Phenotypic characteristics of successful parental pairs in white-tailed deer: evidence of non-random mating

Based on principles of natural selection, high-quality individuals may benefit by selecting mates of similar phenotypic quality when given the opportunity; that is, individuals may benefit by engaging in a form of non-random mating referred to as positive assortative mating. In ungulates, the idea of mate selectivity is still highly debated, with few studies providing evidence of positive assortative mating. In white-tailed deer (Odocoileus virginianus), recent studies have suggested minimal mate selectivity based on phenotypic characteristics. However, findings from these previous studies may be due to the fact that study populations had young age structures and female skewed sex-ratios that may have limited opportunities for preferential mating. From 2008 to 2019 we studied a captive population of white-tailed deer to examine characteristics of 184 parental pairs that successfully recruited fawns, while the population underwent changes in age structure and sex ratio, and we found evidence of non-random mating in parental pairs. Specifically, for each 1-year increase in male age there was a 0.32-year increase in female age among parental pairs, and for every 1.00 cm increase in male skeletal body size there was a 0.18 cm increase in female skeletal body size. We also observed that as the male and female age structures increased over time, the slope of the relationship between male and female lifetime body percentiles of parental pairs had a greater positive increase over time. Furthermore, we observed that breeding success by younger males decreased as the age structure matured. Although, even with evidence for non-random mating, we were unable to firmly conclude these findings as assortative mating due to the fact we still observed cases where there was a large difference in age between mates, indicating some plasticity with mating. Overall, our results suggest that individuals of an older age and greater body size may select for individuals of a similar condition if provided with the opportunity. However, choosiness appears dynamic, with a decrease in selectivity when preferential mates are limited due to a young age structure or when mating opportunities decrease for poor competitors in a population with a mature age structure.