Conspicuous colours reduce predation rates in fossorial uropeltid snakes

Temperature differently affects body pigmentation of the paper wasp Polistes dominula along an urban and a wider geographical gradient

In insects, different pigments, such as melanins and pterins, are involved in thermoregulation. The degree of melanisation often varies along geographical gradients, according to the so-called thermal melanism hypothesis, i.e. darker forms are found in colder places because they can warm up more quickly. Similarly, pterins work as heat sinks and thus are expected to be more abundant in colder sites. Cities, which are warmer than surrounding areas (Urban Heat Island (UHI) effect), might also be expected to influence pigmentation, although studies are lacking. Here, we sampled workers of the social paper wasp Polistes dominula (Christ, 1791) (Vespidae) across an urbanisation gradient in an Italian metropolis and used iNaturalist pictures of this species across Italy to study pigmentation patterns at both urban and larger geographical scales. We found a lower yellow intensity of abdominal spots at warmer locations. Scanning Electron Microscopy strongly suggested that yellow colouration is due xanthopterin, known to be the heat sink molecule in other social vespids. Thus, wasps from warmer (i.e., urban) environments are likely to have fewer xanthopterin granules, in line with the lack of need for heat storage due to the local thermal gradient (UHI effect). At the country level, we found that wasps at higher latitudes had smaller yellow spots on the thorax and only two spots instead of four at higher altitudes, in full accordance with the thermal melanism hypothesis. In conclusion, climatic conditions seem to affect insect colour patterns both along urban and wider geographical gradients, although colour changes may affect different body parts and pigments likely according to different needs.

Embracing the diversity in diverse warning signals

Positive frequency-dependent selection should theoretically lead to monomorphic warning coloration. Instead, numerous examples of polymorphic warning signals exist. Biases - for example, in human perception - hinder our appreciation and research of understanding warning signal diversity. We propose strategies to counter such biases and objectively move our field forward.

Correlated evolution of parental care with dichromatism, colors, and patterns in anurans

Parental care is widespread and has fitness benefits. But caregiving parents incur costs including higher predation, and this may lead to selection for body colors or patterns that help mitigate the risks of caring. The evolution of coloration, including sexual dichromatism, however, can be driven by other factors, such as sexual selection. Therefore, examining the associations between parental care and color patterns may provide key insights into evolutionary patterns and selection pressures for parental care. Our comparative analysis of 988 anuran species reveals that dichromatic species are less likely to provide parental care, irrespective of the caregiving sex, and are more likely to breed in aquatic habitats. We then examined whether dorsal colors and patterns that enhance crypticity or function as aposematic signals are associated with the caregiving sex, and the modality of care (transport or stationary). Only caregiving males are more likely to have dorsal Stripes, but none of the colors (Green-Brown, Red, Yellow, Blue-Black) and other patterns (Plain, Bands, Spots, Mottled-Patches) were associated with caregiving females or the modality of care. Overall, sexual dichromatism, breeding ecology, and parental care are associated, but the evolution of caregiving behavior does not appear to influence the myriad colors and patterns characteristic of anurans globally.

Conspicuous cruciform silk decorations deflect avian predator attacks

Although camouflage as an effective antipredator defence strategy is widespread across animals, highly conspicuous color patterning is not uncommon either. Many orb-web spiders adorn their webs with extra, bright white silk. These conspicuous decorations are hypothesized to deter predators by warning the presence of sticky webs, camouflaging spiders, acting as a decoy, or intimidating predators by their apparent size. The decorations may also deflect predator attacks from spiders. However, empirical evidence for this deflection function remains limited. Here we tested this hypothesis using the X-shaped silk cruciform decorations built by females of Argiope minuta. We employed visual modelling to quantify the conspicuousness of spiders and decorations from a perspective of avian predators. Then we determined actual predation risk on spiders using naïve chicks as predators. Spider bodies and decorations were conspicuous against natural backgrounds to the avian visual systems. Chicks attacked the spider main bodies significantly less frequently on the decorated webs than on the undecorated webs, thus reducing predation risk. When both spiders and decorations were present, chicks also attacked the spider main bodies and their legs or decorations, and not randomly: they attacked the legs or decorations sooner and more frequently than they attacked the main bodies, independence of the ratio of the surface area between the decoration and spider size. Despite the increase in detectability, incorporating a conspicuous cruciform decoration to the web effectively defends the spider by diverting the attack towards the decoration or leg, but not by camouflaging or intimidating, thus, supporting the deflection hypothesis. This article is protected by copyright. All rights reserved.

The effect of albinism on avian predator attack rates in eastern garter snakes

Albinism is a conspicuous and distinctive phenotype arising from the absence of melanin in the integument that has been documented in all major vertebrate groups. With few exceptions, albinism is rare in natural populations, suggesting that it incurs significant fitness costs as compared to wild-type phenotypes. One possible explanation for this rarity is that albinos experience higher predation risk as compared to wild-type individuals. We tested this hypothesis by comparing rates of attack by avian predators on immobile clay model proxies that mimicked wild-type and albino eastern garter snakes (Thamnophis s. sirtalis) at Bond Swamp in central Georgia, USA; two mixed pattern treatments provided intermediate phenotypes for comparison. Surprisingly, we found no difference in attack rate across all four model treatments, nor among pairs of treatments (i.e., wild-type-like vs. albino-like models). This indicates that albino garter snakes are not subject to higher predation risk from birds due to coloration alone, and that other factors (correlated pathologies of albinism, thermoregulatory challenges, mate selection) are likely to be more important in explaining the rarity of albinism in snakes and perhaps other vertebrate groups.

Untangling the structural and molecular mechanisms underlying colour and rapid colour change in a lizard, Agama atra

With functions as diverse as communication, protection and thermoregulation, coloration is one of the most important traits in lizards. The ability to change colour as a function of varying social and environmental conditions is thus an important innovation. While colour change is present in animals ranging from squids, to fish and reptiles, not much is known about the mechanisms behind it. Traditionally, colour change was attributed to migration of pigments, in particular melanin. More recent work has shown that the changes in nanostructural configuration inside iridophores are able to produce a wide palette of colours. However, the genetic mechanisms underlying colour, and colour change in particular, remain unstudied. Here we use a combination of transcriptomic and microscopic data to show that melanin, iridophores and pteridines are the main colour-producing mechanisms in Agama atra, and provide molecular and structural data suggesting that rapid colour change is achieved via melanin dispersal in combination with iridophore organization. This work demonstrates the power of combining genotypic (gene expression) and phenotypic (microscopy) information for addressing physiological questions, providing a basis for future studies of colour change.

Warning signals promote morphological diversification in fossorial uropeltid snakes (Squamata: Uropeltidae)

Many species possess warning colourations that signal unprofitability to predators. Warning colourations are also thought to provide prey with a ‘predator-free space’ and promote niche expansion. However, how such strategies release a species from environmental constraints and facilitate niche expansion is not clearly understood. Fossoriality in reptiles imposes several morphological limits on head and body size to facilitate burrowing underground, but many fossorial snakes live close to the surface and occasionally move above ground, exposing them to predators. In such cases, evolving antipredator defences that reduce predation on the surface could potentially relax the morphological constraints associated with fossoriality and promote morphological diversification. Fossorial uropeltid snakes possess varying degrees of conspicuous warning colourations that reduce avian predation when active above ground. We predicted that species with more conspicuous colourations will exhibit more robust body forms and show faster rates of morphological evolution because constraints imposed by fossoriality are relaxed. Using a comparative phylogenetic approach on the genus Uropeltis, we show that more conspicuous species tend to have more robust morphologies and have faster rates of head-shape evolution. Overall, we find that the evolution of warning colourations in Uropeltis can facilitate niche expansion by influencing rates of morphological diversification.