The effect of against-background contrast on female preferences for a polymorphic colour sexual signal

The role of boundary length and adjacent patch contrast in guppy mate choice

The presence of various combinations of adjacent colors within polymorphic species’ color pattern could have a major impact on mate choice. We studied the role of pattern geometry in predicting mate choice in guppies using boundary strength analysis (BSA). BSA estimates the visual contrast intensity between two adjacent color patches (ΔS) weighted by the lengths of the boundaries between these adjacent color patches. We measured both the chromatic (hue and saturation) and achromatic (luminance) ΔS for each pair of adjacent patches. For each male’s color pattern, we measured BSA as both mean (mΔS) and coefficient of variation (cvΔS) of all ΔS weighted by their corresponding boundary lengths. We also determined if specific color patch boundaries had an impact on female preferences and whether these predicted overall male contrast (mΔS). We found that males with a higher mΔS were more attractive to females and that six boundaries containing either fuzzy black or black as one of the pair colors significantly affected female preferences, indicating that 1) females favored highly conspicuous males and 2) melanin-based patches could be used as a signal amplifier, not only for orange but for other colors.

Designing mate choice experiments

The important role that mate choice plays in the lives of animals is matched by the large and active research field dedicated to studying it. Researchers work on a wide range of species and behaviours, and so the experimental approaches used to measure animal mate choice are highly variable. Importantly, these differences are often not purely cosmetic; they can strongly influence the measurement of choice, for example by varying the behaviour of animals during tests, the aspects of choice actually measured, and statistical power. Consideration of these effects are important when comparing results among studies using different types of test, or when using laboratory results to predict animal behaviour in  natural populations. However, these effects have been underappreciated by the mate choice literature to date. I focus on five key experimental considerations that may influence choice: (i) should mating be allowed to occur, or should a proxy behavioural measure of preference be used instead? (ii) Should subjects be given a choice of options? (iii) Should each subject be tested more than once, either with the same or different stimuli? (iv) When given a choice, how many options should the subject choose between? (v) What form should the experimental stimuli take? I discuss the practical advantages and disadvantages of common experimental approaches, and how they may influence the measurement of mate choice in systematic ways. Different approaches often influence the ability of animals to perceive and compare stimuli presented during tests, or the perceived costs and benefits of being choosy. Given that variation in the design of mate choice experiments is likely unavoidable, I emphasise that there is no single 'correct' approach to measuring choice across species, although ecological relevance is crucial if the aim is to understand how choice acts in natural populations. I also highlight the need for quantitative estimates of the sizes of potentially important effects, without which we cannot make informed design decisions.

Social runaway: Fisherian elaboration (or reduction) of socially selected traits via indirect genetic effects

Our understanding of the evolutionary stability of socially selected traits is dominated by sexual selection models originating with R. A. Fisher, in which genetic covariance arising through assortative mating can trigger exponential, runaway trait evolution. To examine whether nonreproductive, socially selected traits experience similar dynamics-social runaway-when assortative mating does not automatically generate a covariance, we modeled the evolution of socially selected badge and donation phenotypes incorporating indirect genetic effects (IGEs) arising from the social environment. We establish a social runaway criterion based on the interaction coefficient, ψ, which describes social effects on badge and donation traits. Our models make several predictions. (1) IGEs can drive the original evolution of altruistic interactions that depend on receiver badges. (2) Donation traits are more likely to be susceptible to IGEs than badge traits. (3) Runaway dynamics in nonsexual, social contexts can occur in the absence of a genetic covariance. (4) Traits elaborated by social runaway are more likely to involve reciprocal, but nonsymmetrical, social plasticity. Models incorporating plasticity to the social environment via IGEs illustrate conditions favoring social runaway, describe a mechanism underlying the origins of costly traits, such as altruism, and support a fundamental role for phenotypic plasticity in rapid social evolution.