Influence of social dominance on the secondary sex ratio and factors affecting hierarchy in Holstein dairy cows

In several studies of different species, the proportions of male and female offspring did not match the statistical assumption of 50:50. Aside from environmental and genetic factors, biological parameters were found to influence the sex ratio. Three hypotheses (Trivers-Willard, advantaged daughter, and the local resource competition hypothesis) have been proposed, stating that the sex of progeny might be influenced by sex-biased maternal investment in the raising of offspring. The results of existing studies are still conflicting, even within studies on the same species. The aim of the present study was to investigate whether the dominance status of cows influences the secondary sex ratio. A herd of lactating cows was observed directly for 126 h, and a dominance index (DI) was established for 71 individuals (aged between 2.6 and 9.8 yr) for which at least 8 encounters had been observed to categorize individuals as subordinate, intermediate, or dominant. Furthermore, the number and sex of calves were assessed, and further individual characteristics of the animals (e.g., age, milk yield, body condition, and lameness scores) were analyzed for interrelationships with dominance status as well as the influence of the DI on the secondary sex ratio. The results indicated that the DI was independent of age, but a negative correlation was found between DI and lameness, whereas a positive correlation was shown for DI and body condition score. In general, more dominant animals had lower lameness scores and better body conditions. No sex ratio bias was found for primi- or for pluriparous animals and no seasonal influences were detected. The proportion of male offspring was significantly correlated with the DI of lactating cows, where animals with a higher social rank produced a lower proportion of male progeny. The pairwise comparison between dominant and subordinate lactating cows revealed a significant difference in the sex ratios of progeny, with subordinate animals having more male calves. These results indicate that the maternal dominance rank was associated with the secondary sex ratio in cattle. These observations are generally in agreement with the advantaged daughter hypothesis.