FLUCTUATING ASYMMETRY AND LONG TAILS - THE MECHANICAL EFFECTS OF ASYMMETRY MAY ACT TO ENFORCE HONEST ADVERTISEMENT

It has been suggested that low-quality individuals will be more suscep tible to fluctuating asymmetries than high-quality individuals. This l eads to high-quality individuals, which produce large ornaments, also producing symmetrical ones, and likewise, low-quality individuals prod uce small asymmetrical ones. Negative relations between ornament size and asymmetry have been demonstrated. However, these results are also consistent with the view that individuals producing larger ornaments a re constrained to more symmetrical development. This will be true when ever possession of an ornament imposes mechanical or aerodynamic costs on its bearer. In this paper I suggest a new hypothesis which aims to explain the observed patterns of asymmetry in such ornaments. In any signalling system there must be constraints on cheating to prevent low -quality individuals from advertising to the same level as high-qualit y individuals, as it will be in the interests of low-quality individua ls to cheat. Therefore, high-quality individuals should seek signallin g dimensions in which cheating is not profitable. It has been shown th at in ornaments such as elongated tails there are aerodynamic and mech anical reasons to expect the cost of a given amount of asymmetry to in crease with tail length. This disproportionate increase in costs induc ed by asymmetry might act as a barrier against the development of larg e traits by low-quality individuals, because these individuals will pr oduce more asymmetrical ornaments. This will result in low-quality ind ividuals developing short, asymmetrical tails, and high-quality indivi duals long, symmetrical ones. This interpretation implies that asymmet ry is not a signal in itself, but a part of the evolutionary design of the signalling system which constrains cheating. Receivers can still obtain information on individual quality from the degree of asymmetry, but only if trait size is also assessed, because the asymmetry of an ornament is not independent of the size of the trait. Predictions whic h arise from this hypothesis regarding patterns of asymmetry in variou s types of ornament are discussed.