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To investigate the effects of familiarity with opponents on the
activation of aggression and dominance by testosterone (T) in
white-throated sparrows Zonotrichia albicollis, we studied birds
in groups of five or six in outdoor aviaries and in a free-living
population.
Experiments were conducted between January and April, when plasma levels
of endogenous gonadal hormones were low. Previously published
experiments (Archawaranon & Wiley, 1988) showed that when subjects were
given subcutaneous implants of T, held individually in cages for one
week, and then grouped with unfamiliar opponents, they established
dominance over controls with empty implants.
In contrast, the present study showed that low-ranking birds given
subcutaneous implants of T and returned to their original groups and
aviaries did not change in dominance rank nor in aggression scores over a
period of two weeks. When these birds were regrouped with unfamiliar
opponents in new aviaries, their dominance ranks and aggression scores
increased.
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This rise in dominance rankings and aggression scores did not result from
any effects of regrouping in the absence of hormonal treatments. The
sexes did not differ in their responses to these treatments.
In the field, T implants markedly increased some individuals' frequencies
of aggression but had little or no effect on dominance relationships.
The absence of behavioral changes in stable groupings after hormonal
treatment, called social inertia, indicates that birds recognize the
relative dominance of previous opponents, at least when encountered in
familiar locations. Results from the field experiment thus suggest that
white-throated sparrows can recognize at least 20 opponents.
Hormonal state has more influence on dominance relationships of strangers
than on those of opponents familiar with each other in familiar
locations.
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