Estimates of sex ratio require the incorporation of unequal catchability between sexes

Abstract

Context Estimates of the sex ratio of a population are a common summary statistic used for ecological studies and conservation planning. However, methods to determine the sex ratio often ignore capture probability, which can lead to a perceived bias in the sex ratio when the sexes are detected at different rates. Aims To illustrate the bias from conventional count-based analysis methods for determining sex ratio by comparison with analytical methods that include capture probability. Methods Closed-population mark–recapture analysis was used to determine the population size of each sex within a population of green and golden bell frogs (Litoria aurea). This was then compared with the traditional count-based methods of estimating sex ratio to determine the effect of incorporating capture probability on the sex ratio estimate. Key results More males than females were detected during surveys, producing a male-biased sex ratio when there was no incorporation of capture probability. Mark–recapture results indicated a similar population size between the two sexes, suggesting that the sex ratio is closer to even. Conclusions Methods to estimate sex ratio that incorporate capture probability can significantly reduce the bias obtained from count data. Implications We suggest that population studies must incorporate capture probability to determine the sex ratio of a population.