What maintains signal honesty in animal colour displays used in mate choice?

Abstract

Many of the colour displays of animals are proposed to have evolved in response to female mate choice for honest signals of quality, but such honest signalling requires mechanisms to prevent cheating. The most widely accepted and cited mechanisms for ensuring signal honesty are based on the costly signalling hypothesis, which posits that costs associated with ornamentation prevent low-quality males from being highly ornamented. Alternatively, by the index hypothesis, honesty can be achieved via cost-free mechanisms if ornament production is causally linked to core physiological pathways. In this essay, we review how a costly signalling framework has shaped empirical research in mate choice for colourful male ornaments and emphasize that alternative interpretations are plausible under an index signalling framework. We discuss the challenges in both empirically testing and distinguishing between the two hypotheses, noting that they need not be mutually exclusive. Finally, we advocate for a comprehensive approach to studies of colour signals that includes the explicit consideration of cost-free mechanisms for honesty.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Do carotenoid‐based ornaments entail resource trade‐offs? An evaluation of theory and data

“…Correlations between carotenoid levels, coloration and physiological performance can only yield so much information without a better understanding of underlying mechanisms. The goal of this study was not to review and evaluate alternative hypotheses for the resource trade‐off hypothesis (Hill, ; Weaver, Koch, & Hill, ), but it is important to keep alternatives in mind when interpreting results that are non‐significant or do not match the predictions of carotenoid resource trade‐offs. Tackling old questions with new approaches—be they new genetic techniques or new perspectives on the role of carotenoids in physiological function—will be key to substantiating or questioning the carotenoid resource trade‐off hypothesis across systems.…”

“…To summarize, widespread uncertainty exists in the carotenoid literature about whether we define high-quality individuals as those that are (1) currently healthy, (2) currently possessing large quantities of carotenoids, some combination of both or (3) inherently superior in one or more physiological metrics. The result of this uncertainty is that a wide range of observations can be used to support resource trade-off hypothesis even if alternative explanations exist, such as index hypotheses that propose trait quality as an indicator of internal conditions rather than as a direct product of costly tradeoffs (Biernaskie, Grafen, & Perry, 2014;Hill, 2011;Weaver, Koch & Hill, 2017). While it is not erroneous to discuss how results fit (or fail to fit) particular frameworks, the danger is in suggesting most every observation to be supportive of a favoured hypothesis.…”

“…As technology continues to advance and allow for manipulation of target genes within vertebrate systems, studying knock-outs in carotenoid absorption (Toomey et al, 2017) or transformation (Lopes et al, 2016;Mundy et al, 2016) genes will yield transformative information regarding the costs or benefits of specific types of carotenoids and the pro- Correlations between carotenoid levels, coloration and physiological performance can only yield so much information without a better understanding of underlying mechanisms. The goal of this study was not to review and evaluate alternative hypotheses for the resource trade-off hypothesis (Hill, 2011;Weaver, Koch, & Hill, 2017), but it is important to keep alternatives in mind when interpreting results that are non-significant or do not match the predictions of carotenoid resource trade-offs. Tackling old questions with new approaches-be they new genetic techniques or new perspectives on the role of carotenoids in physiological function-will be key to substantiating or questioning the carotenoid resource trade-off hypothesis across systems.…”

More yellow more toxic? Sex rather than alkaloid content is correlated with yellow coloration in the fire salamander

“…In line with the previous study of Balogov a & Uhrin (2015), we found that male fire salamanders display a higher proportion of yellow than do females (Table 1a). Sexual dichromatism is widespread across the animal kingdom, and is generally interpreted as a signal involved in mate attraction (Finkbeiner, Briscoe & Reed, 2014;Weaver, Koch & Hill, 2017). Males displaying highly conspicuous patterns are more effective in foraging, have lower parasite loads, and occupy higher social ranks (Godin & Dugatkin, 1996).…”

Developing a transcriptomic framework for testing testosterone-mediated handicap hypotheses

“…Similarly, if testosterone influences an individual’s susceptibility or exposure to oxidative stress, then we predict that testosterone treatment will cause a decrease in the expression of genes with annotated functions in antioxidant protection and/or an increase in genes that are expressed in response to oxidative stress. An important caveat is that no support for either hypothesis does not exclude the possibility that these pathways enforce the honestly of sexually selected traits independent of testosterone’s effects, as has been suggested before (Metcalfe and Alonso-Alvarez, 2010; Weaver et al, 2017).…”