Sociality of sable island horses: population, group, and individual interactions
Medill, Sarah A 1977-
The social structure of feral horses (Equus ferus caballus) is strongly influenced by their female-defense polygynous mating system, and in turn, social structure is known to impact individual fitness. What regulates structure of social groups is not as well understood. I examine how population density and adult sex ratio (ASR) influence sociality in the feral horse population of Sable Island, (Nova Scotia, Canada), and characterize the duration of associations and the differences in social characteristics between reproductive and non-reproductive adult members of the population. I also explore how concentrations of testosterone and cortisol recovered from tail hair relate to physiological and sociological correlates in this free-living population. Density and ASR influence a number of social characteristics in this population. High local density is associated with larger harems and increased probability of adult females switching between social groups. Male ability to compete for sexual opportunities is reached 2–3 years after females leading to a typically female biased ASR. However, as ASR becomes more neutral or male-biased group size decreases and adult females become less likely to change bands. Foal production decreased with increasingly male-biased ASR while the same conditions improved foal survival. Female reproductive success increased when they maintained long-term associations with specific males and minimized their overall associations. While several studies suggest that female–female associations in the harem are important, I present evidence to suggest that could be a side-effect of females attempting to remain in association with the same male and to avoid antagonistic interactions associated with establishing their position in the social hierarchy of a new group. Investigation of hair cortisol concentrations (HCC) also revealed that the endocrine response to particular stressors may be different between males and females. Top models describing male HCC included only physiological factors of age and body condition and also year. While the best model describing female HCC included body condition, age, and presence of a foal, they also included social variables of harem size and abundance of males in vicinity. Males in reproductive roles as dominant harem stallions had higher hair testosterone concentrations than non-reproductive males. This research provides a better understanding of the interactions between population level processes and indirect actions on individual fitness, opportunities for sexual selection, and endocrinology as they relate to the social and mating structure of an island-bound population of feral horses.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorMcLoughlin, Philip D
CommitteeLane, Jeffrey E; Janz, David M; Johnstone, Jill F; Chivers, Doug P; Bollinger, Trent K
Copyright DateMay 2018
Equus ferus caballus