Coevolution of elaborated male display traits in the spotted bowerbird: an experimental test of the threat reduction hypothesis.

Elaborated male sexual displays commonly involve multiple elements that may differ greatly among closely related species, but there have been few studies of the causes of this divergence. Male spotted bowerbirds, Chlamydera maculata, have unusually intense, aggressive courtship displays and highly divergent bowers. Male and female courtship positions differ from related species in that males court females separated by a modified see-through bower wall. Here we experimentally tested hypotheses that could explain the unique features of spotted bowerbird display relative to other Chlamydera species. Our results support the threat reduction hypothesis, which suggests that accessory traits evolve because they mitigate the threat associated with intense, aggressive male displays that are most effective in causing females to become sexually receptive. In spotted bowerbird males, the highly modified and unique see-through bower walls allow females to view intense displays while reducing threat that would otherwise be associated with these same displays. We found that (1) females preferred males with the most intense displays, (2) males and females at experimentally manipulated bowers consistently moved to courtship positions so that the standing bower wall separated them and (3) males reduced display intensity when not separated from the female by an intact bower wall. Comparisons with other Chlamydera species suggest that bower architecture coevolves with other display elements to allow maximally effective male displays. Such coordinated adaptive changes of display elements suggest an alternative to the runaway divergence of arbitrary traits as the cause of rapid divergence in multifaceted male display traits among closely related species. Copyright 1998 The Association for the Study of Animal Behaviour.