Visual ecology: How glossy colours shine and confuse

Most colours in nature are matte, but across the tree of life glossiness has evolved numerous times, suggesting that glossiness can be beneficial. Recent research finds that glossiness may confuse observers and protect against predators.

Color in motion: Generating 3-dimensional multispectral models to study dynamic visual signals in animals

Analyzing color and pattern in the context of motion is a central and ongoing challenge in the quantification of animal coloration. Many animal signals are spatially and temporally variable, but traditional methods fail to capture this dynamism because they use stationary animals in fixed positions. To investigate dynamic visual displays and to understand the evolutionary forces that shape dynamic colorful signals, we require cross-disciplinary methods that combine measurements of color, pattern, 3-dimensional (3D) shape, and motion. Here, we outline a workflow for producing digital 3D models with objective color information from museum specimens with diffuse colors. The workflow combines multispectral imaging with photogrammetry to produce digital 3D models that contain calibrated ultraviolet (UV) and human-visible (VIS) color information and incorporate pattern and 3D shape. These “3D multispectral models” can subsequently be animated to incorporate both signaler and receiver movement and analyzed in silico using a variety of receiver-specific visual models. This approach—which can be flexibly integrated with other tools and methods—represents a key first step toward analyzing visual signals in motion. We describe several timely applications of this workflow and next steps for multispectral 3D photogrammetry and animation techniques.

Sexual selection for flight performance in hummingbirds

Among size-dimorphic animals, a few clades such as hummingbirds show “reversed” sexual size dimorphism: females tend to be the larger sex. What selects for this pattern? Sexual selection for flight performance could drive the evolution of smaller, more agile males, either for male-male combat or female choice for aerial courtship displays. Alternately, natural selection can select for female fecundity (e.g., egg size influences female body size), or sex differences in foraging niche could favor body size differences. The sexual selection hypotheses predict that dimorphism extends to other aspects of flight morphology (e.g., flight muscle size) whereas the natural selection hypotheses predict that male and female flight morphologies are isometric, and the niche differentiation hypothesis predicts that bill dimorphism is correlated with size dimorphism. We tested these predictions through phylogenetic comparative analyses of flight morphology, wingbeat frequency, and courtship behaviors, focused on 30 species within the “bee” hummingbird clade (tribe Mellisugini). There is no correlation between bill morphology and dimorphism. Relative to females, males tend to be smaller, have proportionately shorter wings and higher hovering wingbeat frequencies, but also longer keels and larger flight muscles. Male wingbeat frequencies are greatly elevated during aerial displays, and the species with the greatest wingbeat frequencies have the greatest dimorphism. Of the four hypotheses for dimorphism, the data best support the hypothesis that female choice for courtship displays has selected for aerial agility and small size in male hummingbirds.

Properties of an attention-grabbing motion signal: a comparison of tail and body movements in a lizard

Animals signals must be detected by receiver sensory systems, and overcome a variety of local ecological factors that could otherwise affect their transmission and reception. Habitat structure, competition, avoidance of unintended receivers and varying environmental conditions have all been shown to influence how animals signal. Environmental noise is also crucial, and animals modify their behavior in response to it. Animals generating movement-based visual signals have to contend with wind-blown plants that generate motion noise and can affect the detection of salient movements. The lizard Amphibolurus muricatus uses tail flicking at the start of displays to attract attention, and we hypothesized that tail movements are ideally suited to this function. We compared visual amplitudes generated by tail movements with push-ups, which are a key component of the rest of the display. We show that tail movement amplitudes are highly variable over the course of the display but consistently greater than amplitudes generated by push-ups and not constrained by viewing position. We suggest that these features, combined with the tail being a light structure that does not compromise other activities, provide an ideal introductory component for attracting attention in the ecological setting in which they are generated.

Feather iridescence of Coeligena hummingbird species varies due to differently organized barbs and barbules

Hummingbirds are perhaps the most exquisite bird species because of their prominent iridescence, created by stacks of melanosomes in the feather barbules. The feather colours crucially depend on the nanoscopic dimensions of the melanosome, and the displayed iridescence can distinctly vary, dependent on the spatial organization of the barbs and barbules. We have taken the genus Coeligena as a model group, with species having feathers that strongly vary in their spatial reflection properties. We studied the feather morphology and the optical characteristics. We found that the coloration of Coeligena hummingbirds depends on both the Venetian-blind-like arrangement of the barbules and the V-shaped, angular arrangement of the barbules at opposite sides of the barbs. Both the nanoscopic and microscopic organization of the hummingbird feather components determine the bird's macroscopic appearance.

How signaling geometry shapes the efficacy and evolution of animal communication systems

Animal communication is inherently spatial. Both signal transmission and signal reception have spatial biases-involving direction, distance and position-that interact to determine signaling efficacy. Signals, be they visual, acoustic, or chemical, are often highly directional. Likewise, receivers may only be able to detect signals if they arrive from certain directions. Alignment between these directional biases is therefore critical for effective communication, with even slight misalignments disrupting perception of signaled information. In addition, signals often degrade as they travel from signaler to receiver, and environmental conditions that impact transmission can vary over even small spatiotemporal scales. Thus, how animals position themselves during communication is likely to be under strong selection. Despite this, our knowledge regarding the spatial arrangements of signalers and receivers during communication remains surprisingly coarse for most systems. We know even less about how signaler and receiver behaviors contribute to effective signaling alignment over time, or how signals themselves may have evolved to influence and/or respond to these aspects of animal communication. Here, we first describe why researchers should adopt a more explicitly geometric view of animal signaling, including issues of location, direction, and distance. We then describe how environmental and social influences introduce further complexities to the geometry of signaling. We discuss how multimodality offers new challenges and opportunities for signalers and receivers. We conclude with recommendations and future directions made visible by attention to the geometry of signaling.

The Paradox of Iridescent Signals

Signals reliably convey information to a receiver. To be reliable, differences between individuals in signal properties must be consistent and easily perceived and evaluated by receivers. Iridescent objects are often striking and vivid, but their appearance can change dramatically with viewing geometry and illumination. The changeable nature of iridescent surfaces creates a paradox: how can they be reliable signals? We contend that iridescent color patches can be reliable signals only if accompanied by specific adaptations to enhance reliability, such as structures and behaviors that limit perceived hue shift or enhance and control directionality. We highlight the challenges of studying iridescence and key considerations for the evaluation of its adaptive significance.

Flies Exploit Predictable Perspectives and Backgrounds to Enhance Iridescent Signal Salience and Mating Success

Communication requires both the encoding of information and its effective transmission, but little is known about display traits that primarily serve to enhance efficacy. Here we examined the visual courtships of Lispe cana, a cursorial fly that lives and mates in heterogeneous foreshores, and tested the prediction that males should seek to enhance signal salience and consequent fitness through the flexible choice of display locations. We show that courting males access the field of view of females by straddling them and holding their wings closed before moving ahead to present their structurally colored faces in ritualized dances. Males preferentially present these UV-white signals against darker backgrounds and the magnitude of contrast predicts female attention, which in turn predicts mating success. Our results demonstrate a striking interplay between the physical and attentional manipulation of receivers and reveal novel routes to the enhancement of signal efficacy in noisy environments.

Plumage iridescence is associated with distinct feather microbiota in a tropical passerine

Birds present a stunning diversity of plumage colors that have long fascinated evolutionary ecologists. Although plumage coloration is often linked to sexual selection, it may impact a number of physiological processes, including microbial resistance. At present, the degree to which differences between pigment-based vs. structural plumage coloration may affect the feather microbiota remains unanswered. Using quantitative PCR and DGGE profiling, we investigated feather microbial load, diversity and community structure among two allopatric subspecies of White-shouldered Fairywren, Malurus alboscapulatus that vary in expression of melanin-based vs. structural plumage coloration. We found that microbial load tended to be lower and feather microbial diversity was significantly higher in the plumage of black iridescent males, compared to black matte females and brown individuals. Moreover, black iridescent males had distinct feather microbial communities compared to black matte females and brown individuals. We suggest that distinctive nanostructure properties of iridescent male feathers or different investment in preening influence feather microbiota community composition and load. This study is the first to point to structural plumage coloration as a factor that may significantly regulate feather microbiota. Future work might explore fitness consequences and the role of microorganisms in the evolution of avian sexual dichromatism, with particular reference to iridescence.

Synchronization of speed, sound and iridescent color in a hummingbird aerial courtship dive

Many animal signals are complex, often combining multimodal components with dynamic motion. To understand the function and evolution of these displays, it is vital to appreciate their spatiotemporal organization. Male broad-tailed hummingbirds (Selasphorus platycercus) perform dramatic U-shaped courtship dives over females, appearing to combine rapid movement and dive-specific mechanical noises with visual signals from their iridescent gorgets. To understand how motion, sound and color interact in these spectacular displays, we obtained video and audio recordings of dives performed by wild hummingbirds. We then applied a multi-angle imaging technique to estimate how a female would perceive the male's iridescent gorget throughout the dive. We show that the key physical, acoustic and visual aspects of the dive are remarkably synchronized-all occurring within 300 milliseconds. Our results highlight the critical importance of accounting for motion and orientation when investigating animal displays: speed and trajectory affect how multisensory signals are produced and perceived.

Quantitative characterization of iridescent colours in biological studies: a novel method using optical theory

Iridescent colours are colours that change with viewing or illumination geometry. While they are widespread in many living organisms, most evolutionary studies on iridescence do not take into account their full complexity. Few studies try to precisely characterize what makes iridescent colours special: their angular dependency. Yet, it is likely that this angular dependency has biological functions and is therefore submitted to evolutionary pressures. For this reason, evolutionary biologists need a repeatable method to measure iridescent colours as well as variables to precisely quantify the angular dependency. In this study, we use a theoretical approach to propose five variables that allow one to fully describe iridescent colours at every angle combination. Based on the results, we propose a new measurement protocol and statistical method to reliably characterize iridescence while minimizing the required number of time-consuming measurements. We use hummingbird iridescent feathers and butterfly iridescent wings as test cases to demonstrate the strengths of this new method. We show that our method is precise enough to be potentially used at intraspecific level while being also time-efficient enough to encompass large taxonomic scales.

Iridescent colouration of male Anna's hummingbird (Calypte anna) caused by multilayered barbules

The male Anna’s hummingbird features a brightly reddish-pink reflecting gorget, due to large stacks of melanosomes in the feather barbules, arranged in layers separated by keratin. Direct observations together with detailed scatterometry demonstrated that the barbules reflect incident light in an approximately specular manner. The structural colouration is iridescent, i.e. varies with a changing angle of light incidence. Spectrophotometrical measurements of the barbule reflectance and absorbance can be well interpreted with calculated spectra obtained with a transfer matrix method for optical multilayers, using anatomical data and measured refractive index spectra. The organization of the reflectors as a Venetian blind presumably functions to create a high spectral contrast of the male’s plumage during courtship.