Fluctuating asymmetry and long tails: The mechanical effects of asymmetry may act to enforce honest advertisement

Abstract

It has been suggested that low-quality individuals will be more susceptible to fluctuating asymmetries than high-quality individuals. This leads to high-quality individuals, which produce large ornaments, also producing symmetrical ones, and likewise, low-quality individuals produce 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 are constrained to more symmetrical development. This will be true whenever 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-quality individuals, as it will be in the interests of low-quality individuals to cheat. Therefore, high-quality individuals should seek signalling dimensions in which cheating is not profitable. It has been shown that in ornaments such as elongated tails there are aerodynamic and mechanical reasons to expect the cost of a given amount of asymmetry to increase with tail length. This disproportionate increase in costs induced by asymmetry might act as a barrier against the development of large traits by low-quality individuals, because these individuals will produce more asymmetrical ornaments. This will result in low-quality individuals developing short, asymmetrical tails, and high-quality individuals long, symmetrical ones. This interpretation implies that asymmetry 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 which arise from this hypothesis regarding patterns of asymmetry in various types of ornament are discussed.