Social costs are an underappreciated force for honest signalling in animal aggregations

The physiological cost of colour change: evidence, implications and mitigations

Animals benefit from phenotypic plasticity in changing environments, but this can come at a cost. Colour change, used for camouflage, communication, thermoregulation and UV protection, represents one of the most common plastic traits in nature and is categorised as morphological or physiological depending on the mechanism and speed of the change. Colour change has been assumed to carry physiological costs, but current knowledge has not advanced beyond this basic assumption. The costs of changing colour will shape the evolution of colour change in animals, yet no coherent research has been conducted in this area, leaving a gap in our understanding. Therefore, in this Review, we examine the direct and indirect evidence of the physiological cost of colour change from the cellular to the population level, in animals that utilise chromatophores in colour change. Our Review concludes that the physiological costs result from either one or a combination of the processes of (i) production, (ii) translocation and (iii) maintenance of pigments within the colour-containing cells (chromatophores). In addition, both types of colour change (morphological and physiological) pose costs as they require energy for hormone production and neural signalling. Moreover, our Review upholds the hypothesis that, if repetitively used, rapid colour change (i.e. seconds-minutes) is more costly than slow colour change (days-weeks) given that rapidly colour-changing animals show mitigations, such as avoiding colour change when possible. We discuss the potential implications of this cost on colour change, behaviour and evolution of colour-changing animals, generating testable hypotheses and emphasising the need for future work to address this gap.

Vocal modulation in human mating and competition

The human voice is dynamic, and people modulate their voices across different social interactions. This article presents a review of the literature examining natural vocal modulation in social contexts relevant to human mating and intrasexual competition. Altering acoustic parameters during speech, particularly pitch, in response to mating and competitive contexts can influence social perception and indicate certain qualities of the speaker. For instance, a lowered voice pitch is often used to exert dominance, display status and compete with rivals. Changes in voice can also serve as a salient medium for signalling a person's attraction to another, and there is evidence to support the notion that attraction and/or romantic interest can be distinguished through vocal tones alone. Individuals can purposely change their vocal behaviour in attempt to sound more attractive and to facilitate courtship success. Several findings also point to the effectiveness of vocal change as a mechanism for communicating relationship status. As future studies continue to explore vocal modulation in the arena of human mating, we will gain a better understanding of how and why vocal modulation varies across social contexts and its impact on receiver psychology. This article is part of the theme issue 'Voice modulation: from origin and mechanism to social impact (Part I)'.

Submission signals in animal groups

Aggression is costly, and animals have evolved tactics to mitigate these costs. Submission signals are an underappreciated example of such adaptations. Here we review submissive behaviour, with an emphasis on non-primates. We highlight the design of submission signals and how such signals can reduce costs. Animal societies necessitate frequent social interactions, which can increase the probability of conflict. Where maintaining group proximity is essential, animals cannot avoid aggression by fleeing. Mutual interest between group members may also select for efficient conflict avoidance and resolution mechanisms. As a result, submission signals may be especially well developed among group living species, helping social animals to overcome potential costs of recurring conflict that could otherwise counter the benefits of group living. Therefore, submission signalling can be a crucial aspect of social living and is deserving of specific attention within the broader context of social evolution and communication.

The role of social costs as a mechanism enforcing the honesty of ultraviolet-reflecting signals in a lizard

According to animal signalling theory, social costs incurred by aggressive conspecifics are one mechanism maintaining signal honesty. Although our understanding of signal evolution has much improved for pigment-based colours, the mechanisms maintaining the honesty of structural colour signals, such as ultraviolet (UV), remain elusive. Here, we used the common lizard (Zootoca vivipara) to test whether the honesty of UV-reflecting signals displayed on male throats is under social control. To do so, we staged agonistic interactions between non-manipulated focal males and opponents of either larger or smaller body size. We manipulated the UV component of the male throat colour patch to create small cheaters with UV-enhanced throats, large cheaters with UV-reduced throats, and their respective controls. In support of a conventional signal hypothesis, focal males were aggressive towards large cheaters and became submissive when these large cheaters retaliated, and were less submissive against small cheaters. However, that focal males were not more aggressive towards small cheaters contradicts our initial predictions. We confirm that male UV reflectance and bite force were good predictors of contest outcomes in control conditions. Overall, we provide partial evidence suggesting that social costs enforce UV signal honesty in common lizards.

The benefits of being dominant: health correlates of male social rank and age in a marmot

The benefits of dominance may not come without costs, particularly for males. For example, the “immunocompetence handicap hypothesis” states that males with enhanced mating success allocate resources to enhance reproductive output at a cost to their current health, whereas the “resource quality hypothesis” predicts that high-ranking males may benefit from increased reproduction and good health. Whereas the predictions from each have been well tested in captive animals and in a variety of highly social primates, fewer studies have been carried out in free-living, facultatively social animals. Using adult male yellow-bellied marmots (Marmota flaviventer), we evaluated predictions of these hypotheses by examining the relationship between social rank and 2 health indicators—fecal glucocorticoid metabolite (FCM) levels, and neutrophil/lymphocyte (N/L) ratios—after accounting for variation explained by age, body mass, and seasonality. We found that higher-ranking males tended to have a lower N/L ratio (reflecting good health) than lower-ranking individuals, whereas FCM levels were not significantly related to rank. In addition, heavier male marmots had lower N/L ratios, whereas body mass was not associated with FCM levels. We also found that older adult males had lower FCM levels (reflecting less physiological stress) but higher N/L ratios than younger adults. Finally, we found that FCM levels decreased as the active season progressed and FCM levels were associated with the time of the day. Overall, our results suggest that socially-dominant male marmots enjoyed better, not worse health in terms of lower N/L ratios.

Microstructures amplify carotenoid plumage signals in tanagers

Brilliantly-colored birds are a model system for research into evolution and sexual selection. Red, orange, and yellow carotenoid-colored plumages have been considered honest signals of condition; however, sex differences in feather pigments and microstructures are not well understood. Here, we show that microstructures, rather than carotenoid pigments, seem to be a major driver of male-female color differences in the social, sexually-dimorphic tanager genus Ramphocelus. We comprehensively quantified feather (i) color (using spectrophotometry), (ii) pigments (using liquid chromatography-mass spectrometry (LC-MS)), and (iii) microstructures (using scanning electron microscopy (SEM) and finite-difference time-domain (FDTD) optical modeling). Males have significantly more saturated color patches than females. However, our exploratory analysis of pigments suggested that males and females have concordant carotenoid pigment profiles across all species (MCMCglmm model, female:male ratio = 0.95). Male, but not female, feathers have elaborate microstructures which amplify color appearance. Oblong, expanded feather barbs in males enhance color saturation (for the same amount of pigment) by increasing the transmission of optical power through the feather. Dihedral barbules (vertically-angled, strap-shaped barbules) in males reduce total reflectance to generate "super black" and "velvet red" plumage. Melanin in females explains some, but not all, of the male-female plumage differences. Our results suggest that a widely cited index of honesty, carotenoid pigments, cannot fully explain male appearance. We propose that males are selected to evolve amplifiers-in this case, microstructures that enhance appearance-that are not necessarily themselves linked to quality.

Physiological costs and age constraints of a sexual ornament: an experimental study in a wild bird

Sexual ornaments are often considered honest signals of quality because potential costs or constraints prevent their display by low-quality individuals. Testing for potential physiological costs of ornaments is difficult, as this requires experimentally forcing individuals to produce and display elaborate ornaments. We use this approach to test whether a sexually selected trait is physiologically costly to male superb fairy-wrens (Malurus cyaneus). Male fairy-wrens molt from brown to blue breeding plumage at different times of the year, and females strongly prefer the few males that are blue early, during winter. We used short-acting testosterone implants to stimulate males to produce “early-blue” plumage and assessed costs during and after molt using a panel of physiological indices. Testosterone-implanted, T-males molted in winter and produced blue plumage 6 weeks before control-implanted, C-males. T-males molted while in lower body condition, tended to have lower fat reserves, and were more likely to be parasitized by lice. However, we detected no negative effects on immune function, blood parasites, exposure to stressors, or survival. Juvenile males never naturally display early-blue plumage, but we found no evidence for increased costs paid by juvenile T-males. Instead, juvenile T-males molted later than adult T-males, suggesting that age presents an absolute constraint on ornament exaggeration that cannot be fully overcome by testosterone treatment. Together, these small costs and large, age-related constraints may enforce signal honesty, and explain female preference for early-blue males.

Male fairy-wrens produce and maintain vibrant breeding colors irrespective of individual quality

Conspicuous colors may signal individual quality if high-quality individuals produce more elaborate colors or have a greater capacity to invest in color maintenance. We investigate these hypotheses using repeated within-individual observations and experimentally induced color production in a wild bird, the superb fairy-wren (Malurus cyaneus). Male superb fairy-wrens undergo an annual molt from brown, nonbreeding plumage to an ultraviolet-blue and black breeding plumage. Color maintenance is especially relevant for this species because structural, ultraviolet-blue plumage colors are particularly susceptible to fading. Further, only the most sexually attractive males molt to breeding plumage early (before spring) and thereby keep their colors for an extended time before the breeding season. Our results show that (i) sexually attractive, early-molting males do not have higher quality breeding colors and (ii) breeding colors are not impacted by experimentally inducing males to molt early and while in low body condition. We found that (iii) breeding colors do not fade but remain consistent or become more saturated within individuals over time. Despite this, (iv) males do not spend more time preening while in breeding plumage. Instead, males keep their colors in pristine condition by re-molting parts of their breeding plumage throughout the breeding season, suggesting an alternative, potential cost of maintaining ornamental colors. We conclude that variation in structural breeding colors is unlikely to indicate individual quality in superb fairy-wrens.

Melanin-based ornament darkness positively correlates with across-season nutritional condition

Sexually dimorphic ornamental traits are widely regarded as indicators of nutritional condition. However, variation of nutritional condition outside the reproductive and the ornament production seasons has rarely been considered, although it affects the generality of information content, especially for ornaments that may be used across the year. We measured several indicators of migratory and molt condition in male and female blackcaps (Sylvia atricapilla) during their autumn migration, and quantified their crown reflectance. We detected robust correlations between migratory and molt condition indices, and the correlation structure was similar in the two sexes. Furthermore, the across-season measure of body condition was positively related to the darkness of the black crown in males, while being unrelated to reflectance traits of the reddish crown in females. Our results reinforce the possibility that some melanin-based ornaments may be year-round indicators of individual quality via their dependence on nutritional condition.

Testosterone induces plumage ornamentation followed by enhanced territoriality in a female songbird

We know little of the proximate mechanisms underlying the expression of signaling traits in female vertebrates. Across males, the expression of sexual and competitive traits, including ornamentation and aggressive behavior, is often mediated by testosterone. In the white-shouldered fairywren (Malurus alboscapulatus) of New Guinea, females of different subspecies differ in the presence or absence of white shoulder patches and melanic plumage, whereas males are uniformly ornamented. Previous work has shown that ornamented females circulate more testosterone and exhibit more territorial aggression than do unornamented females. We investigated the degree to which testosterone regulates the expression of ornamental plumage and territorial behavior by implanting free-living unornamented females with testosterone. Every testosterone-treated female produced a male-like cloacal protuberance, and 15 of 20 replaced experimentally plucked brown with white shoulder patch feathers but did not typically produce melanic plumage characteristic of ornamented females. Testosterone treatment did not elevate territorial behavior prior to the production of the plumage ornament or during the active life of the implant. However, females with experimentally induced ornamentation, but exhausted implants, increased the vocal components of territory defense relative to the pretreatment period and also to testosterone-implanted females that did not produce ornamentation. Our results suggest that testosterone induces partial acquisition of the ornamental female plumage phenotype and that ornament expression, rather than testosterone alone, results in elevations of some territorial behaviors.

The Handicap Principle: how an erroneous hypothesis became a scientific principle

The most widely cited explanation for the evolution of reliable signals is Zahavi's so-called Handicap Principle, which proposes that signals are honest because they are costly to produce. Here we provide a critical review of the Handicap Principle and its theoretical development. We explain why this idea is erroneous, and how it nevertheless became widely accepted as the leading explanation for honest signalling. In 1975, Zahavi proposed that elaborate secondary sexual characters impose 'handicaps' on male survival, not due to inadvertent signalling trade-offs, but as a mechanism that functions to demonstrate males' genetic quality to potential mates. His handicap hypothesis received many criticisms, and in response, Zahavi clarified his hypothesis and explained that it assumes that signals are wasteful as well as costly, and that they evolve because wastefulness enforces honesty. He proposed that signals evolve under 'signal selection', a non-Darwinian type of selection that favours waste rather than efficiency. He maintained that the handicap hypothesis provides a general principle to explain the evolution of all types of signalling systems, i.e. the Handicap Principle. In 1977, Zahavi proposed a second hypothesis for honest signalling, which received many different labels and interpretations, although it was assumed to be another example of handicap signalling. In 1990, Grafen published models that he claimed vindicated Zahavi's Handicap Principle. His conclusions were widely accepted and the Handicap Principle subsequently became the dominant paradigm for explaining the evolution of honest signalling in the biological and social sciences. Researchers have subsequently focused on testing predications of the Handicap Principle, such as measuring the absolute costs of honest signals (and using energetic and other proximate costs as proxies for fitness), but very few have attempted to test Grafen's models. We show that Grafen's models do not support the handicap hypothesis, although they do support Zahavi's second hypothesis, which proposes that males adjust their investment into the expression of their sexual signals according to their condition and ability to bear the costs (and risks to their survival). Rather than being wasteful over-investments, honest signals evolve in this scenario because selection favours efficient and optimal investment into signal expression and minimizes signalling costs. This idea is very different from the handicap hypothesis, but it has been widely misinterpreted and equated to the Handicap Principle. Theoretical studies have since shown that signalling costs paid at the equilibrium are neither sufficient nor necessary to maintain signal honesty, and that honesty can evolve through differential benefits, as well as differential costs. There have been increasing criticisms of the Handicap Principle, but they have focused on the limitations of Grafen's model and overlooked the fact that it is not a handicap model. This model is better understood within a Darwinian framework of adaptive signalling trade-offs, without the added burden and confusing logic of the Handicap Principle. There is no theoretical or empirical support for the Handicap Principle and the time is long overdue to usher this idea into an 'honorable retirement'.

Social interactions do not drive territory aggregation in a grassland songbird

Understanding the drivers of animal distributions is a fundamental goal of ecology and informs habitat management. The costs and benefits of colonial aggregations in animals are well established, but the factors leading to aggregation in territorial animals remain unclear. Territorial animals might aggregate to facilitate social behavior such as (1) group defense from predators and/or parasites, (2) cooperative care of offspring, (3) extra-pair mating, and/or (4) mitigating costs of extra-pair mating through kin selection. Using experimental and observational methods, we tested predictions of all four hypotheses in a tallgrass prairie in northeast Kansas, United States. Grasshopper Sparrow (Ammodramus savannarum) males formed clumps of territories in some parts of the site while leaving other apparently suitable areas unoccupied. Despite substantial sampling effort (653 territories and 223 nests), we found no support for any hypothesized social driver of aggregation, nor evidence that aggregation increases nest success. Our results run counter to previous evidence that conspecific interactions shape territory distributions. These results suggest one of the following alternatives: (1) the benefits of aggregation accrue to different life-history stages, or (2) the benefits of territory aggregation may be too small to detect in short-term studies and/or the consequences of aggregation are sufficiently temporally and spatially variable that they do not always appear to be locally adaptive, perhaps exacerbated by changing landscape contexts and declining population sizes.

Endless forms of sexual selection

In recent years, the field of sexual selection has exploded, with advances in theoretical and empirical research complementing each other in exciting ways. This perspective piece is the product of a "stock-taking" workshop on sexual selection and sexual conflict. Our aim is to identify and deliberate on outstanding questions and to stimulate discussion rather than provide a comprehensive overview of the entire field. These questions are organized into four thematic sections we deem essential to the field. First we focus on the evolution of mate choice and mating systems. Variation in mate quality can generate both competition and choice in the opposite sex, with implications for the evolution of mating systems. Limitations on mate choice may dictate the importance of direct vs. indirect benefits in mating decisions and consequently, mating systems, especially with regard to polyandry. Second, we focus on how sender and receiver mechanisms shape signal design. Mediation of honest signal content likely depends on integration of temporally variable social and physiological costs that are challenging to measure. We view the neuroethology of sensory and cognitive receiver biases as the main key to signal form and the 'aesthetic sense' proposed by Darwin. Since a receiver bias is sufficient to both initiate and drive ornament or armament exaggeration, without a genetically correlated or even coevolving receiver, this may be the appropriate 'null model' of sexual selection. Thirdly, we focus on the genetic architecture of sexually selected traits. Despite advances in modern molecular techniques, the number and identity of genes underlying performance, display and secondary sexual traits remains largely unknown. In-depth investigations into the genetic basis of sexual dimorphism in the context of long-term field studies will reveal constraints and trajectories of sexually selected trait evolution. Finally, we focus on sexual selection and conflict as drivers of speciation. Population divergence and speciation are often influenced by an interplay between sexual and natural selection. The extent to which sexual selection promotes or counteracts population divergence may vary depending on the genetic architecture of traits as well as the covariance between mating competition and local adaptation. Additionally, post-copulatory processes, such as selection against heterospecific sperm, may influence the importance of sexual selection in speciation. We propose that efforts to resolve these four themes can catalyze conceptual progress in the field of sexual selection, and we offer potential avenues of research to advance this progress.

Dynamic eye colour as an honest signal of aggression

Animal eyes are some of the most widely recognisable structures in nature. Due to their salience to predators and prey, most research has focused on how animals hide or camouflage their eyes [1]. However, across all vertebrate Classes, many species actually express brightly coloured or conspicuous eyes, suggesting they may have also evolved a signalling function. Nevertheless, perhaps due to the difficulty with experimentally manipulating eye appearance, very few species beyond humans [2] have been experimentally shown to use eyes as signals [3]. Using staged behavioural trials we show that Trinidadian guppies (Poecilia reticulata), which can rapidly change their iris colour, predominantly express conspicuous eye colouration when performing aggressive behaviours towards smaller conspecifics. Furthermore, using a novel, visually realistic robotic system to create a mismatch between signal and relative competitive ability, we show that eye colour is used to honestly signal aggressive motivation. Specifically, robotic 'cheats' (that is, smaller, less-competitive robotic fish that display aggressive eye colouration when defending a food patch) attracted greater food competition from larger real fish. Our study suggests that eye colour may be an under-appreciated aspect of signalling in animals, shows the utility of our biomimetic robotic system for investigating animal behaviour, and provides experimental evidence that socially mediated costs towards low-quality individuals may maintain the honesty of dynamic colour signals.

Self-deception in nonhuman animals: weak crayfish escalated aggression as if they were strong

Humans routinely deceive themselves when communicating to others, but no one knows whether other animals do the same. We ask whether dishonest signaling between crayfish meets a condition required for self-deception: dishonest individuals and honest individuals escalate aggression according to their signals of strength rather than actual strength. Using game theory, we predicted how an animal’s knowledge of its strength should affect its decision to escalate aggression. At the evolutionary equilibrium, an animal that knows its strength should escalate aggression according to its strength, relative to the expected strength of its opponent. By contrast, an animal that knows only its size should escalate aggression according to its size, relative to the size of its opponent. We tested these predictions by staging encounters between male crayfish (Cherax dispar) of known sizes and strengths. Consistent with a model of self-deception, crayfish escalated aggression based on the sizes of their claws relative to those of their opponents, despite the fact that size poorly predicts strength. Males who were weak for their size escalated disputes less often, but their aggression far exceeded the level predicted by a model of self-awareness, suggesting these crayfish were largely ignorant of their deception. Animals that fail to recognize their own dishonest signals may win disputes with stronger opponents without engaging in costly combat. Our game-theoretical approach can be used to identify potential cases of self-deception in nonhuman animals, enabling comparative studies of this behavior.