Relatedness and the effect of inbreeding on mortality in Sable Island horses

Abstract

It is often difficult to asses the social and mating structure of populations through the usage of observational methods alone. Molecular methods give greater insight into patterns of relatedness within populations, and with the recent application of network theory, allows for the quantification of those patterns of relatedness. This can be used to examine aspects of population such as mating patterns, reproductive success, dispersal and inbreeding. Small populations will lose genetic diversity faster than larger populations through genetic drift and inbreeding, and this genetic decline can lead to issues with fitness. Sable Island is a small island located off the eastern coast of Nova Scotia, which contains a small population of feral horses. Little is known about their social structure, or the factors behind the fluctuations of population size that occasionally occur. This study aimed to assess their natal dispersal patterns and reproductive success, as well as to assess the degree to which inbreeding affects mortality in harsh winters. DNA was extracted from samples collected from 208 individuals that died in a large mortality event in 2003. Genotype data from these individuals were used to estimate relatedness. A network was generated, and used to test for correlation between relatedness patterns and habitat use patterns, and to test the reproductive success of males and females. No correlation between groups of related individuals and location in females was seen, indicating that females are not remaining in natal areas as expected. A negative correlation between the two was seen in males, indicating that they are likely avoiding areas with close kin. Males were also had overall lower reproductive success, and more variation in reproductive success than females, which is expected of a polygynous mating system. No connection between inbreeding and mortality was seen, as levels of inbreeding were similar across all age classes.