Tazzyman, SJ; Iwasa, Y; Pomiankowski, A; (2014) THE HANDICAP PROCESS FAVOURS EXAGGERATED, RATHER THAN REDUCED, SEXUAL ORNAMENTS. Evolution , 68 (9) pp. 2534-2549. 10.1111/evo.12450.

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Abstract

Why are traits that function as secondary sexual ornaments generally exaggerated in size compared to the naturally selected optimum, and not reduced? Since they deviate from the naturally selected optimum, traits that are reduced in size will handicap their bearer, and could thus provide an honest signal of quality to a potential mate. Thus if secondary sexual ornaments evolve via the handicap process, current theory suggests that reduced ornamentation should be as frequent as exaggerated ornamentation, but this is not the case. To try to explain this discrepancy, we analyse a simple model of the handicap process. Our analysis shows that asymmetries in costs of preference or ornament with regard to exaggeration and reduction cannot fully explain the imbalance. Rather, the bias towards exaggeration can be best explained if either the signalling efficacy or the condition dependence of a trait increases with size. Under these circumstances, evolution always leads to more extreme exaggeration than reduction: though the two should occur just as frequently, exaggerated secondary sexual ornaments are likely to be further removed from the naturally selected optimum than reduced ornaments. This article is protected by copyright. All rights reserved.

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