Dominance hierarchies in female mountain goats:: Stability, aggressiveness and determinants of rank

To investigate the importance of dominance relationships in the social organization of large mammals, I studied the aggressive behaviour of marked adult female mountain goats (Oreamnos americanus) during four years in west-central Alberta, Canada. Despite large group size, the 38-45 adult females in the population were organized in a strong and very stable linear hierarchy. Social rank was strongly related to age and did not decrease for the oldest females. The presence of a kid did not affect the aggressive behaviour of females, suggesting that aggressiveness probably did not evolve fur offspring defense in mountain goats. Initiators won most encounters, except when the receiver was an older female. When age was accounted for, body mass, horn length, and body size were not related to female rank. A new measure of aggressiveness, controlling for the number of opportunities for interactions, revealed that aggressiveness towards younger adult females increased with both age and social rank. On the other hand, age and social rank did not affect aggressiveness towards other females of the same age or older. Goats interacted more often with individuals of similar ranks than with individuals that were distant in the dominance hierarchy. Social rank of adult daughters was not related to the social rank of mothers. Although central positions in a group may decrease predation risk, dominant females did not occupy central positions more often than subordinates. Because age was the main determinant of rank, the only effective way to increase social rank was to survive.