ABSTRACT
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. All previous demonstrations of social inertia have returned treated and untreated birds to their original aviaries and opponents and thus have not separated effects of familiarity with the location from those of familiarity with opponents. The present experiment 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 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.
STATUS SIGNALS AND RECOGNITION OF OPPONENTS (from the Discussion)
Studies of status signalling in a congener of the white-throated sparrow, the Harris' sparrow Zonotrichia querula, have included manipulations of hormonal state or appearance or both (a double treatment). For instance, one individual implanted with testosterone and then released where it had been captured failed to increase in dominance; in contrast, two others observed before and after they were both implanted and painted to look like older males did increase in rank (Rohwer & Rohwer 1978). The bird that was only implanted received more attacks from dominant opponents but apparently did not initiate any more attacks (Rohwer & Rohwer 1978). In white-throated sparrows, some implanted birds returned to wild flocks also engage in more frequent aggression, in part initiated by themselves (Archawaranon et al. 1991). Birds released into wild flocks confront opponents that no doubt vary in familiarity. Presumably, as in stage III of the present experiment, testosterone activates aggression when opponents are unfamiliar with each other.		Painting a bird might well affect its previous acquaintances' ability to recognize it. This new situation might also quickly affect the subject's behaviour. As a result, the rise in dominance of Harris' sparrows following double treatment (with both implants and paint) might result either from direct effects of the painting on its status signals or from indirect effects of the painting on recognition of opponents. In the latter case, when recognition is diminished, testosterone could produce an increase in dominance. Such double-treatment experiments might thus be explained by the effects of testosterone on the behaviour of unfamiliar opponents, in effect by an absence of social inertia.

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