataset containing the demographic data of over 1 million zoo-housed birds for species-specific offspring sex ratio biases. The offspring sex ratios of 19,867 zoo-hatched dams were calculated, and the median offspring sex ratio of 277 species from 25 orders was calculated across two 20-year time periods. We used a Wilcoxon signed rank tests with a Holm Bonferroni alpha criterion in order to determine if species medians differed from parity. Only one species in one time period displayed a significant species median offspring sex ratio. The limits of our dataset are discussed. We propose that future research should further explore the determinants of sex allocation strategies for species held within zoological facilities.