The carotenoid redshift: Physical basis and implications for visual signaling

Carotenoid pigments are the basis for much red, orange, and yellow coloration in nature and central to visual signaling. However, as pigment concentration increases, carotenoid signals not only darken and become more saturated but they also redshift; for example, orange pigments can look red at higher concentration. This occurs because light experiences exponential attenuation, and carotenoid-based signals have spectrally asymmetric reflectance in the visible range. Adding pigment disproportionately affects the high-absorbance regions of the reflectance spectra, which redshifts the perceived hue. This carotenoid redshift is substantial and perceivable by animal observers. In addition, beyond pigment concentration, anything that increases the path length of light through pigment causes this redshift (including optical nano- and microstructures). For example, male Ramphocelus tanagers appear redder than females, despite the same population and concentration of carotenoids, due to microstructures that enhance light-pigment interaction. This mechanism of carotenoid redshift has sensory and evolutionary consequences for honest signaling in that structures that redshift carotenoid ornaments may decrease signal honesty. More generally, nearly all colorful signals vary in hue, saturation, and brightness as light-pigment interactions change, due to spectrally asymmetrical reflectance within the visible range of the relevant species. Therefore, the three attributes of color need to be considered together in studies of honest visual signaling.

Flower Size as an Honest Signal in Royal Irises (Iris Section Oncocyclus, Iridaceae)

Flower traits, such as flower size or color changes, can act as honest signals indicating greater rewards such as nectar; however, nothing is known about shelter-rewarding systems. Large flowers of Royal irises offer overnight shelter as a reward to Eucera bees. A black patch might signal the entrance to the tunnel (shelter) and, together with the flower size, these might act as honest signals. We hypothesize that larger flowers and black patches indicate larger tunnels, and larger tunnels will increase pollinator visits, enhancing the plants' reproductive success. We measured seven species in a controlled environment and two species from three natural populations varying in flower size. Fruit and seed sets were assessed in these natural populations. We found a positive correlation between the flower, patch size, and tunnel volume, suggesting that the flowers and patch size act as honest signals, both under controlled conditions and in the wild. However, in natural populations, this positive relationship and its effect on fitness was population-specific. Flower size increased the fitness in YER I. petrana, and interactions between flower/patch size and tunnel size increased the fitness in YER and I. atropurpurea NET populations. This suggests that the honesty of the signal is positively selected in these two populations. This study supports the hypothesis that pollinator-mediated selection leads to the honest signaling of flower advertisement.

Negative body size-dependent resource allocation underlies conspicuous sexual ornaments in a territorial damselfly

Sexual ornaments, signalling individual quality to choosy females or rival males, often show steeper body size scaling compared with non-sexually selected traits. Theory posits such steeper body size scaling is the result of differential resource allocation, reflecting trade-offs between different components of fitness. Yet, the process of resource allocation towards body size-dependent sexual ornaments has been rarely understood empirically. Using the Neotropical territorial damselfly Megaloprepus caerulatus, whose males and females carry wax-based, sex-specific white wing bands and white wing tips respectively, we investigated nutrition sensitivity and body size scaling of both traits by manipulating larval food availability and directly quantified both the fat allocated to wing ornaments and the fat reserve from which allocations are made. Both colour traits exhibited sensitivity to food availability during larval development and steeper body size scaling compared with control traits. Although the absolute amount of fat invested in developing the colour ornaments increased with body size, the proportion of total fat allocated to the ornaments decreased with body size, making exaggerated ornaments less affordable for smaller individuals. Our data demonstrate that measuring the proportion of resource pool from which an individual's ornaments are derived (i.e. its affordability) is essential for understanding the maintenance of honesty of sexual signals.

A sensory bias overrides learned preferences of bumblebees for honest signals in Mimulus guttatus

Insect pollinators readily learn olfactory cues, and this is expected to select for 'honest signals' that provide reliable information about floral rewards. However, plants might alternatively produce signals that exploit pollinators' sensory biases, thereby relaxing selection for signal honesty. We examined the innate and learned preferences of Bombus impatiens for Mimulus guttatus floral scent phenotypes corresponding to different levels of pollen rewards in the presence and absence of the innately attractive floral volatile compound β-trans-bergamotene. Bees learned to prefer honest signals after foraging on live M. guttatus flowers, but only exhibited this preference when presented floral scent phenotypes that did not include β-trans-bergamotene. Our results suggest that a sensory bias for β-trans-bergamotene overrides the ability of B. impatiens to use honest signals when foraging on M. guttatus. This may represent a deceptive pollination strategy that allows plants to minimize investment in costly rewards without incurring reduced rates of pollinator visitation.

Intraspecific relationships between floral signals and rewards with implications for plant fitness

Within-species variation in traits such as petal size or colour often provides reliable information to pollinators about the rewards offered to them by flowers. In spite of potential disadvantages of allowing pollinators to discriminate against less-rewarding flowers, examples of informative floral signals are diverse in form and widely distributed across plant taxa, apparently having evolved repeatedly in different lineages. Although hypotheses about the adaptive value of providing reward information have been proposed and tested in a few cases, a unified effort to understand the evolutionary mechanisms favouring informative floral signals has yet to emerge. This review describes the diversity of ways in which floral signals can be linked with floral rewards within plant species and discusses the constraints and selective pressures on floral signal-reward relationships. It focuses particularly on how information about floral rewards can influence pollinator behaviour and how those behavioural changes may, in turn, affect plant fitness, selecting either for providing or withholding reward information. Most of the hypotheses about the evolution of floral signal-reward relationships are, as yet, untested, and the review identifies promising research directions for addressing these considerable gaps in knowledge. The advantages and disadvantages of sharing floral reward information with pollinators likely play an important role in floral trait evolution, and opportunities abound to further our understanding of this neglected aspect of floral signalling.

The Pastoral Origin of Semiotically Functional Tonal Organization of Music

This paper presents a new line of inquiry into when and how music as a semiotic system was born. Eleven principal expressive aspects of music each contains specific structural patterns whose configuration signifies a certain affective state. This distinguishes the tonal organization of music from the phonetic and prosodic organization of natural languages and animal communication. The question of music’s origin can therefore be answered by establishing the point in human history at which all eleven expressive aspects might have been abstracted from the instinct-driven primate calls and used to express human psycho-emotional states. Etic analysis of acoustic parameters is the prime means of cross-examination of the typical patterns of expression of the basic emotions in human music versus animal vocal communication. A new method of such analysis is proposed here. Formation of such expressive aspects as meter, tempo, melodic intervals, and articulation can be explained by the influence of bipedal locomotion, breathing cycle, and heartbeat, long before Homo sapiens. However, two aspects, rhythm and melodic contour, most crucial for music as we know it, lack proxies in the Paleolithic lifestyle. The available ethnographic and developmental data leads one to believe that rhythmic and directional patterns of melody became involved in conveying emotion-related information in the process of frequent switching from one call-type to another within the limited repertory of calls. Such calls are usually adopted for the ongoing caretaking of human youngsters and domestic animals. The efficacy of rhythm and pitch contour in affective communication must have been spontaneously discovered in new important cultural activities. The most likely scenario for music to have become fully semiotically functional and to have spread wide enough to avoid extinctions is the formation of cross-specific communication between humans and domesticated animals during the Neolithic demographic explosion and the subsequent cultural revolution. Changes in distance during such communication must have promoted the integration between different expressive aspects and generated the basic musical grammar. The model of such communication can be found in the surviving tradition of Scandinavian pastoral music - kulning. This article discusses the most likely ways in which such music evolved.

Structural colours reflect individual quality: a meta-analysis

Colourful ornaments often communicate salient information to mates, and theory predicts covariance between signal expression and individual quality. This has borne out among pigment-based signals, but the potential for 'honesty' in structural coloration is unresolved. Here, I synthesized the available evidence to test this prediction via meta-analysis and found that, overall, the expression of structurally coloured sexual signals is positively associated with individual quality. The effects varied by the measure of quality, however, with body condition and immune function reliably encoded across taxa, but not age nor parasite resistance. The relationship was apparent for both the colour and brightness of signals and was slightly stronger for iridescent ornaments. These results suggest diverse pathways to the encoding and exchange of information among structural colours while highlighting outstanding questions as to the development, visual ecology and evolution of this striking adornment.

Why has transparency evolved in aposematic butterflies? Insights from the largest radiation of aposematic butterflies, the Ithomiini

Defended species are often conspicuous and this is thought to be an honest signal of defences, i.e. more toxic prey are more conspicuous. Neotropical butterflies of the large Ithomiini tribe numerically dominate communities of chemically defended butterflies and may thus drive the evolution of mimetic warning patterns. Although many species are brightly coloured, most are transparent to some degree. The evolution of transparency from a warning-coloured ancestor is puzzling as it is generally assumed to be involved in concealment. Here, we show that transparent Ithomiini species are indeed less detectable by avian predators (i.e. concealment). Surprisingly, transparent species are not any less unpalatable, and may in fact be more unpalatable than opaque species, the latter spanning a larger range of unpalatability. We put forth various hypotheses to explain the evolution of weak aposematic signals in these butterflies and other cryptic defended prey. Our study is an important step in determining the selective pressures and constraints that regulate the interaction between conspicuousness and unpalatability.

Honey Bee Alarm Pheromone Mediates Communication in Plant-Pollinator-Predator Interactions

Honey bees play a crucial role in pollination, and in performing this critical function, face numerous threats from predators and parasites during foraging and homing trips. Back in the nest, their defensive behavior drives some individuals to sacrifice themselves while fighting intruders with their stingers or mandibles. During these intense conflicts, bees release alarm pheromone to rapidly communicate with other nest mates about the present danger. However, we still know little about why and how alarm pheromone is used in plant–pollinator–predator interactions. Here, we review the history of previously detected bee alarm pheromones and the current state of the chemical analyses. More new components and functions have been confirmed in honey bee alarm pheromone. Then, we ask how important the alarm pheromones are in intra- and/or inter-species communication. Some plants even adopt mimicry systems to attract either the pollinators themselves or their predators for pollination via alarm pheromone. Pheromones are honest signals that evolved in one species and can be one of the main driving factors affecting co-evolution in plant–pollinator–predator interactions. Our review intends to stimulate new studies on the neuronal, molecular, behavioral, and evolutionary levels in order to understand how alarm pheromone mediates communication in plant–pollinator–predator interactions.

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

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

Experimental stress during molt suggests the evolution of condition-dependent and condition-independent ornaments in the king penguin

Sexual selection and social selection are two important theories proposed for explaining the evolution of colorful ornamental traits in animals. Understanding signal honesty requires studying how environmental and physiological factors during development influence the showy nature of sexual and social ornaments. We experimentally manipulated physiological stress and immunity status during the molt in adult king penguins (Aptenodytes patagonicus), and studied the consequences of our treatments on colourful ornaments (yellow‐orange and UV beak spots and yellow‐orange auricular feather patches) known to be used in sexual and social contexts in this species. Whereas some ornamental features showed strong condition‐dependence (yellow auricular feather chroma, yellow and UV chroma of the beak), others were condition‐independent and remained highly correlated before and after the molt (auricular patch size and beak UV hue). Our study provides a rare examination of the links between ornament determinism and selection processes in the wild. We highlight the coexistence of ornaments costly to produce that may be honest signals used in mate choice, and ornaments for which honesty may be enforced by social mediation or rely on genetic constraints.

Variation in reward quality and pollinator attraction: the consumer does not always get it right

Nearly all bees rely on pollen as the sole protein source for the development of their larvae. The central importance of pollen for the bee life cycle should exert strong selection on their ability to locate the most rewarding sources of pollen. Despite this importance, very few studies have examined the influence of intraspecific variation in pollen rewards on the foraging decisions of bees. Previous studies have demonstrated that inbreeding reduces viability and hence protein content in Mimulus guttatus (seep monkeyflower) pollen and that bees strongly discriminate against inbred in favour of outbred plants. We examined whether variation in pollen viability could explain this preference using a series of choice tests with living plants, artificial plants and olfactometer tests using the bumble bee Bombus impatiens. We found that B. impatiens preferred to visit artificial plants provisioned with fertile anthers over those provisioned with sterile anthers. They also preferred fertile anthers when provided only olfactory cues. These bumble bees were unable to discriminate among live plants from subpopulations differing dramatically in pollen viability, however. They preferred outbred plants even when those plants were from subpopulations with pollen viability as low as the inbred populations. Their preference for outbred plants was evident even when only olfactory cues were available. Our data showed that bumble bees are able to differentiate between anthers that provide higher rewards when cues are isolated from the rest of the flower. When confronted with cues from the entire flower, their choices are independent of the quality of the pollen reward, suggesting that they are responding more strongly to cues unassociated with rewards than to those correlated with rewards. If so, this suggests that a sensory bias or some level of deception may be involved with advertisement to pollinators in M. guttatus.

Pheromonal control: reconciling physiological mechanism with signalling theory

Pheromones are intraspecific chemical signals. They can have profound effects on the behaviour and/or physiology of the receiver, and it is still common to hear pheromones described as controlling of the behaviour of the receiver. The discussion of pheromonal control arose initially from a close association between hormones and pheromones in the comparative physiological literature, but the concept of a controlling pheromone is at odds with contemporary signal evolution theory, which predicts that a manipulative pheromonal signal negatively affecting the receiver's fitness should not be stable over evolutionary time. Here we discuss the meaning of pheromonal control, and the ecological circumstances by which it might be supported. We argue that in discussing pheromonal control it is important to differentiate between control applied to the effects of a pheromone on a receiver's physiology (proximate control), and control applied to the effects of a pheromone on a receiver's fitness (ultimate control). Critically, a pheromone signal affecting change in the receiver's behaviour or physiology need not necessarily manipulate the fitness of a receiver. In cases where pheromonal signalling does lead to a reduction in the fitness of the receiver, the signalling system would be stable if the pheromone were an honest signal of a social environment that disadvantages the receiver, and the physiological and behavioural changes observed in the receiver were an adaptive response to the new social circumstances communicated by the pheromone.

Colored nectar as an honest signal in plant-animal interactions

Many flowering plants obtain the services of pollinators by using their floral traits as signals to advertise the rewards they offer to visitors-such as nectar, pollen and other food resources. Some plants use colorful pigments to draw pollinators' attention to their nectar, instead of relying on the appeal of nectar taste. Although this rare floral trait of colored nectar was first recorded by the Greek poet Homer in the Odyssey, it has only recently received the attention of modern science. This mini-review focuses on recent findings about some of the species that use colored nectar; topics include its function as an honest signal for pollinators, as well as the pigments responsible for the nectar coloration. Such research of the ecology and physiology of colored nectar expands our understanding of the role and evolution of pollinator signaling in plants.

Sex-biased parental investment is correlated with mate ornamentation in eastern bluebirds

Males typically have greater variance in reproductive success than females, so mothers should benefit by producing sons under favorable conditions. Being paired with a better-than-average mate is one such favorable circumstance. High-quality fathers can improve conditions for their offspring by providing good genes, good resources, or both, so females paired to such males should invest preferentially in sons. Ornamentation has been linked to male quality in many birds and, in support of differential allocation theory, females of several avian species invest more in entire broods when paired to attractive mates. Additionally, the females of some bird species apparently manipulate the primary sex-ratio of their broods in relation to the attractiveness of their mates. However, empirical support for a link between mate ornamentation and preferential feeding of sons (another form of biased investment) is lacking. We tested for correlations between sex-biased parental investment and mate plumage colour in the eastern bluebird (Sialia sialis), a species in which juveniles have sexually dichromatic UV-blue plumage. We found that the proportion of maternal feeding attempts to fledgling sons (versus fledgling daughters) was positively correlated with structurally coloured plumage ornamentation of fathers. Additionally, paternal feeding attempts to sons were correlated with plumage ornamentation of mothers and increased in fathers exhibiting breast plumage characteristics typical of older males. These results provide further support for the idea that parental strategies are influenced by mate attractiveness and provide the first evidence that mate ornamentation can influence parental behavior even after offspring have left the nest.