Female ornamentation, mate choice and sexual selection

Abstract

The recent TREE review of female ornamentation by Amundsen1xWhy are female birds ornamented?. Amundsen, T. Trends Ecol. Evol. 2000; 15: 149–155Abstract | Full Text | Full Text PDF | PubMed | Scopus (347)See all References1 admirably covered diverse hypotheses concerning correlative expression of male traits in females and direct selection of conspicuous traits in females. However, one important hypothesis, based on female choice2xSee all References2, was not mentioned. In addition, the male-choice hypothesis assumed that sexual selection can operate on females exactly as it does on males, without specifying the limited conditions under which this would be true.Amundsen is not alone in ignoring the Trivers’ hypothesis that female choice can generate sexual dimorphism with expression of the male character in females2.xSee all References, 3.xAsymmetry in the evolution of female mating preferences. Seger, J. and Trivers, R. Nature. 1986; 319: 771–773Crossref | Scopus (20)See all References. This hypothesis identifies asymmetry in female choice: females that prefer males with characters beneficial to their daughters, even if they are detrimental to their sons, will prevail over females that prefer males with characters beneficial to their sons at the expense of their daughters. Female choice for such characters might well drive elaboration of those characters in males, and the elaboration might be increased further if the characters subsequently become used for male–male competition. To my knowledge, this hypothesis has never been mentioned in the extensive sexual selection literature, except once as a reference without emphasis4xSee all References4. The hypothesis directly pertains to female ornamentation because it predicts higher fitness in females with greater expression of the male secondary sexual character, exactly the opposite of the nonadaptive correlated-response hypothesis5xSexual dimorphism, sexual selection, and adaptation in polygenic characters. Lande, R. Evolution. 1980; 34: 292–305CrossrefSee all References5 covered in the review.Second, I think the assertion in the review, that male choice for female ornamentation involves the same sexual selection process as female choice for male secondary sexual characteristics, is potentially misleading. The process can certainly be the same for some forms of direct female–female competition. However, there is a fundamental difference between male choice and female choice – male choice can be based entirely on sexual selection in males for the highest quality females, whereas female choice must lead to sexual selection in males.How can male choice for ornamentation in females cause sexual selection in females for that ornamentation? I see two ways. First, females with lesser expression might be denied the opportunity to reproduce because males avoid them. Second, male quality might vary and females with greater expression might be more likely to attract males of higher quality, with consequences for reproductive success and survival, as documented for barn swallows (Hirudo rustica, reproductive success only)6xSexual selection in the barn swallow (Hirundo rustica). III. Female tail ornaments. Moller, A.P. Evolution. 1993; 47: 417–430CrossrefSee all References6. Each of these conditions results in biased reproductive success among females because of the males they are able to attract, or be accepted by, with their ornamentation. Females are indeed competing with each other for the attention of males even if this competition does not involve contest. However, if ornamentation in females is an indicator of female quality, and males prefer females with greater ornamentation, then the only sexual selection is in males competing for the best mates, unless one or both conditions apply. These conditions were not explicitly developed in the review, but pertain to many species. Imagine the conditions under which a territorial male, fixed in space, would be favored to reject females until one with suitable ornamentation appeared.