Social inertia in white-throated sparrows results from recognition of opponents.

Social inertia is a term for the stability of dominance relationships despite changes in the intrinsic dominating abilities of opponents. In a standard test for social inertia, low-ranking birds in an established hierarchy receive implants with testosterone (treated) and high-ranking birds receive empty implants (untreated). Social inertia occurs when the treated birds remain subordinate to untreated opponents in these groups, despite evidence that similarly treated birds dominate untreated strangers. In previous demonstrations of social inertia, however, treated and untreated birds were returned to their original aviaries and tested with familiar opponents, and thus the effects of familiarity with the location and those of familiarity with opponents were not separated. To address this issue, we investigated social inertia in 16 groups of white-throated sparrows Zonotrichia albicollis. When low-ranking treated birds were placed in new aviaries with familiar high-ranking, untreated opponents (treatment S, same opponents), dominance relationships showed social inertia. When such birds were placed in new aviaries with unfamiliar opponents (treatment N, new opponents), testosterone influenced dominance. When groups of high-ranking, untreated birds acquainted with each other were placed with unfamiliar treated opponents (treatment G, grouped dominants), 'coat-tail' effects (dominance by association with high-ranking individuals) sometimes outweighed the effects of testosterone. Social inertia in this species is thus a result of familiarity with opponents, rather than familiarity with locations of encounters. Measurements of aggressive tendencies confirmed a previous report that social inertia suppresses activation of aggression by testosterone. White-throated sparrows can thus recognize their opponents, and this ability affects the expression of both dominance and aggression. Copyright 1999 The Association for the Study of Animal Behaviour.