The handicap process favors exaggerated, rather than reduced, sexual ornaments.
Why are traits that function as secondary sexual ornaments generally exaggerated in size compared to the naturally selected optimum, and not reduced? Because 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 analyze 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 toward exaggeration can be best explained if either the signaling efficacy or the condition dependence of a trait increases with size. Under these circumstances, evolution always leads to more extreme exaggeration than reduction: although 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.
CoMPLEX, ,  University College London, London WC1E 6BT, United Kingdom; Institute of Integrative Biology (IBZ), ETH Zürich, Zürich 8092, Switzerland; The Galton Laboratory, ,  Department of Genetics, Environment, and Evolution, , University College London, London WC1E 6BT, United Kingdom. samuel.tazzyman@env.ethz.ch.
Samuel J
SJ
Tazzyman
Yoh
Y
Iwasa
Andrew
A
Pomiankowski
D000818:Animals; D005075:Biological Evolution; D008297:Male; D052586:Mating Preference, Animal; D008957:Models, Genetic; D012641:Selection, Genetic / Q000235:genetics*; D012727:Sex Characteristics; D017603:Signal Detection, Psychological
Handicap process; mate choice; mate preference; sexual dimorphism; sexual selection; signaling/courtship