Abstract
Why are there so few small secondary sexual characters? Theoretical models predict that sexual selection should lead to reduction as often as exaggeration, and yet we mainly associate secondary sexual ornaments with exaggerated features such as the peacock's tail. We review the literature on mate choice experiments for evidence of reduced sexual traits. This shows that reduced ornamentation is effectively impossible in certain types of ornamental traits (behavioral, pheromonal, or color-based traits, and morphological ornaments for which the natural selection optimum is no trait), but that there are many examples of morphological traits that would permit reduction. Yet small sexual traits are very rarely seen. We analyze a simple mathematical model of Fisher's runaway process (the null model for sexual selection). Our analysis shows that the imbalance cannot be wholly explained by larger ornaments being less costly than smaller ornaments, nor by preferences for larger ornaments being less costly than preferences for smaller ornaments. Instead, we suggest that asymmetry in signaling efficacy limits runaway to trait exaggeration.
Highlights
How the flamboyant ornamental traits used to attract mates in various species evolve is a question that has been debated since Darwin’s time (Cronin 1991)
We extend a classic model of Fisherian runaway (Pomiankowski et al 1991; Pomiankowski and Iwasa 1993) to explore whether asymmetry in the cost of preference, the cost of the male ornament, or signaling efficiency could explain why reduced traits are less likely to evolve
It is a general observation that sexual traits are exaggerated rather than reduced, even though models of sexual selection predict that deviations from the natural selection optimum should occur in both directions (Andersson 1994; Mead and Arnold 2004; Kuijper et al 2012)
Summary
The terms Gt and G p are the additive genetic variances for male ornament size and female preference, respectively, and are assumed to be constant. The terms βt and βp are the selection gradients on male ornament size and female preference, respectively. The first term describes the effect of sexual selection on the male This is a product of the average female preference p , the difference between the male’s ornament and the average ornament (t − t), and the signaling efficacy of the male’s ornament, a[t]. The second term in curly braces on the right-hand side of equation (2a) describes the natural selection cost of bearing the ornament. It is again a function of t, c[t]. In the curly braces on the right-hand side is the function b[p] This is the cost to a female of having a preference p.
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