What maintains signal honesty in animal colour displays used in mate choice? rstb.royalsocietypublishing.org Ryan J. Weaver, Rebecca E. Koch and Geoffrey E. Hill

Department of Biological Sciences, Auburn University, Auburn, AL 36849, USA RJW, 0000-0002-6160-4735 Review Many of the colour displays of animals are proposed to have evolved in Cite this article: Weaver RJ, Koch RE, Hill GE. response to female mate choice for honest signals of quality, but such 2017 What maintains signal honesty in animal honest signalling requires mechanisms to prevent . The most widely accepted and cited mechanisms for ensuring signal honesty are colour displays used in mate choice? Phil. based on the costly signalling hypothesis, which posits that costs associated Trans. R. Soc. B 372: 20160343. with ornamentation prevent low-quality males from being highly ornamen- http://dx.doi.org/10.1098/rstb.2016.0343 ted. Alternatively, by the index hypothesis, honesty can be achieved via cost-free mechanisms if ornament production is causally linked to core Accepted: 29 January 2017 physiological pathways. In this essay, we review how a costly signalling fra- mework has shaped empirical research in mate choice for colourful male ornaments and emphasize that alternative interpretations are plausible One contribution of 19 to a theme issue under an index signalling framework. We discuss the challenges in both ‘: production, perception, empirically testing and distinguishing between the two hypotheses, noting and application’. that they need not be mutually exclusive. Finally, we advocate for a compre- hensive approach to studies of colour signals that includes the explicit consideration of cost-free mechanisms for honesty. Subject Areas: This article is part of the themed issue ‘Animal coloration: production, , behaviour, theoretical perception, function and application’.

Keywords: handicap, costly signal, resource trade-off, index signal, honest signal, cost-free signal 1. Introduction Female mate choice is hypothesized to be an important form of selection on Author for correspondence: bold and brilliant colour displays of male animals [1–3]. In particular, it is widely proposed that choosing females assess prospective males by their col- Ryan J. Weaver ourful ornaments because colour displays can be honest signals of individual e-mail: [email protected] quality [4–6]. For signals of quality to be effective and evolutionarily stable, they must be reliable such that there is a consistent correlation between orna- ment expression and some unobservable male quality that is of interest to the female [7–10]. Thus, it has been proposed that ornaments should evolve and be the object of female assessment only if they remain honest signals of the true condition or quality of a male (i.e. condition-dependent ornaments) [8]. The logical challenge to the concept of honest signalling is cheating. During mate choice, the interests of a choosing female and a displaying male are fun- damentally different [11,12]. It is to each male’s advantage to present a signal that maximizes his chances of mating—even if it is a dishonest reflection of his quality—while it is to each female’s advantage to sort prospective mates by aspects of quality that will affect her reproductive success [13,14]. A wealth of empirical data from diverse taxa shows that coloration can be an honest signal of male quality [15–17], so the fundamental question is: what maintains the honesty of colourful ornaments in mate choice systems [9,18,19]? In this essay, we review two hypothesized mechanisms for the enforcement of the honesty of colour signals assessed by females during mate choice: (i) the costly signalling hypothesis [10] and (ii) the index hypothesis [20,21]. The former is the foundation for the most widely accepted and cited hypotheses for how colour signals remain honest, proposing that costs entailed in the pro- duction or maintenance of colour displays ensure the honesty of the signals. On the other hand, the index hypothesis, which proposes that signal honesty is maintained by cost-free mechanisms that arise from the inexorable links

& 2017 The Author(s) Published by the Royal Society. All rights reserved. between male condition and signal expression, is an often- full ornamentation [8]. This validation bolstered Zahavi’s 2 overlooked but important alternative as the basis for honest as the paradigm for the evolution and rstb.royalsocietypublishing.org signalling from coloration. We first briefly review the frame- persistence of honest animal signals in the wild. work of the costly signaling and index hypotheses and Many recent critiques of the handicap principle argue that consider their application to colour signals. We assess how the realized costs of signalling—the actual decrease in including the perspective from the index hypothesis provides incurred by giving the signal—required by the handicap important alternative explanations for experimental results principle are not wholly sufficient, or required, to prevent from two empirical studies of coloured ornaments. Finally, cheating. [19,41–45]. The key points of these critiques are we discuss how future studies can better tease apart whether that a high signal cost is not the only mechanism that main- honest colour signals used in mate choice are shaped by tains honesty and that cheap or cost-free signals (i.e. signals costly or cost-free mechanisms—or a combination of both. imposing little or no reduction in fitness) can be honest. For

example, when the potential costs of cheating are sufficiently B Soc. R. Trans. Phil. high—such as risk of punishment through harassment of 2. Mechanisms of honest signalling cheats—the realized costs of honest signals could be zero [41,45,46]; in other words, a signal that is not difficult to pro- (a) The costly signaling hypothesis duce or maintain can still be honest if cheats run the risk of The costly signalling hypothesis requires that signals be paying high costs through social mediation or other means. costly to be reliable [10,22,23]. In the realm of animal color- As long as high-quality individuals and not low-quality indi- 372 ation, costs come in a variety of forms such as increased viduals gain a net fitness benefit from giving a high-quality 20160343 : risk (1a, table 1) or resource allocation towards signal, costs paid for giving the signal are arbitrary in main- ornamentation instead of body maintenance (1b, table 1). taining signal honesty [44–46]. Despite the mounting However, for signalling to be evolutionarily stable with evidence that handicap-like costs are perhaps unlikely to little cheating, these costs associated with ornamentation maintain honesty in most signalling systems [41,44,47–49], must ultimately result in a decrease in fitness [8–10,20,30]. research on animal coloration frequently cites costly mechan- Moreover, a key premise of costly is that isms, like those of the handicap principle, for maintaining ornamentation will impose a higher cost for low-quality signal honesty [26,50–52]. males than high-quality males [8,31–33]. Consequently, sig- While Zahavi’s handicap principle of honest signalling nals remain honest because males of low quality simply originally focused on maintenance costs and increased mor- cannot recover sufficient fitness from dishonest signalling to tality as the means to ensure signal honesty [10], research compensate for the high cost of ornamentation. For example, on honest signalling via colour displays in recent decades bold and conspicuous coloration may slightly increase the has largely extended the concept of costly signals to include predation risk for a high-quality male with good stamina production costs and especially resource trade-offs (discussed and agility, but the same coloration may greatly increase the in detail in §3) as the potential mechanisms for maintaining predation risk for a low-quality male that is less able to honesty in colour signals [50,53]. The costly signalling evade predators [34]. Similarly, high- and low-quality males hypothesis is an overarching concept that extends the handi- may suffer differential effects of allocating resources to orna- cap principle (table 1) to propose that the costs associated ment production and away from physiological functions that with ornament production or maintenance are key to signal contribute to fitness [35,36] (1b, table 1). As with predation honesty. However, others have shown that alternative risk, allocating critical resources to ornamentation rather cheap or cost-free mechanisms can also maintain signal hon- than body maintenance may cause low-quality males to esty [4,21,41,43,45,46,54] but have largely been ignored in the suffer higher fitness costs than high-quality males animal coloration field. [24,37,38]. The unifying theme of costly signalling is that the fitness costs of ornamentation maintain signal honesty because only high-quality males gain a net fitness benefit from producing the colour displays most preferred by (b) The index hypothesis females [8]. The second major explanation for honest signalling in the Zahavi [10] proposed the first conceptualization of the context of mate choice is the index hypothesis, which pro- costly signalling hypothesis in biology with his handicap poses that coloration can serve as an uncheatable signal of principle, which states that ornaments pose a survival cost individual quality if it is inexorably linked to individual con- on the signaler such that only high-quality males can with- dition [20,21,27]. By this idea, signal honesty is not stand the handicap of being fully ornamented. Bearing a maintained by costs but is instead ensured because signal large ornament is a strategic cost—a cost in excess of what production relies directly on the function of internal pro- is needed to unambiguously convey information [9]—and cesses that cannot be faked. Proponents of the index thus this form of honest signalling has been termed wasteful hypothesis recognize that all behaviours and physiological signalling [39]. Zahavi’s hypothesis proposed survival costs processes inevitably require energy, resources and/or time as an intuitive mechanism for maintaining signal reliability, that could potentially have been allocated towards other but his verbal model was met with much early criticism functions. However, the index hypothesis proposes that the (reviewed in [9]). The plausibility of the handicap principle requirements of producing or maintaining the signal are was not validated until the 1980s through mathematical mod- not sufficient to impose handicaps or other costly constraints elling by Enquist [40], Pomiankowski [33] and especially described by the costly signaling hypothesis. Under the index Grafen [8]. Grafen’s game-theoretic approach showed that hypothesis, signals are honest because their expression is fun- strategic costs could maintain honesty if lower-quality indi- damentally linked to core physiological processes—not viduals paid more than high-quality individuals to achieve because of costs associated with that signal. Table 1. Key hypotheses proposed to maintain honesty from animal colour signals. 3 rstb.royalsocietypublishing.org overarching key principle underlying concepts description reference(s)

1. costly signalling (a) Zahavi handicap principle signals are honest because they act as handicaps in contexts other [7,10] principle than mating that are too costly for low-quality males. (b) resource trade-off the production of ornaments requires the irreversible allocation of [24] hypothesis resources that could otherwise serve a beneficial function; as a result, only individuals in good health and otherwise good condition can ‘afford’ to produce high-quality ornaments. B Soc. R. Trans. Phil. (c) immunocompetence testosterone both suppresses immune function and enhances [25] handicap hypothesis ornament production, so only healthy individuals can withstand the high testosterone required for good ornamentation. (d) oxidative handicap testosterone mediates a trade-off between ornament expression and [26] 372 hypothesis internal system function by posing an oxidative challenge, such 20160343 : that only good-condition individuals can withstand the increased oxidative burden of high testosterone and still produce high- quality ornaments. 2. index (a) good genes (the full expression of colourful ornaments is an ‘unbluffable display’ [4] hypothesis Hamilton-Zuk hypothesis) dependent on male condition that signals genetic parasite resistance. (b) shared pathway honest signalling of male condition from colourful ornaments arises [27] hypothesis because of physiological links between vital cellular processes and coloration. (c) mitochondrial function mitochondrial function is the key shared pathway that integrates [28,29] hypothesis immunocompetence, oxidative stress maintenance and other aspects of individual condition with ornament expression.

The concept of index traits has long been accepted as a colourful ornaments can arise when the underlying physio- cost-free mechanism to ensure honesty of vocal and behav- logical pathways that produce coloration are dependent on ioural displays [20,40,55,56]. For example, in red , the the same core cellular processes that are the key determinants pitch of roaring vocalizations is causally linked to body size of individual condition (2b, table 1). By this ‘shared pathway’ (larger males have a longer vocal apparatus that produces a concept, colour displays can be fully executed only if the bio- deeper pitch) and thus fighting ability (larger males are chemical and cellular processes of an individual are also more successful in intrasexual competition) [57]. In this functioning well. System functionality, in turn, depends on example, roaring is an honest signal of fighting ability the combination of genetic, environmental and gene by because it is an index of body size that cannot be faked [9]. environment interactions that determine an individual’s con- However, the concept of index traits has rarely been invoked dition [27,28]. Ornament expression matches the condition of to explain the signal honesty of animal coloration [18,58]. the male because maximum expression is trivial if the indi- Hamilton & Zuk’s essay on plumage colour as an vidual is functioning well, but impossible if the individual honest signal of parasite resistance was the first time a is functioning poorly. Mitochondrial function in particular cost-free mechanism was proposed to maintain honesty in has been proposed to be a key physiological process that animal colour signals (2a, table 1) [4]. They concluded links ornamentation to overall condition (2c, table 1) [29]. that bright coloration acts as an honest signal of heri- For example, several recent investigations have shown table parasite resistance. While the mechanisms of honestly that colourful ornament expression can be honest signals of signalling genetic parasite resistance through feather color- condition because the themselves are sensitive to ation were not explicitly discussed, Hamilton & Zuk oxidative stress from free radicals; such high oxidative rejected the idea that females would select for a handicap. stress levels would reduce the colourfulness of the pigments Instead, they proposed that females would select for a ‘dem- [59,60]. Oxidative stress arises when production of free onstration of health that cannot be bluffed’ (note 20 in [4]), radicals exceeds the capacity of antioxidant systems [61,62]. which is precisely what we and others [20] argue to be an Because core system processes, including the immune index signal. response and mitochondrial respiration, can be major contri- More recently, Hill [27] revived the idea that a cost-free butors to oxidative stress when they are dysfunctional [6], mechanism could ensure honesty from animal colour signals the index hypothesis predicts that colourful ornaments by proposing that condition-dependent expression of can honestly signal condition because of these inherent connections between oxidative stress, system functionality for most animals with -coloured signals and 4 and ornamentation [28,29]. whether they are relevant to animal antioxidant and immune rstb.royalsocietypublishing.org systems remains largely unresolved [70,76–78]. This resource trade-off hypothesis has been applied less 3. Assessing studies of honest signaling via extensively to melanin pigmentation, but the substrates needed for production of melanic ornaments are also ulti- coloration mately sourced from an animal’s diet and therefore may be While the costly signalling and index hypotheses are both limited resources requiring differential allocation among theoretically viable and not mutually exclusive, research on functions. For example, the melanin pigments that produce honest signalling via coloration has been dominated by an black, brown and rufous coloration are synthesized from an assumption of the necessity of costs. In this section, we briefly essential amino acid that is also used for myriad physiologi- review how signalling costs have been specifically proposed cal functions, including hormonal cascades and immune B Soc. R. Trans. Phil. to maintain signal honesty in male coloration, focusing on responses [79]. An attempt to manipulate the access to key the prominent concept of resource trade-offs (1b, table 1). precursor amino acids in the diets of house sparrows We then introduce how a consideration of cost-free index (Passer domesticus), however, failed to affect black melanin mechanisms can both refocus interpretation of experimental coloration [80]. In addition, melanin synthesis requires trace and correlational studies and offer new opportunities for metals that have been proposed to be limited in the diet, future study. which could maintain honesty if there is a cost to allocating 372

The resource trade-off hypothesis (1b, table 1) posits that those metals to melanin synthesis instead of to internal main- 20160343 : honest signalling from coloration is due to differential allo- tenance functions [64]. However, the role of these potential cation of the substrates used to produce ornamental resource limitations and physiological trade-offs in pro- coloration, and it is among the most commonly invoked duction of melanin ornamentation remains contentious mechanisms to maintain the honesty of colour signals [81,82]. under the costly signalling hypothesis [25,63–65]. Pigments, In contrast to pigmentary coloration, much less attention the substrates used to synthesize pigments, or the com- has been focused on honesty-maintaining mechanisms for ponents needed for integumentary microstructures structural coloration, though these ornaments may also func- potentially have uses within the body other than colour pro- tion as honest signals of quality. Structural coloration duction, so individuals might face a direct allocation trade-off requires the precise arrangement of integumentary microstruc- [36]. By this idea, only high-quality males can afford the cost tures, and the integrity of microstructures has been shown to of allocating resources to produce a good colour signal and be an honest signal of male body condition in some systems still meet the demands of body maintenance [66]. Thus, this [83–85]. The costly signaling hypothesis has also been adopted resource trade-off conceptualization of the costly signaling to explain honesty from structurally coloured ornaments with hypothesis has two key assumptions: (i) the resources proposed costs related to production and precise organization needed for coloration are scarce or difficult to obtain, and of the components that produce coloration [86]. (ii) the same resources needed to produce coloration also While the importance of resource trade-offs to signal serve important physiological functions. honesty is widely accepted, definitive testing of how differ- Costly signalling through differential resource allocation is ential resource allocation shapes condition-dependent perhaps most often applied to carotenoid-based ornaments coloration is challenging. It is typically extremely difficult because carotenoid pigments cannot be synthesized de novo to know the quantity of pigments or other resources avail- within the animal body and must be acquired from the diet able to the individual producing the ornament, how [67]. Whether or not are a limited resource in the resources are allocated within the body, and diets of most animals has been debated for 25 years [68–70], whether patterns of allocation change in the manners pre- with still surprisingly few direct measurements of the caroten- dicted by trade-off hypotheses. For example, an individual oid contents of diets of wild animals and carotenoid may vary the amount of pigments it has available for requirements of ornaments [71–73]. Moreover, carotenoids ornamental use at any of the various stages of the pigmen- are thought to be relevant, powerful antioxidants and impor- tation process, including absorption from the diet (in the tant boosters of immune response such that allocating case of strictly dietary pigments, like carotenoids), transpor- carotenoids towards coloration comes at the cost of those pig- tation to target tissues, metabolic transformation (if ments not being available for these other functions [63,74,75]. necessary), as well as deposition in the integument [87]. Carotenoid coloration may therefore be an honest signal of oxi- The complexities of allocation make the interpretations of dative stress or function because allocating whole-animal experimentation problematic because non- potentially scarce and important carotenoid resources to color- terminal investigations generally rely on the quantities of ation is thought to be too costly for males of poor quality. pigments in circulation as a proxy for the internal pigment Related hypotheses propose that testosterone may be a critical stores. Such methods, however, do not allow researchers to mediator of the relationship between ornament colour and distinguish between re-allocation versus disruption of individual quality by affecting both carotenoid allocation uptake, transport or metabolism. This problem of ‘blackbox- and immune function (1c, table 1) or oxidative stress (1d, ing’ the physiological and biochemical processes that table 1); in this case, only high-quality individuals could determine pigment usage within the body of an animal pre- afford to pay potential costs of high testosterone—costs that sents a major challenge to understanding the mechanism could potentially be offset by the beneficial function of caro- that gives rise to honest signalling. tenoids. Although the concept of the resource trade-off is For example, a foundational experiment performed by widely cited as the mechanism that maintains honesty from Faivre et al. [88] is often cited as compelling evidence for caro- carotenoid signals, whether carotenoids are limiting resources tenoid pigment allocation away from ornamental coloration and towards immune and antioxidant function during framework in interpretation of experimental results is not 5 immune activation. In this experiment, male European Black- always warranted because experimental designs often rstb.royalsocietypublishing.org birds (Turdus merula) were injected with a foreign substance cannot rule out alternative explanations. However, just as (sheep red blood cells) to induce an innate immune response it is challenging to clearly demonstrate that resource allo- that was quantified via a blood haemagglutination test; the cation is a cost to ornamentation, it can be equally coloration of each male’s characteristically orange bill was difficult to definitively demonstrate links to biochemical measured before and after injection. Faivre et al. [88] reported pathways that enforce honesty in signalling systems. We that bill coloration became duller after injection in all males propose that future studies work to clarify the mechanisms and that dullness of bill coloration correlated positively of honest signalling by performing targeted experimental with strength of haemagglutination response. These results manipulations that can better isolate the importance of were interpreted as being supportive of the costly signaling costs versus physiological function in determining orna- hypothesis, with carotenoids being allocated away from ment expression. Indeed, we argue that better awareness B Soc. R. Trans. Phil. coloration and towards an immune response. However, with- of the index hypothesis and its potential application to out understanding the specific allocation processes occurring male coloration used in mate choice displays will lead within the blackbirds, the results of Faivre et al. [88] are also to more decisive experimental design and effective consistent with the index hypothesis. Challenged birds may interpretation of results. have exhibited duller ornaments because of a change in Specifically, recent and ongoing advancements in bio- underlying system functionality independent of reallocation medical, molecular and genetic technologies present new 372 of carotenoids for immune defense. For instance, challenged opportunities for testing the roles of resources, energy, cel- 20160343 : blackbirds may have absorbed fewer carotenoid pigments lular processes and other key factors in maintaining from the diet to avoid potential pro-oxidative effects of caro- honesty in coloured ornaments. For example, the primary tenoids [89]. The relationship between dullness of bill and roadblock to definitively demonstrating the role of differen- strength of haemagglutination response is also potentially tial resource allocation in condition-dependent carotenoid consistent with the index hypothesis because a decreased colour expression is the inability to clarify where, when haemagglutination reaction may indicate a bird that was and in what quantities carotenoids are moved through the less challenged by an antigen and therefore may have bodies of animals. Relatively unexplored techniques, such better functioning core cellular processes that also enable it as designing radiolabels for carotenoid pigments, may be to produce a bright orange bill. By the index hypothesis, key to isolating the actual size of the pool of carotenoid bill coloration is a signal of the core system functionality resources to which an individual has access and how that underlies processes mediating both immunocompetence those resources are allocated under changing conditions. and ornament production [27]. Studies of pigment resource abundance and movement Experiments with melanin pigmentation are also incon- can be paired with the use of new genetic discoveries— clusive with regard to the mechanisms that link such as key genes in the biosynthesis of melanin [91] and ornamentation to condition. For example, the dark melanin the metabolism of red carotenoids [92,93]—to consider patches on the wings of male damselflies are sexually selected how the genetic regulation of pigment uptake and/or trans- traits that have been shown to honestly signal parasite resist- formation may interact with resource use in defining both ance [90]. Importantly, melanin production in this and other signal expression and individual performance. To test the insect species is dependent on a shared pathway with role of allocation or resource trade-offs in maintaining hon- immune defense, the phenoloxidase (PO) pathway. Using esty in colour signalling, we must isolate specific trade-offs, an experimental parasite challenge, Siva-Jothy [90] found their fitness costs to males of differing quality and their role that males with higher parasite burdens after inoculation in colour signalling systems. also had elevated PO levels but lighter, more heterogeneous Testing the index hypothesis is particularly challenging wing patches. The author concluded that melanic wing pig- because the physiological processes and potential shared mentation is an honest signal of male parasite resistance pathways that define individual quality are difficult to isolate. because the cost of diverting resources to melanin production Perhaps the most effective means to test a potential index was too great for poor-condition males. However, alternative signal is to focus on the mechanisms generating that signal conclusions are plausible under the index hypothesis. If we and to consider how such mechanisms may overlap with consider individual condition to comprise the functionality other physiological pathways involved in individual per- of core physiological processes—in this case, the PO formance (such as the immune system) [29]. By performing immune response pathway—then we might expect an indi- a targeted experimental manipulation that alters the function- vidual in poor condition to also exhibit poor functionality ality of the hypothesized controlling mechanism and then of melanin production pathways. Melanin ornaments can measuring the resulting effects on ornament expression, we thus serve as honest signals of condition through cost-free may be able to elucidate whether central pathways drive mechanisms because melanin production is an index of variation in ornament production. immunocompetence through a shared pathway between For example, it was recently proposed that mitochon- melanin pigment production and immune function. drial cellular respiration is the core pathway that links ornamentation to aspects of condition (2c, table 1) [27]. Many aspects of individual condition that are usually 4. Discerning costly trade-offs from cost-free invoked with respect to condition-dependent ornaments, such as immune function and reproductive success, can be indices traced through pathways that intersect at the level of the We have used two empirical examples to demonstrate mitochondrion [28,29,94,95]. This is important because that the acceptance of the costly signaling hypothesis the mitochondrion is both the core energy producer of the animal body and also a critical integrator organelle that 5. Conclusion 6 interprets, transduces and responds to a multitude of rstb.royalsocietypublishing.org internal signals. As such, functionality of an individual’s The diversity of...mechanisms...producing cost-free, minimal-cost mitochondria may be a primary mechanistic basis for the signals, and handicaps at the equilibrium, will reveal itself in its full index hypothesis; mitochondrial processes could be con- shape and beauty when researchers are primed to look for it [41]. sidered the fundamental physiological processes that The entrenchment of the paradigm that signals must be costly determine both ornament and individual quality, thereby to be honest has resulted in a widespread acceptance that maintaining uncheatable signals [28]. Basic understanding allocation trade-offs define colour signals, while experimental of mitochondrial function is progressing rapidly and lead- data are often equivocal and correlational evidence does not ing to novel means to manipulate mitochondrial function rule out alternative explanations. To advance understanding [96–98], providing new means to test the idea that colourful of the mechanisms that maintain signal honesty, we encou- ornaments act as index signals because mitochondrial func- rage scientists to design experiments and collect data that B Soc. R. Trans. Phil. tion potentially mediates both individual condition and can disentangle competing hypotheses and to consider ornamentation. Future research into how ornament quality whether alternative hypotheses may provide valid alternative varies with mitochondrial function as well as with the per- explanations to observations. The costly signaling and the formance of traditional measures of individual condition, index hypotheses are not mutually exclusive; it may prove such as immunocompetence or oxidative stress balance, that both the functionality of core pathways and costs result- will be important to test ideas related to honest signalling. ing from resource limitations contribute to colour expression 372

Laboratory techniques, such as quantifying mitochondrial and establish the observed links between condition and orna- 20160343 : ATP production efficiency or production of free radicals mentation. It is time, however, to open the black box that has [99–101] or performing biochemical manipulations that been erected around many physiological systems so as to inhibit various aspects of mitochondrial function with or better understand the mechanisms that underlie honest without corresponding changes to free radical production signalling in animal coloration. [96–98], will be critical to advancing these ideas. For example, detecting a change in carotenoid coloration after Authors’ contributions. All authors contributed to the design, writing and experimental inhibition of mitochondrial function in the revising of this essay. absence of increased free radical production would indicate Competing interests. We have no competing interests. that colour expression is dependent on internal functional- Funding. R.J.W. and G.E.H. acknowledge support from the College of ity rather than on costs of using the pigments as Mathematics and Sciences and the Department of Biological Sciences at Auburn University. antioxidants. Determining a direct mechanistic link between Acknowledgements. We thank two anonymous reviewers for helpful mitochondrial function and ornament production will be comments on a previous version of this manuscript and the Hill important to determine whether coloured signals are true and Hood lab groups for valuable discussion during the preparation indices of cellular function. of this manuscript.

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