New slants on ornament asymmetry

Abstract

Elaborate sexual ornaments are found in many species, and some of them have been shown to have important functional roles. What limits the development of such ornaments? The fluctuating asymmetry hypothesis proposes that high-quality individuals will produce large symmetrical ornaments, the converse being true for low-quality individuals. Negative relations between ornament size and asymmetry have been demonstrated and have been taken as supporting evidence for this hypothesis. Asymmetry in the tail ornament of male scarlet-tufted malachite sunbirds (Nectarinia johnstoni) was investigated. During this process we discovered various problems in the theory and methods of analysis; we illustrate some of these using the sunbird data. We show that, because there are various ways of presenting the data, it is impossible to fail to obtain relations between asymmetry and some measures of trait size. This illustrates the problem that there is no clear null hypothesis for these analyses, and we discuss various possibilities. Fluctuating asymmetry theory cannot necessarily be applied to traits which are composed of more than one unit, and we discuss why similar trends have been found in these multi-component traits and single-unit traits. It is possible that aerodynamic and mechanical constraints may force symmetry on larger ornaments; this may also explain why similar trends have sometimes been found in the ornaments of the two sexes. We conclude that this hypothesis may not provide an adequate explanation for the honesty of ornamentation.