Stability of Social Status in Wild Rats: Age and the Role of Settled Dominance

Abstract

Abstract One way of understanding the evolution of social dominance is to establish which factors determine an animal's ability to dominate conspecifics. The dynamics of dominance between 20 adult male wild rats were investigated in a multi-generational, free-breeding colony in a large outdoor enclosure. Dominance relations between the adult males were stable and organised in a near-linear hierarchy. Dyadic interactions not fitting the social hierarchy, as well as challenges by subordinates and overt aggression by dominants were rare (< 5%) and principally occurred between animals of similar social rank. The correlates of social status within the colony show, for the first time in adult small mammals, that despite the significant role of body weight on the probability of winning contests, age was the most reliable indicator of adult dominance, with the higher ranking males being older but not necessarily heavier. Age also explained the outcome of 85% of agonistic encounters between dyads, compared with 65% for weights. The proximate mechanisms of age-related dominance fit better the 'previous outcome' hypothesis than the alternative 'fighting skill' or 'site dominance' hypotheses. The stability of dominance relations and the role of age, which in stable groups is equivalent with time spent in the colony, suggest that rats remain dominant over individuals that they have beaten in the past, long after initial body weight asymmetries have disappeared. The functional significance of the acceptance of subordinate social status is consistent with the fact that dominant individuals generally could not monopolize food resources or mates.