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Simple Summary: The study of animal coloration has long been a central theme in biological science. Bright colors that often derived from pigments serve as vital social signals across species, conveying information about individual quality, social status, and reproductive fitness. Pterins as a group of endogenous pigments capable of producing bright colors, have received relatively little attention so far. Whether pterin-based colors represent signals of individual quality remains unclear. The vibrant red coloration displayed by male Guinan toad-headed lizards offers an ideal model to investigate this question. Through an integrated approach combining metabolomics, colorimetry, and behavioral experiments, we found that this red coloration was not correlated with body size, bite force, or testosterone level, and it did not influence female mate choice. However, red intensity predicted male–male competition outcomes, with deeper red males more likely to win contests. Our findings suggest that pterin-based coloration can serve as a signal of male quality. This study provides novel insights into the role of pterin-based colors in animals. Animal coloration offers a unique opportunity to explore the evolutionary mechanisms underlying phenotypic diversity. Conspicuous coloration caused by pigments plays a crucial role in social signaling across multiple species by conveying information about individual quality, social ranks, or reproductive condition. Nevertheless, most previous studies have focused predominantly on colors produced by the exogenous pigments—carotenoids. Pterins are another prevalent group of conspicuous pigments, which can be produced endogenously and have received comparatively little attention. Whether pterin-based colors represent reliable signals remains elusive. The remarkable red ventrolateral coloration exhibited by males of the Guinan toad-headed lizard (Phrynocephalus guinanensis) in the Mugetan Desert presents an ideal model for investigating pterin-based coloration. Through electron microscopy and metabolomic identification, we discovered three types of pterin pigments within xanthophores. Integrating a series of morphological measurements and behavioral experiments, we found that this red coloration was not correlated with body size, bite force, and testosterone level, nor did females show a preference bias toward it. However, the red intensity predicted male–male competition outcomes, with deeper red males being more likely to emerge as winners. Our results indicated that the pterin-based coloration could convey information about male quality, suggesting its potential role in honest signaling, given the vital importance of pterin metabolism in physiological processes. This study provides a novel case into the understanding of pterin-based colors in animals. [ABSTRACT FROM AUTHOR]

Abstract Background Fleshy fruits evolved to be attractive to seed dispersers through various signals such as color and scent. Signals can evolve through different trajectories and have various degrees of reliability. The strongest substrate on which reliable signals can evolve is when there is an inherent link between signal and reward, rendering cheating costly or impossible. It was recently proposed that aliphatic esters in fruit scent may be predictive of sugar content due to their synthesis from products of sugar fermentation. We test this hypothesis on a case study of wild fig species (Ficus tiliifolia) from Madagascar, which relies on seed dispersal by lemurs. Results We found a strong positive correlation between signal (esters) and reward (sugar). We also found that non-esters, including direct fermentation products, in fruit scent do not indicate sugar levels, which implies that this relationship is not simply a product of fruit maturation wherein more mature fruits emit more scent and contain more sugar. Conclusions While based on a single taxon, these results strongly support the hypothesis that a biochemical link between ester synthesis and sugar may render the ester fraction of fruit scent an honest signal for fruit quality, with consequences for animal sensory and feeding ecology, and the evolution of plants in the context of seed dispersal.

In some aposematic species the conspicuousness of an individual's warning signal and the concentration of its chemical defense are positively correlated. Several mechanisms have been proposed to explain this phenomenon, including resource allocation trade‐offs where the same limiting resource is needed to produce both the warning signal and chemical defense. Here, the large milkweed bug (Oncopeltus fasciatus: Heteroptera, Lygaeinae) was used to test whether allocation of antioxidants, that can impart color, trade against their availability to prevent self‐damage caused by toxin sequestration. We investigated if (i) the sequestration of cardenolides is associated with costs in the form of changes in oxidative state; and (ii) oxidative state can affect the capacity of individuals to produce warning signals. We reared milkweed bugs on artificial diets with increasing quantities of cardenolides and examined how this affected signal quality (brightness and chroma) across different instars. We then related the expression of warning colors to the quantity of sequestered cardenolides and indicators of oxidative state—oxidative lipid damage (malondialdehyde), and two antioxidants: total superoxide dismutase and total glutathione. Bugs that sequestered more cardenolides had significantly lower levels of the antioxidant glutathione, and bugs with less total glutathione had less luminant orange warning signals and reduced chroma of their black patches compared to bugs with more glutathione. Bugs that sequestered more cardenolides also had reduced red–green chroma of their black patches that was unrelated to oxidative state. Our results give tentative support for a physiological cost of sequestration in milkweed bugs and a mechanistic link between antioxidant availability, sequestration, and warning signals. [ABSTRACT FROM AUTHOR]

In many species, offspring display conspicuous coloration already early in life, even though they might be very vulnerable to predation at this stage. However, most attention has been drawn to the conspicuous plumage displayed by adult individuals in a sexual context, while other signaling functions have been explored much less. Here, we investigated whether the yellow breast plumage of blue tit (Cyanistes caeruleus) nestlings shows patterns of condition dependence and hence signals individual quality, as has been described for adult birds. During three consecutive breeding seasons, we, therefore, explored the association between nestling body mass and three color components of the yellow breast plumage (i.e., UV chroma, carotenoid chroma, and total brightness), considering both within and among nest effects. Variation in carotenoid chroma was not related to body mass. However, UV chroma and total brightness varied with body mass on an among‐nest level, suggesting that they might signal aspects of genetic quality or parental rearing capacity. Interestingly, we also found a within‐nest effect of body mass on total brightness, suggesting that this is a good candidate for a condition‐dependent signal within the family. Thus, other family members could rely on brightness to adjust their behavioral strategies, such as feeding behavior in parents. Our study thus reveals that certain color components of the yellow breast plumage might signal different aspects of offspring quality, and they might have a correlated signaling value across life‐history stages. [ABSTRACT FROM AUTHOR]

Sexual selection is a major force shaping morphological and behavioral diversity. Existing theory focuses on courtship display traits such as morphological ornaments whose costs and benefits are assumed be to fixed across individuals' lifetimes. In contrast, empirically observed displays are often inherently dynamic, as vividly illustrated by the acrobatic dances, loud vocalizations, and vigorous motor displays involved in courtship behavior across a broad range of taxa. One empirically observed form of display flexibility occurs when signalers adjust their courtship investment based on the number of rival signalers. The predictions of established sexual selection theory cannot readily be extended to such displays because display expression varies between courtship events, such that any given display may not reliably reflect signaler quality. We thus lack an understanding of how dynamic displays coevolve with sexual preferences and how signalers should tactically adjust their display investment across multiple courtship opportunities. To address these questions, we extended an established model of the coevolution of a female sexual preference and a male display trait to allow for flexible, dynamic displays. We find that such a display can coevolve with a sexual preference away from their naturally selected optima, though display intensity is a weaker signal of male quality than for non-flexible displays. Furthermore, we find that males evolve to decrease their display investment when displaying alongside more rivals. This research represents a first step towards generalizing the findings of sexual selection theory to account for the ubiquitous dynamism of animal courtship. Significance statement: Animal courtship displays are typically costly for survival: songs attract predators; dances are exhausting; extravagant plumage is cumbersome. Because of the trade-off between mating benefits and survival costs, displaying individuals often vary their displays across time, courting more intensely when the potential benefit is higher or the cost is lower. Despite the ubiquity of such adjustment in nature, existing theory cannot account for how this flexibility might affect the coevolution of displays with sexual preferences, nor for the patterns of tactical display adjustment that might result, because those models treat displays as static, with fixed costs and benefits. Generalizing a well-studied model of sexual selection, we find that a static display and a flexible display can evolve under similar conditions. Our model predicts that courtship should be less intense when more competitors are present. [ABSTRACT FROM AUTHOR]

Abstract In many species, offspring display conspicuous coloration already early in life, even though they might be very vulnerable to predation at this stage. However, most attention has been drawn to the conspicuous plumage displayed by adult individuals in a sexual context, while other signaling functions have been explored much less. Here, we investigated whether the yellow breast plumage of blue tit (Cyanistes caeruleus) nestlings shows patterns of condition dependence and hence signals individual quality, as has been described for adult birds. During three consecutive breeding seasons, we, therefore, explored the association between nestling body mass and three color components of the yellow breast plumage (i.e., UV chroma, carotenoid chroma, and total brightness), considering both within and among nest effects. Variation in carotenoid chroma was not related to body mass. However, UV chroma and total brightness varied with body mass on an among‐nest level, suggesting that they might signal aspects of genetic quality or parental rearing capacity. Interestingly, we also found a within‐nest effect of body mass on total brightness, suggesting that this is a good candidate for a condition‐dependent signal within the family. Thus, other family members could rely on brightness to adjust their behavioral strategies, such as feeding behavior in parents. Our study thus reveals that certain color components of the yellow breast plumage might signal different aspects of offspring quality, and they might have a correlated signaling value across life‐history stages.

After they diverged from panins, hominins evolved an increasingly committed terrestrial lifestyle in open habitats that exposed them to increased predation pressure from Africa's formidable predator guild. In the Pleistocene, Homo transitioned to a more carnivorous lifestyle that would have further increased predation pressure. An effective defense against predators would have required a high degree of cooperation by the smaller and slower hominins. It is in the interest of predator and potential prey to avoid encounters that will be costly for both. A wide variety of species, including carnivores and apes and other primates, have therefore evolved visual and auditory signals that deter predators by credibly signaling detection and/or the ability to effectively defend themselves. In some cooperative species, these predator deterrent signals involve highly synchronized visual and auditory displays among group members. Hagen and Bryant (Human Nature, 14(1), 21–51, 2003) proposed that synchronized visual and auditory displays credibly signal coalition quality. Here, this hypothesis is extended to include credible signals to predators that they have been detected and would be met with a highly coordinated defensive response, thereby deterring an attack. Within-group signaling functions are also proposed. The evolved cognitive abilities underlying these behaviors were foundations for the evolution of fully human music and dance. [ABSTRACT FROM AUTHOR]

Animal vocalizations may provide information about a sender's condition or motivational state and, hence, mediate social interactions. In this study, we examined whether vocalizations of gray mouse lemurs (Microcebus murinus) emitted in aggressive contexts (grunts, tsaks) co-vary with physical condition, which would underly and indicate honest signaling. We recorded calls from captive individuals that were subjected to a caloric restricted (CR) or ad libitum (AL) diet, assuming that individuals on an ad libitum dietary regime were in better condition. We analyzed 828 grunts produced by seven CR and nine AL individuals and 270 tsaks by eight CR and five AL individuals. Grunts consisted of two separate elements, with the 1st element having more energy in higher frequencies than the 2nd element. Body mass correlated negatively with acoustic features of grunts, and heavier individuals produced lower-frequency grunts. Acoustic features of grunts did not differ between sexes. Acoustic features of tsaks were predicted by neither body mass nor sex. However, tsaks produced by AL individuals were noisier than those of CR individuals. Hence, manipulation of body condition via dietary regimes affected acoustic features of calls given during aggression in different ways: acoustic features of grunts varied according to the rule of acoustic allometry, and can be considered as honest signals. Acoustic features of tsaks, however, varied according to motivational structural rules. Longitudinal studies are now indicated to examine whether intra-individual changes in body mass are also reflected in the acoustic structure of calls, allowing callers to signal more flexible variation in condition. [ABSTRACT FROM AUTHOR]

Birdsong is commonly associated with sexual selection. It can influence mate choice through honestly signaling the emitter's quality. Such quality may be reflected, among other factors, in body condition and bite force. The Green-winged Saltator (Saltator similis) is a good model species to assess such relationships because males defend their territories throughout the year and sing a stereotyped song, which is known to be associated with sexual selection. The objective of this work is to assess the relationships among song structure, vocal individuality, and possible fitness indicators (body condition, morphology, and bite force) in captive male Green-winged Saltators. We conducted a linear discriminant analysis (LDA) to assess vocal individuality and to highlight the principal acoustic variables associated with it. We conducted Y-Aware PCAs to evaluate the relationship between the body condition index, bite force, and morphometric and bioacoustic variables. The morphometric variables, particularly head width and length, explained 23% of the bite force variation. The acoustic variation explained 36.17% of body condition index variation, but a small nonsignificant amount of bite force (12%). From the acoustic variables, peak frequency and high frequency of the last syllable, slope of the first syllable, and emission rate showed significant negative relationships with body condition. The LDA model based on acoustic variables had a 97.47% accuracy, although this result might also reflect regional song dialects. The acoustic variables most relevant to individual classification (first syllable duration, last syllable duration, whole-song slope, and whole-song duration) are not the same variables that had a stronger relationship to body condition. Besides that, there can be a weaker mixed signaling, as variables related to frequency modulations were important in both contexts. Our results suggest that the song may act as an honest signal of an individual's body condition in this species. [ABSTRACT FROM AUTHOR]

Background Depression occurs about twice as often in women as in men, a disparity that remains poorly understood. In a previous publication, Hagen and Rosenström predicted and found that grip strength, a highly sexually dimorphic index of physical formidability, mediated much of the effect of sex on depression. Striking results like this are more likely to be published than null results, potentially biasing the scientific record. It is therefore critical to replicate and extend them. Methodology Using new data from the 2013–14 cycle of the National Health and Nutrition Examination Survey, a nationally representative sample of US households (n = 3650), we replicated models of the effect of sex and grip strength on depression reported in Hagen and Rosenström, along with additional potential confounds and a new detailed symptom-level exploration. Results Overall, the effects from the original paper were reproduced although with smaller effect sizes. Grip strength mediated 38% of the effect of sex on depression, compared to 63% in Hagen and Rosenström. These results were extended with findings that grip strength had a stronger association with some depression symptoms, like suicidality, low interest and low mood than with other symptoms, like appetite changes. Conclusions Grip strength is negatively associated with depression, especially its cognitive–affective symptoms, controlling for numerous possible confounds. Although many factors influence depression, few of these reliably occur cross-culturally in a sex-stratified manner and so are unlikely to explain the well-established, cross-cultural sex difference in depression. The sex difference in upper body strength occurs in all populations and is therefore a candidate evolutionary explanation for some of the sex difference in depression. Lay summary: Why are women at twice the risk of developing depression as men? Depression typically occurs during social conflicts, such as physical or sexual abuse. Physically strong individuals can often single-handedly resolve conflicts in their favor, whereas physically weaker individuals often need help from others. We argue that depression is a credible cry for help. Because men generally have greater strength than women, we argue that men may be more likely to resolve conflicts using physical formidability and women to signal others for help. We find that higher grip strength is associated with lower depression, particularly symptoms like feeling down or thoughts of suicide and that strength accounts for part of the sex difference in rates of depression. [ABSTRACT FROM AUTHOR]

Abstract The ability to assess food quality is crucial to all organisms. Fleshy fruits are a major source of nutrients to various animals, and unlike most food sources, have evolved to be attractive and to be consumed by animals to promote seed dispersal. It has recently been established that fruit scent—the bouquet of volatile chemicals emitted by ripe fruit—is an evolved communication system between plants and animals. Further, it has been argued that chemicals that are synthesized from sugar and its products may be an honest signal for sugar content and fruit quality. Elephants are important seed dispersers for numerous species and possess an olfactory system that is likely to outperform most other animals. We tested the hypothesis that fruit scent signifies sugar content and that elephants are capable of assessing fruit sugar levels based on scent alone. Using a paired‐choice test of marula fruits (Sclerocarya birrea) by semitame African elephants, we show that elephants are capable of identifying more sugar‐rich fruits based on scent alone and that this is likely based on two chemical compounds: ethanol and ethyl acetate, both downstream products of sugar fermentation. These results shed light on the mechanisms driving elephant feeding ecology, plant signaling, and the coevolutionary process between angiosperms and animal seed dispersers.

Abstract Color‐based visual signals are important aspects of communication throughout the animal kingdom. Individuals evaluate color to obtain information about age and condition and to behave accordingly. Birds display a variety of striking, conspicuous colors and make ideal subjects for the study of color signaling. While most studies of avian color focus on plumage, bare unfeathered body parts also display a wide range of color signals. Mate choice and intrasexual competitive interactions are easily observed in lekking grouse, which also signal with prominent unfeathered color patches. Most male grouse have one pair of colorful bare part ornaments (combs), and males of several species also have inflatable air sacs in their throat. Previous studies have mostly focused on comb color and size, but little is known about the signaling role of air sac color. We measured comb size and the color properties of combs and air sacs in the Lesser and Greater Prairie‐Chickens (Tympanuchus pallidicinctus and T. cupido, respectively), and investigated whether these properties varied with age and mass. We found that mass predicted color properties of air sacs and that age predicted comb size in the Greater Prairie‐Chicken, suggesting that these ornaments indicate condition dependence. No conclusive relationships between color and age or size were detected in the Lesser Prairie‐Chicken. Color properties of both ornaments differed between the two species. Further research is needed to determine mechanisms that link condition to color and whether the information advertised by color signals from these ornaments is intended for males, females, or both.

Abstract Honest signaling mechanisms can function to appropriate care to hungry offspring and avoid misdirected care of unrelated offspring. Begging, the behavior by which offspring solicit food and parental care, may be an honest signaling mechanism for need, as well as association of parents and offspring. Roseate terns (Sterna dougallii) exhibit prolonged parental care during the postbreeding staging period, offering an ideal system in which to study begging as an honest signaling mechanism. We conducted focal sampling during two premigratory staging seasons (2014 and 2015) at Cape Cod National Seashore, Massachusetts, USA to determine whether postfledging tern begging behavior was an honest signal for need and parent–offspring association. Based on honest signaling theory, we expected begging behavior to be highest during times of high perceived need, and we expected to see a decrease in begging behavior as young terns became increasingly independent of the care‐giving parent. Also, we predicted that young terns would be more likely to beg at parents than nonparents. We found that young roseate terns begged at their parents more often than nonparents; however, they did not always beg at parents. Model predictions of begging probability showed a linear relationship between begging and time of day and date of season, such that begging increased with time of day and decreased with date of season, respectively. Our results provide evidence for honest parent–offspring interactions and are inconsistent with parent–offspring conflict theory but suggest that begging may play a complex role in postfledging parent–offspring interactions. OPEN RESEARCH BADGES This article has been awarded Open Data, Open materials Badges. All materials and data are publicly accessible via the Open Science Framework at https://doi.org/10.5281/zenodo.2656718.

Bird songs often display musical acoustic features such as tonal pitch selection, rhythmicity, and melodic contouring. We investigated higher-order musical temporal structure in bird song using an experimental method called “music scrambling” with human subjects. Recorded songs from a phylogenetically diverse group of 20 avian taxa were split into constituent elements (“notes” or “syllables”) and recombined in original and random order. Human subjects were asked to evaluate which version sounded more “musical” on a per-species basis. Species identity and stimulus treatment were concealed from subjects, and stimulus presentation order was randomized within and between taxa. Two recordings of human music were included as a control for attentiveness. Participants varied in their assessments of individual species musicality, but overall they were significantly more likely to rate bird songs with original temporal sequence as more musical than those with randomized temporal sequence. We discuss alternative hypotheses for the origins of avian musicality, including honest signaling, perceptual bias, and arbitrary aesthetic coevolution.

Bird songs often display musical acoustic features such as tonal pitch selection, rhythmicity, and melodic contouring. We investigated higher-order musical temporal structure in bird song using an experimental method called "music scrambling" with human subjects. Recorded songs from a phylogenetically diverse group of 20 avian taxa were split into constituent elements ("notes" or "syllables") and recombined in original and random order. Human subjects were asked to evaluate which version sounded more "musical" on a per-species basis. Species identity and stimulus treatment were concealed from subjects, and stimulus presentation order was randomized within and between taxa. Two recordings of human music were included as a control for attentiveness. Participants varied in their assessments of individual species musicality, but overall they were significantly more likely to rate bird songs with original temporal sequence as more musical than those with randomized temporal sequence. We discuss alternative hypotheses for the origins of avian musicality, including honest signaling, perceptual bias, and arbitrary aesthetic coevolution. [ABSTRACT FROM AUTHOR]

Acoustic signals function in intrasexual mating competition in a wide variety of species, including humans. The low voice pitch of human males has been proposed to represent an honest signal of formidability. Although voice pitch in men affects perceptions of size and dominance, it is relatively weakly associated with objective measures of formidability such as body size and strength. As a result, some authors have argued that low male voice pitch is not a valid signal of formidability but is deceptive and salient only because it hijacks a tendency to perceive lower frequency sounds as emanating from larger sources. In this paper, we consider theoretical and empirical issues associated with this perceptual exploitation hypothesis and ask whether male voice pitch transmits information about formidability. We utilize mediation models to investigate whether male voice pitch is an honest signal of formidability in data collected from university students in the U.S. (n = 231 male speakers, 565 male raters) and Canada (n = 74 male speakers, 108 female raters, 65 male raters). In both data sets, male voice pitch mediated the relationship between objective (measured by height) and perceived formidability. Collectively, these results indicate that men's voice pitch transmits information about formidability from signaler to receiver. • We tested whether men's voice pitch signals formidability in two samples. • Listeners could accurately assess formidability from men's voices. • Male speakers' voice pitch mediated listeners' ability to assess formidability. • Mediation by voice pitch was independent of resonant (formant) frequencies. • Men's voice pitch appears to honestly signal formidability. [ABSTRACT FROM AUTHOR]

At the earliest break of ancient hominins from their primate relatives in vocal communication, we propose a selection pressure on vocal fitness signaling by hominin infants. Exploratory vocalizations, not tied to expression of distress or immediate need, could have helped persuade parents of the wellness and viability of the infants who produced them. We hypothesize that hominin parents invested more in infants who produced such signals of fitness plentifully, neglecting or abandoning them less often than infants who produced the sounds less frequently. Selection for such exploratory vocalization provided a critically important inclination and capability relevant to language, we reason, because the system that encouraged spontaneous vocalization also made vocalization functionally flexible to an extent that has not been observed in any other animal. Although this vocal flexibility did not by itself create language, it provided an essential foundation upon which language would evolve through a variety of additional steps. In evaluating this speculation, we consider presumable barriers to evolving language that are thought to be implications of Darwinian Theory. It has been claimed that communication always involves sender self-interest and that self-interest leads to deceit, which is countered through clever detection by receivers. The constant battle of senders and receivers has been thought to pose an insuperable challenge to honest communication, which has been viewed as a requirement of language. To make communication honest, it has been proposed that stable signaling requires costly handicaps for the sender, and since language cannot entail high cost, the reasoning has suggested an insurmountable obstacle to the evolution of language. We think this presumed honesty barrier is an illusion that can be revealed by recognition of the fact that language is not inherently honest and in light of the distinction between illocutionary force and semantics. Our paper also considers barriers to the evolution of language (not having to do with honesty) that we think may have actually played important roles in preventing species other than humans from evolving language.

At the earliest break of ancient hominins from their primate relatives in vocal communication, we propose a selection pressure on vocal fitness signaling by hominin infants. Exploratory vocalizations, not tied to expression of distress or immediate need, could have helped persuade parents of the wellness and viability of the infants who produced them. We hypothesize that hominin parents invested more in infants who produced such signals of fitness plentifully, neglecting or abandoning them less often than infants who produced the sounds less frequently. Selection for such exploratory vocalization provided a critically important inclination and capability relevant to language, we reason, because the system that encouraged spontaneous vocalization also made vocalization functionally flexible to an extent that has not been observed in any other animal. Although this vocal flexibility did not by itself create language, it provided an essential foundation upon which language would evolve through a variety of additional steps. In evaluating this speculation, we consider presumable barriers to evolving language that are thought to be implications of Darwinian Theory. It has been claimed that communication always involves sender self-interest and that self-interest leads to deceit, which is countered through clever detection by receivers. The constant battle of senders and receivers has been thought to pose an insuperable challenge to honest communication, which has been viewed as a requirement of language. To make communication honest, it has been proposed that stable signaling requires costly handicaps for the sender, and since language cannot entail high cost, the reasoning has suggested an insurmountable obstacle to the evolution of language. We think this presumed honesty barrier is an illusion that can be revealed by recognition of the fact that language is not inherently honest and in light of the distinction between illocutionary force and semantics. Our paper also considers barriers to the evolution of language (not having to do with honesty) that we think may have actually played important roles in preventing species other than humans from evolving language. [ABSTRACT FROM AUTHOR]

The ability to assess food quality is crucial to all organisms. Fleshy fruits are a major source of nutrients to various animals, and unlike most food sources, have evolved to be attractive and to be consumed by animals to promote seed dispersal. It has recently been established that fruit scent—the bouquet of volatile chemicals emitted by ripe fruit—is an evolved communication system between plants and animals. Further, it has been argued that chemicals that are synthesized from sugar and its products may be an honest signal for sugar content and fruit quality. Elephants are important seed dispersers for numerous species and possess an olfactory system that is likely to outperform most other animals. We tested the hypothesis that fruit scent signifies sugar content and that elephants are capable of assessing fruit sugar levels based on scent alone. Using a paired‐choice test of marula fruits (Sclerocarya birrea) by semitame African elephants, we show that elephants are capable of identifying more sugar‐rich fruits based on scent alone and that this is likely based on two chemical compounds: ethanol and ethyl acetate, both downstream products of sugar fermentation. These results shed light on the mechanisms driving elephant feeding ecology, plant signaling, and the coevolutionary process between angiosperms and animal seed dispersers. [ABSTRACT FROM AUTHOR]

When a measure becomes a target, it often ceases to be a good measure – an effect familiar from the declining usefulness of standardized testing in schools. This economic principle also applies to mate choice and, perhaps surprisingly, pregnancy. Just as females screen potential mates under many metrics, human mothers unconsciously screen embryos for quality. 'Examinees' are under intense selection to improve test performance by exaggerating formerly 'honest' signals of quality. Examiners must change their screening criteria to maintain useful information (but cannot abandon old criteria unilaterally). By the resulting 'proxy treadmill', new honest indicators arise while old degraded indicators linger, resulting in trait elaboration and exaggeration. Hormone signals during pregnancy show extreme evolutionary escalation (akin to elaborate mating displays). Mate choice by honest signaling is a classic explanation for elaborate traits in nature. Many researchers have: (i) observed deceptive signaling, and (ii) wondered how honest signals relate to trait elaboration. Honest signaling is analogous to high-stakes testing. Quality is hard to measure directly, so proxies (tests) are used. High-stakes testing causes 'teaching to the test' without improving educational outcomes. Embryo choice is another high-stakes test. Mothers select healthy embryos and terminate sub-par embryos automatically. Embryos are selected to pass maternal tests without improving their quality. The resulting arms race causes extreme and elaborate signals during pregnancy. We can better understand elaborate traits in nature if we interpret mate selection, and embryo choice, as a dynamic give-and-take between two parties with conflicting fitness interests. [ABSTRACT FROM AUTHOR]

How anthropogenic change affects animal social behavior, including communication is an important question. Urban noise often drives shifts in acoustic properties of signals but the consequences of noise for the honesty of signals—that is, how well they predict signaler behavior—is unclear. Here we examine whether honesty of aggressive signaling is compromised in male urban song sparrows (Melospiza melodia). Song sparrows have two honest close-range signals: the low amplitude soft songs (an acoustic signal) and wing waves (a visual signal), but whether the honesty of these signals is affected by urbanization has not been examined. If soft songs are less effective in urban noise, we predict that they should predict attacks less reliably in urban habitats compared to rural habitats. We confirmed earlier findings that urban birds were more aggressive than rural birds and found that acoustic noise was higher in urban habitats. Urban birds still sang more soft songs than rural birds. High rates of soft songs and low rates of loud songs predicted attacks in both habitats. Thus, while urbanization has a significant effect on aggressive behaviors, it might have a limited effect on the overall honesty of aggressive signals in song sparrows. We also found evidence for a multimodal shift: urban birds tended to give proportionally more wing waves than soft songs than rural birds, although whether that shift is due to noise-dependent plasticity is unclear. These findings encourage further experimental study of the specific variables that are responsible for behavioral change due to urbanization. Soft song, the low amplitude songs given in close range interactions, is an honest threat signal in urban song sparrows. Given its low amplitude, soft songs may be a less effective signal in noisy urban habitats. However, we found that soft song remained an honest signal predicting attack in urban habitats. We also found that birds may use more visual signals (rapid fluttering of wings) in urban habitats to avoid masking from acoustic noise. [ABSTRACT FROM AUTHOR]

Male secondary sexual traits potentially function as indicators of direct or indirect fitness benefits to females. Direct benefits, such as paternal care, may be especially important to females in species with biparental care. In an experimental field study of the golden rocket frog (Anomaloglossus beebei), a Neotropical species with biparental care, we tested predictions from four hypotheses proposed to explain the evolutionary relationship between male secondary sexual traits and paternal care quality (the "good parent," "differential allocation," "trade-off," and "essential male care" hypotheses). We examined: 1) the influence of paternal care on offspring survival, 2) the relationships between male calls and paternal care, maternal care, and opportunities for males to acquire multiple mates, and 3) female preferences for three acoustic properties of male advertisement calls. Our results reveal that paternal care positively impacts offspring survival, that males producing longer calls also provide higher-quality paternal care in the form of greater egg attendance and territory defense, and that females prefer longer calls. Females did not discriminate among potential mates based on differences in dominant frequency or call rate. These findings, which suggest male advertisement calls are indicators of potential direct benefits to females in the form of paternal care, are consistent with the good parent hypothesis and inconsistent with the trade-off, differential allocation, and essential male care hypotheses. [ABSTRACT FROM AUTHOR]

Penises play a key role in sperm transport and in stimulating female genitals. This should impact post-copulatory competition, and expose penis characteristics to sexual selective pressures. Studies of male genitalia have repeatedly reported negative static allometries, which mean that, within species, large males have disproportionally small genitals when compared with smaller individuals. Males of some sperm-storing bat species may stand as an exception to such a pattern by arousing from hibernation to copulate with torpid females. The selection for large penises might take place, if a long organ provides advantages during post-copulatory competition and/or if females have evolved mechanisms allowing the choice of sire, relying on characters other than pre-copulatory traits (e.g. penis size). In this study, we measured dimensions of the erected penis in 4 sperm-storing bat species. Furthermore, we collected sperm and evaluated the link between penis dimensions and sperm velocity. Our results revealed steep allometric slopes of the erected penis length in Barbastella barbastellus and an inverse allometry of penis head width in Myotis nattereri. More detailed studies of copulatory behavior are urgently needed to explain the range of observed scaling relations. Furthermore, penis head width correlates with sperm velocity in Plecotus auritus. For this last species, we propose that penis shape might act as a marker of male fertility. [ABSTRACT FROM AUTHOR]

Chemical information in canid urine has been implicated in territoriality and influences the spacing of individuals. We identified the key volatile organic compound (VOC) components in dingo (Canis lupus dingo) urine and investigated the potential role of scents in territorial spacing. VOC analysis, using headspace gas chromatography-mass spectrometry (GC-MS), demonstrated that the information in fresh urine from adult male dingoes was sufficient to allow statistical classification into age categories. Discriminant function analyses demonstrated that the relative amounts or combinations of key VOCs from pre-prime (3-4 years), prime (5-9 years), and post-prime (≥10 years) males varied between these age categories, and that scents exposed to the environment for 4 (but not 33) days could still be classified to age categories. Further, a field experiment showed that dingoes spent less time in the vicinity of prime male dingo scents than other scents. Collectively, these results indicate that age-related scent differences may be discriminable by dingoes. Previous authors have suggested the potential to use scent as a management tool for wild canids by creating an artificial territorial boundary/barrier. Our results suggest that identifying the specific signals in prime-age male scents could facilitate the development of scent-based tools for non-lethal management., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Color‐based visual signals are important aspects of communication throughout the animal kingdom. Individuals evaluate color to obtain information about age and condition and to behave accordingly. Birds display a variety of striking, conspicuous colors and make ideal subjects for the study of color signaling. While most studies of avian color focus on plumage, bare unfeathered body parts also display a wide range of color signals. Mate choice and intrasexual competitive interactions are easily observed in lekking grouse, which also signal with prominent unfeathered color patches. Most male grouse have one pair of colorful bare part ornaments (combs), and males of several species also have inflatable air sacs in their throat. Previous studies have mostly focused on comb color and size, but little is known about the signaling role of air sac color. We measured comb size and the color properties of combs and air sacs in the Lesser and Greater Prairie‐Chickens (Tympanuchus pallidicinctus and T. cupido, respectively), and investigated whether these properties varied with age and mass. We found that mass predicted color properties of air sacs and that age predicted comb size in the Greater Prairie‐Chicken, suggesting that these ornaments indicate condition dependence. No conclusive relationships between color and age or size were detected in the Lesser Prairie‐Chicken. Color properties of both ornaments differed between the two species. Further research is needed to determine mechanisms that link condition to color and whether the information advertised by color signals from these ornaments is intended for males, females, or both. [ABSTRACT FROM AUTHOR]

Honest signaling mechanisms can function to appropriate care to hungry offspring and avoid misdirected care of unrelated offspring. Begging, the behavior by which offspring solicit food and parental care, may be an honest signaling mechanism for need, as well as association of parents and offspring. Roseate terns (Sterna dougallii) exhibit prolonged parental care during the postbreeding staging period, offering an ideal system in which to study begging as an honest signaling mechanism. We conducted focal sampling during two premigratory staging seasons (2014 and 2015) at Cape Cod National Seashore, Massachusetts, USA to determine whether postfledging tern begging behavior was an honest signal for need and parent–offspring association. Based on honest signaling theory, we expected begging behavior to be highest during times of high perceived need, and we expected to see a decrease in begging behavior as young terns became increasingly independent of the care‐giving parent. Also, we predicted that young terns would be more likely to beg at parents than nonparents. We found that young roseate terns begged at their parents more often than nonparents; however, they did not always beg at parents. Model predictions of begging probability showed a linear relationship between begging and time of day and date of season, such that begging increased with time of day and decreased with date of season, respectively. Our results provide evidence for honest parent–offspring interactions and are inconsistent with parent–offspring conflict theory but suggest that begging may play a complex role in postfledging parent–offspring interactions. OPEN RESEARCH BADGES: This article has been awarded Open Data, Open materials Badges. All materials and data are publicly accessible via the Open Science Framework at https://doi.org/10.5281/zenodo.2656718. [ABSTRACT FROM AUTHOR]

Advertisement signals can convey information about a sender's characteristics, such as body size. The reliability of signals, however, can be reduced when signal production is partially dependent on the environment. Here, we assess the effect of display-site properties on the production, attractiveness and honesty of sexual signals. We recorded male túngara frogs (Physalaemus pustulosus) while manipulating water levels in order to constrain calling. We found that water level affected male call [ABSTRACT FROM AUTHOR]

It has been argued that men's hunting in many forager groups is not, primarily, a means of family provisioning but is a costly way of signaling otherwise cryptic qualities related to hunting ability. Much literature concerning the signaling value of hunting draws links to Zahavi's handicap principle and the costly signaling literature in zoology. However, although nominally grounded in the same theoretical paradigm, these literatures have evolved separately. Here I review honest signaling theory in both hunter‐gatherer studies and zoology and highlight three issues with the costly signaling literature in hunter‐gather studies: (a) an overemphasis on the demonstration of realized costs, which are neither necessary nor sufficient to diagnose costly signaling; (b) a lack of clear predictions about what specific qualities hunting actually signals; and (c) an insufficient focus on the broadcast effectiveness of hunting and its value as a heuristics for signal recipients. Rather than signaling hunting prowess, hunting might instead facilitate reputation‐building. [ABSTRACT FROM AUTHOR]

Humans frequently perform extravagant and seemingly costly behaviors, such as widely sharing hunted resources, erecting conspicuous monumental structures, and performing dramatic acts of religious devotion. Evolutionary anthropologists and archeologists have used signaling theory to explain the function of such displays, drawing inspiration from behavioral ecology, economics, and the social sciences. While signaling theory is broadly aimed at explaining honest communication, it has come to be strongly associated with the handicap principle, which proposes that such costly extravagance is in fact an adaptation for signal reliability. Most empirical studies of signaling theory have focused on obviously costly acts, and consequently anthropologists have likely overlooked a wide range of signals that also promote reliable communication. Here, we build on recent developments in signaling theory and animal communication, developing an updated framework that highlights the diversity of signal contents, costs, contexts, and reliability mechanisms present within human signaling systems. By broadening the perspective of signaling theory in human systems, we strive to identify promising areas for further empirical and theoretical work. [ABSTRACT FROM AUTHOR]

There are many different sports, and some are more popular for watching than others. The present paper attempts to address the question why sports vary in popularity. To this end, an evolutionary framework is employed that indicates that sports have evolved to enable the reliable exchange of information of unobserved traits. Six of these traits are nominated, namely, physical strength, stamina, speed, dexterity, aggression and team spirit. On this basis, it is predicted that sports which require higher competence in these traits, and thus are better in transferring information on these dimensions, are more popular than sports which require less competence. Analysis on data based on 34 different sports supports this prediction. The implications of these finding are further discussed.

Sexual ornaments found only in females are a rare occurrence in nature. One explanation for this is that female ornaments are costly to produce and maintain and, therefore, females must trade‐off resources related to reproduction to promote ornament expression. Here, we investigate whether a trade‐off exists between female ornamentation and fecundity in the sex‐role reversed, wide‐bodied pipefish, Stigmatopora nigra. We measured two components of the disk‐shaped, ventral‐striped female ornament, body width, and stripe thickness. After controlling for the influence of body size, we found no evidence of a cost of belly width or stripe thickness on female fecundity. Rather, females that have larger ornaments have higher fecundity and thus accurately advertise their reproductive value to males without incurring a cost to fecundity. We also investigated the relationship between female body size and egg size and found that larger females suffer a slight decrease in egg size and fecundity, although this decrease was independent of female ornamentation. More broadly, considered in light of similar findings in other taxa, lack of an apparent fecundity cost of ornamentation in female pipefish underscores the need to revisit theoretical assumptions concerning the evolution of female ornamentation. In the wide‐bodied pipefish, females attract males with enormous striped bellies. Do females suffer a cost to offspring production by producing such an elaborate ornament? Larger females have larger ornaments that honestly signal their fecundity to males. [ABSTRACT FROM AUTHOR]

Abstract: Sexual ornaments contribute substantially to phenotypic diversity and it is particularly relevant to understand their evolution. Ornaments can assume the function of signals‐of‐quality that the choosy sex uses to evaluate potential mating partners. Often there are no obvious direct benefits and investment into mate choice is primarily rewarded by beneficial alleles that are inherited to the offspring. Inter‐sexual communication via sexual ornaments requires honesty of the sexual signal, yet the question of what maintains honesty remains only partially solved. One solution is that honesty is maintained by trait expression being dependent on individual condition, since condition‐dependent trait expression offers an effectively inexhaustible source of genetic variability. Here we test in the highly sexually dimorphic club‐legged grasshopper Gomphocerus sibiricus if putative sexual ornaments, in particular the striking front‐leg clubs, are more strongly affected by a lipopolysaccharide (LPS) immune challenge than putatively not sexually selected traits. Our results show overall little condition‐dependent expression of morphological and song traits, with sexually selected traits exhibiting effects comparable to nonsexually selected traits (with the possible exception of stridulatory file length and syllable‐to‐pause ratio in advertisement songs). Interestingly, field observations of individuals of lethally parasitized individuals suggest that a very strong environmental challenge can specifically affect the expression of the front‐leg clubs. The presence of 1% of males in natural populations with missing or heavily deformed clubs plus 5% with minor club deformations furthermore indicate that there are risks associated with club development during final ecdysis and this might act as a filter against deleterious alleles. [ABSTRACT FROM AUTHOR]

Abstract: The distinctive black and red wing pattern of six‐spot burnet moths (Zygaena filipendulae, L.) is a classic example of aposematism, advertising their potent cyanide‐based defences. While such warning signals provide a qualitatively honest signal of unprofitability, the evidence for quantitative honesty, whereby variation in visual traits could provide accurate estimates of individual toxicity, is more equivocal. Combining measures of cyanogenic glucoside content and wing color from the perspective of avian predators, we investigate the relationship between coloration and defences in Z. filipendulae, to test signal honesty both within and across populations. There were no significant relationships between mean cyanogenic glucoside concentration and metrics of wing coloration across populations in males, yet in females higher cyanogenic glucoside levels were associated with smaller and lighter red forewing markings. Trends within populations were similarly inconsistent with quantitative honesty, and persistent differences between the sexes were apparent: larger females, carrying a greater total cyanogenic glucoside load, displayed larger but less conspicuous markings than smaller males, according to several color metrics. The overall high aversiveness of cyanogenic glucosides and fluctuations in color and toxin levels during an individual's lifetime may contribute to these results, highlighting generally important reasons why signal honesty should not always be expected in aposematic species. [ABSTRACT FROM AUTHOR]

Sexual selection for honest behavioral displays of quality has driven the development of remarkably complex courtship behavior in many animal species. Olfactory displays are often overlooked as an area of inquiry compared to auditory and visual displays. Ring-tailed lemur ( Lemur catta) scent marking of substrates has been studied extensively, although the male olfactory displays of anointing and wafting tails to females has received relatively little attention. We studied the role of male olfactory displays to females, evaluating whether such signals function as honest, costly signals of male dominance status in two groups of wild L. catta at Berenty Reserve, Madagascar. Our results suggest that male tail anointing and tail wafting displays to pre-estrous and estrous females are correlated with male dominance rank, and moreover appear to operate as costly signals, as such displays increase aggression received from males and females while other types of scent marking do not. Furthermore, females showed greater mating preference (as measured by sexual presents) for resident males who performed the 'anoint tail' and 'waft tail' displays towards them. When males perform the 'anoint tail' and 'waft tail' displays to females, they receive higher levels of aggression than if they were to perform other types of scent marking. Interestingly, immigrating (peripheral) males performed the 'anoint tail' and 'waft tail' displays at higher rates than resident males, which could honestly indicate their quality or may simply be associated with the alternative mating strategy of transferring between groups to gain mating opportunities. Our finding that tail anointing and tail wafting displays function as honest signals of dominance for resident males-and that these costly displays appear to positively affect female mate choice-is the first evidence of this function for this particular olfactory signal in L. catta. [ABSTRACT FROM AUTHOR]

Vocal communication is a crucial aspect of animal behavior. The mechanism which most mammals use to vocalize relies on three anatomical components. First, air overpressure is generated inside the lower vocal tract. Second, as the airstream goes through the glottis, sound is produced via vocal fold vibration. Third, this sound is further filtered by the geometry and length of the upper vocal tract. Evidence from mammalian anatomy and bioacoustics suggests that some of these three components may covary with an animal's body size. The framework provided by acoustic allometry suggests that, because vocal tract length (VTL) is more strongly constrained by the growth of the body than vocal fold length (VFL), VTL generates more reliable acoustic cues to an animal's size. This hypothesis is often tested acoustically but rarely anatomically, especially in pinnipeds. Here, we test the anatomical bases of the acoustic allometry hypothesis in harbor seal pups Phoca vitulina. We dissected and measured vocal tract, vocal folds, and other anatomical features of 15 harbor seals post-mortem. We found that, while VTL correlates with body size, VFL does not. This suggests that, while body growth puts anatomical constraints on how vocalizations are filtered by harbor seals' vocal tract, no such constraints appear to exist on vocal folds, at least during puppyhood. It is particularly interesting to find anatomical constraints on harbor seals' vocal tracts, the same anatomical region partially enabling pups to produce individually distinctive vocalizations. [ABSTRACT FROM AUTHOR]

Telomere dynamics in natural populations have been linked to survival, reproduction, and energetic investment. Given their putative role in mediating life-history trade-offs, telomeres are also a likely candidate for maintaining honesty in sexually selected signals; few studies to date, however, have demonstrated a correlation between sexual signals and telomere dynamics. Here, we show that plumage coloration in male common yellowthroats ( Geothlypis trichas) is correlated with both relative telomere length and with the rate of telomere loss between years. Elevated antioxidant capacity is also associated with reduced telomere loss, but only among older males. Previous work in this population has demonstrated that males with brighter plumage are in better condition, have higher reproductive success, and are more likely to survive over winter. Thus, the signal attribute associated with mate choice in this system also conveys reliable information about telomere dynamics. At present, it is unclear whether telomere maintenance plays a causal role in maintaining signal honesty or whether the correlation arises due to underlying variation in individual resources or genotypes. We suggest that subsequent work should consider the possibility that fundamental trade-offs between signal investment and cell-level processes that influence aging and reproductive senescence may provide a foundation for understanding the maintenance of sexual signal honesty. [ABSTRACT FROM AUTHOR]

Selection for low male voice pitch is generally assumed to occur because it is a valid cue of formidability. Here we summarize recent empirical challenges to this hypothesis. We also outline an alternative account in which selection for low male voice pitch is a byproduct of sensory exploitation. [ABSTRACT FROM AUTHOR]

There are many different sports, and some are more popular for watching than others. The present paper attempts to address the question why sports vary in popularity. To this end, an evolutionary framework is employed that indicates that sports have evolved to enable the reliable exchange of information of unobserved traits. Six of these traits are nominated, namely, physical strength, stamina, speed, dexterity, aggression and team spirit. On this basis, it is predicted that sports which require higher competence in these traits, and thus are better in transferring information on these dimensions, are more popular than sports which require less competence. Analysis on data based on 34 different sports supports this prediction. The implications of these finding are further discussed. [ABSTRACT FROM AUTHOR]

Visual signals of ovulation vary among primate species. Although slight ovulatory signals are considered primate ancestral traits of which some species still exhibit, some show prominent swelling of their perineal skin (exaggerated sexual swellings) and others do not exhibit any signals of ovulation (concealed ovulation). These signals strongly affect male mating behaviors. I develop an evolutionary model of female ovulatory signals and male–male competition. I assume that each male allocates his effort between attraction of females and male–male competition for dominance. Each female gains a benefit if she is fertile and free from the alpha male who always guards one of the most fertile females in the group, but suffers a cost if she expresses a different ovulatory signal from an ancestral trait. I show that various types of ovulatory signals may evolve and can be multistable. Male–male competition becomes intense when the signal honestly indicates ovulation. Ovulatory signals may evolve to be less exaggerated in unimale groups than in multimale groups and monogamy is more likely to evolve when ovulation is concealed. These results may partly explain why various types of primate ovulatory signals evolved and how they have affected primate societies. [ABSTRACT FROM AUTHOR]

Queen pheromones mediate the reproductive division of labor in social insect colonies and provide novel opportunities for investigating the evolution of animal communication. Previous work found that queens in the ant genus Lasius produce several 3-methylalkanes in greater relative amounts than workers do. At least one of these (3-MeC) is a queen pheromone that regulates worker sterility in two Lasius species, although there are indications that other 3-methylalkanes might also function as queen pheromones. Here, we presented workers from three Lasius species with four different 3-methylalkanes, and measured the effect on worker ovary development. In all three species, only 3-MeC showed clear evidence of inhibiting worker fecundity. Our results suggest that worker ants can discriminate homologous hydrocarbons that differ in chain length and only treat specific homologs as queen pheromones. These results provide insight into the conflicting selective pressures on cuticular hydrocarbons arising from their multiple roles in signaling and adaptation to the abiotic environment. [ABSTRACT FROM AUTHOR]