Effects of population structure and density on calf sex ratio in red deer (Cervus elaphus)—implications for management

Abstract

There is ongoing interest to assess what factors affect offspring sex ratio, especially in ungulates. Wildlife managers might be interested in influencing this sex ratio for two reasons: either in order to limit population growth more effectively by reducing the proportion of females born or to increase revenues by a higher proportion of trophy bearing males in the population. While previous studies mostly focused on how maternal traits affect offspring sex ratio, we included here also male traits in our analysis. We achieved this by investigating data from 30 areas covering entire Lower Austria, collected over the past 12 years from both hunted red deer and those killed in road accidents. We focused our analyses on parameters that can be easily assessed by managers on the population and individual level, i.e. the numbers of animals culled in different age/sex classes and their body mass. We found that the proportion of females among calves increased with population density. Furthermore, we found that calf sex ratio (i.e. the proportion males among calves aged between 2 and 7 months) increased with increasing proportions of adult females and males older than 10 years, independent of the density effect. We conclude that wildlife managers interested in the effective reduction of red deer abundance and/or increasing the proportion of males among offspring should select a culling regime leading to a low population density dominated by adult, prime-aged females and males. This can be achieved by over-proportional removal of young females and warranting that a high number of strong males reach an age of at least 10 years.