Mummy's boys: sex differential maternal-offspring bonds in semi-feral horses

In polygynous species with high reproductive skew in males, mothers often show differential investment between sons and daughters. Although consistent sex differential investment has not been found by previous studies in horses, maternal investment beyond weaning has often been overlooked. We investigated sex differences in mother-offspring relationships in nutritionally independent sub-adult feral ponies, Equus ferus caballus. Stronger affiliative bonds between mothers and their sons than with their daughters were shown by their maintenance of closer proximity, higher rates of affiliative interactions and more frequent suckling bouts. These were associated with higher aggression levels directed towards sub-adult males by other band members, particularly stallions. We suggest that aggression may serve as the proximate mechanism driving male dispersal in feral horses and that the stronger mother son bond signals an attempt to delay dispersal, highlighting conflict within the band concerning dispersal timing. Since we used social network theory to show that males become increasingly central within the band over time, we propose that delaying colts' dispersal allows for further development of social skills in a relatively safe environment, maximising their reproductive success. This study illustrates how social network theory can be used to quantify individuals' social experiences, contributing to a greater understanding of the evolution of group living.