Pattern mimicry of host eggs by the common cuckoo, as seen through a bird's eye

Scorpionfish adjust skin pattern contrast on different backgrounds

The two scorpionfish species Scorpaena maderensis and S. porcus are well camouflaged ambush predators that rapidly change body colouration to adjust to background colour in less than 1 min. We tested whether individuals of both species also adjust body pattern to that of the background. We placed fish on backgrounds of different pattern granularity and quantified the change in fish body pattern over 1 min. We used calibrated image analysis to analyse the patterns from the visual perspective of a prey fish species using a granularity (pattern energy) analysis and an image clustering approach. In our experiment, fish did not change their most contrasting pattern components as defined by the dominant marking size, but changed their average marking size. Moreover, fish responded with a change in pattern in contrast to the different experimental backgrounds, especially when compared to the acclimation phase. These results indicate that scorpionfish have one main pattern that can be adjusted by modulating its internal contrast. A reduction in pattern contrast could thereby improve background matching, while an increase could promote camouflage via disruptive colouration.

Predator selection on phenotypic variability of cryptic and aposematic moths

Natural selection generally favours phenotypic variability in camouflaged organisms, whereas aposematic organisms are expected to evolve a more uniform warning coloration. However, no comprehensive analysis of the phenotypic consequences of predator selection in aposematic and cryptic species exists. Using state-of-the-art image analysis, we examine 2800 wing images of 82 moth species accessed via three online museum databases. We test whether anti-predator strategy (i.e., camouflage or aposematism) explains intraspecific variation in wing colour and pattern across northern hemisphere moths. In addition, we test two mutually non-exclusive, ecological hypotheses to explain variation in colour pattern: diel-activity or dietary-niche. In this work, taking into account phylogenetic relationships, moth phenotypic variability is best explained by anti-predator strategy with camouflaged moths being more variable in wing patterning than aposematic species.

Evolution of Plumage Patterns in a Pattern Morphospace: A Phylogenetic Analysis of Melanerpine Woodpeckers

AbstractPlumage patterns of melanerpine (Melanerpes-Sphyrapicus) woodpeckers are strikingly diverse. Understanding the evolution and function of this diversity is challenging because of the difficulty of quantifying plumage patterns. We use a three-dimensional space to characterize the evolution of melanerpine achromatic plumage patterns. The axes of the space are three pattern features (spatial frequency, orientation, and contrast) quantified using two-dimensional fast Fourier transformation of museum specimen images. Mapping plumage in pattern space reveals differences in how species and subclades occupy the space. To quantify these differences, we derive two new measures of pattern: pattern diversity (diversity across plumage patches within a species) and pattern uniqueness (divergence of patterns from those of other species). We estimate that the melanerpine ancestor had mottled plumage and find that pattern traits across patches and subclades evolve at different rates. We also find that smaller species are more likely to display horizontal face patterning. We promote pattern spaces as powerful tools for investigating animal pattern evolution.

Brood parasitism and egg recognition in three bunting hosts of the cuckoos

Comparative studies of egg recognition and rejection between various sympatric hosts provide insight into the coevolutionary history of the hosts and parasites, as well as the degree of antagonism between the species. Although buntings are widely considered to be a suitable host taxon for cuckoos, there has been relatively little research on this example of parasitism and host antiparasitic behaviour. Here we provided the first report on brood parasitism and egg recognition in three sympatric ground-nesting bunting hosts of the common cuckoo (Cuculus canorus), namely the yellow-throated bunting (Emberiza elegans), south rock bunting (E. yunnanensis), and crested bunting (E. lathami). The results show that for the five breeding seasons during 2018-2022, the parasitism rate by common cuckoos was 0.87% and 0.45% in yellow-throated buntings and south rock buntings, respectively, whereas the parasitism rate by an unidentified parasite was 4% during 2018-2023 in the crested bunting. The rejection rates of the three bunting hosts for blue non-mimetic eggs were 89.3%, 88.9%, and 100% for yellow-throated buntings, south rock buntings, and crested buntings, respectively. The rejection rates for red non-mimetic eggs by yellow-throated buntings and south rock buntings were lower at 76.9% and 82.4%, respectively. All three sympatric bunting hosts examined had high levels of egg recognition and egg rejection, suggesting that it may have been subjected to high parasitic history and that egg recognition ability was retained after the loss of parasitism, which needs to be further verified by future experiments.

Evolutionary Origins and Patterns of Diversification in Animal Brood Parasitism

AbstractBrood parasitism involves the exploitation of host parental care rather than the extraction of resources directly from hosts. We identify defining characteristics of this strategy and consider its position along continua with adjacent behaviors but focus on canonical brood parasites, where parasitism is obligate and hosts are noneusocial (thereby distinguishing from social parasitism). A systematic literature survey revealed 59 independently derived brood parasitic lineages with most origins (49) in insects, particularly among bees and wasps, and other origins in birds (seven) and fish (three). Insects account for more than 98% of brood parasitic species, with much of that diversity reflecting ancient (≥100-million-year-old) brood parasitic lineages. Brood parasites usually, but not always, evolve from forms that show parental care. In insects, brood parasitism often first evolves through exploitation of a closely related species, following Emery's rule, but this is less typical in birds, which we discuss. We conducted lineage-level comparisons between brood parasitic clades and their sister groups, finding mixed results but an overall neutral to negative effect of brood parasitism on species richness and diversification. Our review of brood parasites reveals many unanswered questions requiring new research, including further modeling of the coevolutionary dynamics of brood parasites and their hosts.

Gradual changes in model shape affect egg-directed behaviours by parasitic shiny cowbirds Molothrus bonariensis in captivity

Eggs are critically important for avian reproduction as all birds are oviparous. Accordingly, the recognition and care of own eggs represent the cornerstones of avian breeding, whereas the elimination of foreign objects, including brood-parasitic eggs and non-egg items from the nest are known to also increase fitness by refocusing incubation effort on the parents' own eggs. But egg recognition also plays a role in some avian obligate brood parasites' reproductive strategy through the pecking of already present eggs in the hosts' clutch to reduce nestmate competition with the parasite's own hatchling. Here we tested egg shape recognition in this parasitic egg-pecking context by exposing two different series of 3D printed models to captive obligate brood-parasitic shiny cowbirds (Molothrus bonariensis) in artificial nests. Natural egg-shaped models were pecked more often compared with increasingly thinner models, but there was no effect of increasing angularity on pecking rates, implying that a natural, rather than an artificial, range of variability elicited adaptive responses from parasitic cowbirds.

Experimental evidence that cuckoos choose host nests following an egg matching strategy

The arms race between brood parasites and their hosts provides a classic model to study coevolution. Hosts often reject the parasitic egg, and brood parasites should therefore select host nests in which the colour of the eggs best matches that of their own. Although this hypothesis has received some support, direct experimental evidence is still lacking. Here, we report on a study of Daurian redstarts, which show a distinct egg-colour dimorphism, with females laying either blue or pink eggs. Redstarts are often parasitized by common cuckoos, which lay light blue eggs. First, we showed that cuckoo eggs were more similar in spectral reflectance to the blue than to the pink redstart egg morph. Second, we report that the natural parasitism rate was higher in blue than in pink host clutches. Third, we performed a field experiment in which we presented a dummy clutch of each colour morph adjacent to active redstart nests. In this set-up, cuckoos almost always chose to parasitize a blue clutch. Our results demonstrate that cuckoos actively choose redstart nests in which the egg colour matches the colour of their own eggs. Our study thus provides direct experimental evidence in support of the egg matching hypothesis.

Polymorphism at the nestling stage and host-specific mimicry in an Australasian cuckoo-host arms race

Decades of research have shown that the coevolutionary arms race between avian brood parasites and their hosts can promote phenotypic diversification in hosts and brood parasites. However, relatively little is known about the role of brood parasitism in promoting phenotypic diversification of nestlings. We review field data collected over four decades in Australia, New Caledonia and New Zealand to assess potential for coevolutionary interactions between the shining bronze-cuckoo (Chalcites lucidus) and its hosts, and how diversification at the nestling stage may be generating different subspecies. The shining bronze-cuckoo is a specialist parasite of a few hosts in the family Acanthizidae. It has diversified into subspecies, of which the nestlings closely mimic the respective host nestlings in each region. Additionally, some cuckoo subspecies have polymorphic nestlings. The Acanthizidae hosts have similar breeding and nesting habits and only moderately effective frontline defences against parasitism at cuckoo egg laying or at the egg stages. However, some hosts have developed highly effective defences at the nestling stage by recognising and ejecting cuckoo nestlings from the nest. As with the cuckoo nestlings, some hosts have polymorphic nestlings. The coevolutionary interactions in each region suggest different evolutionary stages of the arms race in which either the parasite or the host is currently in the lead. The presence of moderately effective defences at the egg laying and egg stages might explain why some hosts do not have defences at the nestling stage. The south-Pacific cuckoo - host systems are excellent models to explore the evolutionary mechanisms driving the diversification at the nestling stage in the coevolutionary arms race between avian brood parasites and their hosts.

Recognition of Coat Pattern Variation and Broken Tail Phenomenon in the Asiatic Golden Cat (Catopuma temminckii)

Simple Summary

A variety of new survey technologies are continuously being developed and used in wildlife monitoring. Rapidly advancing and widely used camera trap survey technology has helped to capture data and gain insights into many species. The Eastern Himalayas is a global biodiversity hotspot with exceptionally high species diversity. The Asian golden cat is widely distributed in the Yarlung Zangbo Grand Canyon National Nature Reserve. It inhabits seasonal rain forests from 100 m above sea level to the Rhododendron forest up to 3500 m above sea level. Coat pattern variation in the Asian golden cat is particularly prominent in this region. The common color type is the most widely distributed, followed by nine other types. We found 10 coat pattern variations and two coat patterns with a broken tail made up 0.32% of independent photos taken during a long-term nine-year monitoring program. The variation in coat patterns is indicative of the geography of the region. Environmental conditions regulate and activate the genetic diversity of Asian golden cat phenotypes. This study further strengthened the understanding of the basic knowledge of golden cat color types and lays the foundation for exploring the diversity of golden cat color types at the molecular level.

Abstract

The Asian golden cat (Catopuma temminckii) is the most varied wild cat species in terms of coat color. Understanding coat pattern variation will help to elucidate the mechanisms behind it as well as its relationship with the environment. We conducted long-term (2013–2021) monitoring of Asian golden cats in the Yarlung Zangbo Grand Canyon National Nature Reserve, Tibet, using camera traps at 283 points over 89,991 camera days. A total of 620 cat photos were recorded, including 344 (55.48%) with recognizable color patterns. Vector graphics of the coat patterns were extracted from the field image data, which revealed 10 color types in the ratio common: cinnamon: reddish-brown long hair: ocelot: blackening: melanistic: gray: brown: brown short hair: pure black = 123:76:57:35:22:8:7:7:5:4. The genes for coat pattern variation are widespread in the Asian golden cat population and are relatively stable. The increase in population size intraspecific competition has led to the tail break phenotype in individual cats. The gene encoding for tail breakage in Asian golden cats remains unknown. This study provides basic information for understanding faunal diversity in the Eastern Himalayan biodiversity hotspot and serves as a reference for studies on the formation mechanisms for feline color pattern diversity.

Egg rejection based on egg size recognition as a specific strategy against parasitic cuckoos

In the co-evolutionary interactions between avian brood parasites and their hosts, egg recognition based on color and/or pattern is a common and effective defense to counter parasitism. However, for egg recognition based on size, only a few studies have found affirmative results, and they do not provide unambiguous evidence that egg size recognition in hosts has evolved as an important and specific anti-parasite adaptation against parasite eggs. We studied the brood parasite system between the Asian emerald cuckoo Chrysococcyx maculatus and its host, the chestnut-crowned warbler Phylloscopus castaniceps. The cuckoo parasitizes the warbler using non-mimetic and larger eggs with a parasitism rate of 12.9%. The warbler nests used in this experiment were built in a dark environment with the nest illuminance near 0 lux. Experiments with two types of model eggs with colors and patterns resembling cuckoo eggs different sizes (cuckoo egg size or host egg size) showed that the warblers were able to reject 63.6% of cuckoo model eggs under these dim light conditions. However, model eggs with the same color and pattern similar to the warbler egg size were always accepted. This study provides strong evidence supporting the theory that egg size recognition can be evolve in hosts as a specific anti-parasite adaptation against cuckoos. We suggest that the egg size recognition of the warbler is an outcome of the tradeoff between the costs of violating the parental investment rule and suffering cuckoo parasitism.

Defensive adaptations to cuckoo parasitism in the black-browed reed warbler (Acrocephalus bistrigiceps): recognition and mechanism

Hosts of avian brood parasites suffer a high cost of reproductive loss due to parasitism, driving them to evolve a variety of anti-parasitic defenses. These defenses comprise a series of components, including the recognition of brood parasites and the eggs laid by the parasites, cues used for recognition, and the mechanisms on which these behaviors are based. In this study, we conducted egg recognition and nest intruder experiments to examine these components of anti-parasitic behavior in the black-browed reed warbler (Acrocephalus bistrigiceps), a rare host of the common cuckoo (Cuculus canorus). We found that the host possessed strong recognition capacity, rejecting 100% of parasitic eggs, and used a template-based mechanism for egg recognition. The host birds also rejected 80% of their own eggs on which artificial markings were added to the blunt pole; however, they accepted all eggs with the same manipulation on the sharp pole, implying that the blunt pole was an important recognition cue. Furthermore, the host exhibited stronger aggression to cuckoos than to harmless controls; a behavior specific to the incubation stage rather than the nestling stage. Therefore, the host was able to distinguish the cuckoo from other nest intruders as being a brood parasite. These results together help explain the near absence of cuckoo parasitism in black-browed reed warblers and provide new information concerning anti-parasitic defenses in this host species.

Genetic colour variation visible for predators and conspecifics is concealed from humans in a polymorphic moth

The definition of colour polymorphism is intuitive: genetic variants express discretely coloured phenotypes. This classification is, however, elusive as humans form subjective categories or ignore differences that cannot be seen by human eyes. We demonstrate an example of a ‘cryptic morph’ in a polymorphic wood tiger moth (Arctia plantaginis), a phenomenon that may be common among well‐studied species. We used pedigree data from nearly 20,000 individuals to infer the inheritance of hindwing colouration. The evidence supports a single Mendelian locus with two alleles in males: WW and Wy produce the white and yy the yellow hindwing colour. The inheritance could not be resolved in females as their hindwing colour varies continuously with no clear link with male genotypes. Next, we investigated if the male genotype can be predicted from their phenotype by machine learning algorithms and by human observers. Linear discriminant analysis grouped male genotypes with 97% accuracy, whereas humans could only group the yy genotype. Using vision modelling, we also tested whether the genotypes have differential discriminability to humans, moth conspecifics and their bird predators. The human perception was poor separating the genotypes, but avian and moth vision models with ultraviolet sensitivity could separate white WW and Wy males. We emphasize the importance of objective methodology when studying colour polymorphism. Our findings indicate that by‐eye categorization methods may be problematic, because humans fail to see differences that can be visible for relevant receivers. Ultimately, receivers equipped with different perception than ours may impose selection to morphs hidden from human sight.

Identity signaling, identity reception, and the evolution of social recognition in a Neotropical frog

Animals recognize familiar individuals to perform a variety of important social behaviors. Social recognition is often mediated by communication between signalers who produce signals that contain identity information and receivers who categorize these signals based on previous experience. We tested two hypotheses about adaptations in signalers and receivers that enable the evolution of social recognition using two species of closely related territorial poison frogs. Male golden rocket frogs (Anomaloglossus beebei) recognize the advertisement calls of conspecific territory neighbors and display a "dear enemy effect" by responding less aggressively to neighbors than strangers, whereas male Kai rocket frogs (Anomaloglossus kaiei) do not. Our results did not support the identity signaling hypothesis: both species produced advertisement calls that contain similar amounts of identity information. Our results did support the identity reception hypothesis: both species exhibited habituation of aggression to playbacks simulating the arrival of a new neighbor, but only golden rocket frogs showed renewed aggression when they subsequently heard calls from a different male. These results suggest that an ancestral mechanism of plasticity in aggression common among frogs has been modified through natural selection to be specific to calls of individual males in golden rocket frogs, enabling a social recognition system.

Analysis of Egg Variation and Foreign Egg Rejection in Rüppell’s Weaver (Ploceus galbula)

Egg appearance is notable for its variation and as a source of recognition cues in bird species that are subject to egg-mimicking brood parasitism. Here I analyze the egg appearance of an East African weaverbird species that has variable eggs and is a host of brood parasitism by an egg-mimicking cuckoo, in order to (1) compare population variation to variation within a clutch as a measure of the distinctiveness of eggs; (2) assess modularity versus correlation among egg appearance traits as an indication of the complexity of egg signatures; and (3) address whether the eggs are discretely polymorphic or continuously variable in appearance. I also compare three methods of assessing egg coloration: reduction of spectral data to orthogonal components, targeted spectral shape variables, and avian visual modeling. Then I report the results of egg replacement experiments that assess the relationship between egg rejection behavior and the difference in appearance between own and foreign eggs. Rüppell’s weaver (Ploceus galbula) eggs are variable in appearance between individuals and consistent within a clutch, but vary widely in the distinctiveness of particular traits. Most aspects of color and spotting are decoupled from each other, including coloration likely to derive from different pigments. Egg ground color is bimodal, with a broad continuous class of off-white/UV eggs and another broad class of blue-green eggs. Variation in all other traits is unimodal and usually normal in distribution. Females reject foreign eggs on the basis of the difference in brightness of the ground color and spotting of foreign eggs relative to their own, and the difference in degree to which spots are aggregated at the broad end of the egg. This aggregation is among the most distinctive features of their eggs, but the brightness of the ground color and spotting brightness are not; the birds’ use of brightness rather than the more distinctive chromatic variation to recognize eggs might reflect the salience of achromatic contrast in a dim enclosed nest.

Host and brood parasite coevolutionary interactions covary with comparative patterns of the avian visual system

In coevolutionary arms-races, reciprocal ecological interactions and their fitness impacts shape the course of phenotypic evolution. The classic example of avian host–brood parasite interactions selects for host recognition and rejection of increasingly mimetic foreign eggs. An essential component of perceptual mimicry is that parasitic eggs escape detection by host sensory systems, yet there is no direct evidence that the avian visual system covaries with parasitic egg recognition or mimicry. Here, we used eye size measurements collected from preserved museum specimens as a metric of the avian visual system for species involved in host–brood parasite interactions. We discovered that (i) hosts had smaller eyes compared with non-hosts, (ii) parasites had larger eyes compared with hosts before but not after phylogenetic corrections, perhaps owing to the limited number of independent evolutionary origins of obligate brood parasitism, (iii) egg rejection in hosts with non-mimetic parasitic eggs positively correlated with eye size, and (iv) eye size was positively associated with increased avian-perceived host–parasite eggshell similarity. These results imply that both host-use by parasites and anti-parasitic responses by hosts covary with a metric of the visual system across relevant bird species, providing comparative evidence for coevolutionary patterns of host and brood parasite sensory systems.

Ecology and behavior predict an evolutionary trade-off between song complexity and elaborate plumages in antwrens (Aves, Thamnophilidae)

The environment can impose constraints on signal transmission properties such that signals should evolve in predictable directions (Sensory Drive Hypothesis). However, behavioral and ecological factors can limit investment in more than one sensory modality leading to a trade-off in use of different signals (Transfer Hypothesis). In birds, there is mixed evidence for both sensory drive and transfer hypothesis. Few studies have tested sensory drive while also evaluating the transfer hypothesis, limiting understanding of the relative roles of these processes in signal evolution. Here, we assessed both hypotheses using acoustic and visual signals in male and female antwrens (Thamnophilidae), a species-rich group that inhabits diverse environments and exhibits behaviors, such as mixed-species flocking, that could limit investment in different signal modalities. We uncovered significant effects of habitat (sensory drive) and mixed-species flocking behavior on both sensory modalities, and we revealed evolutionary trade-offs between song and plumage complexity, consistent with the transfer hypothesis. We also showed sex- and trait-specific responses in visual signals that suggest both natural and social selection play an important role in the evolution of sexual dimorphism. Altogether, these results support the idea that environmental (sensory drive) and behavioral pressures (social selection) shape signal evolution in antwrens.

Ecological basis and genetic architecture of crypsis polymorphism in the desert clicker grasshopper (Ligurotettix coquilletti)

Color polymorphic species can offer exceptional insight into the ecology and genetics of adaptation. Although the genetic architecture of animal coloration is diverse, many color polymorphisms are associated with large structural variants and maintained by biotic interactions. Grasshoppers are notably polymorphic in both color and karyotype, which makes them excellent models for understanding the ecological drivers and genetic underpinnings of color variation. Banded and uniform morphs of the desert clicker grasshopper (Ligurotettix coquilletti) are found across the western deserts of North America. To address the hypothesis that predation maintains local color polymorphism and shapes regional crypsis variation, we surveyed morph frequencies and tested for covariation with two predation environments. Morphs coexisted at intermediate frequencies at most sites, consistent with local balancing selection. Morph frequencies covaried with the appearance of desert substrate-an environment used only by females-suggesting that ground-foraging predators are major agents of selection on crypsis. We next addressed the hypothesized link between morph variation and genome structure. To do so, we designed an approach for detecting inversions and indels using only RADseq data. The banded morph was perfectly correlated with a large putative indel. Remarkably, indel dominance differed among populations, a rare example of dominance evolution in nature.

Variations of Hawk Mimicry Traits in the Four Sympatric Cuculus Cuckoos

A well-known visual signal, hawk-like features such as yellow eyes and feet, and barred underparts have been recognized as coevolutionary traits obtained against host defense in Cuculus cuckoos. However, the variation of these traits within and among species remains poorly understood because empirical studies quantifying these traits are limited in terms of the number of studies and the number of species concerned, and mostly depend on museum collections. In this study, we quantified and compared these traits as well as other new features (e.g., inner wing spot and underpart background color) in the four sympatric Cuculus cuckoos (Cuculus poliocephalus, Cuculus micropterus, Cuculus optatus, and Cuculus canorus) that were wild-captured in South Korea. We found that the yellow color of the eye ring and feet was fairly consistent across the four species. However, the iris color appeared to vary within a species (e.g., between sexes) and varied more substantially among species from nearly black in C. micropterus to bright yellow in C. canorus. In addition, there were significant differences among species with respect to the thickness of the underpart bars, from the thinnest in C. canorus to the thickest in C. micropterus. We also found that the underpart color (pure white versus yellowish brown) and the number of inner wing spots varied within and among species. These results indicate that although hawk-like traits are widely present in Cuculus cuckoos, detailed quantitative features of these traits vary across species. We discuss the potential reasons that generate such variations and suggest future directions to increase our understanding of visual signals in avian brood parasitism.

No evidence of host-specific egg mimicry in Asian koels

Avian brood parasitism is costly for the host, in many cases leading to the evolution of defenses like discrimination of parasitic eggs. The parasite, in turn, may evolve mimetic eggs as a counter-adaptation to host egg rejection. Some generalist parasites have evolved host-specific races (gentes) that may mimic the eggs of their main hosts, while others have evolved ‘jack-of-all-trades’ egg phenotypes that mimic key features of the eggs of several different host species. The Asian koel (Eudynamys scolopaceus) is a widely distributed generalist brood parasite that exploits a wide range of host species. Based on human vision, previous studies have described Asian koel eggs as resembling those of its main host, the house crow (Corvus splendens). Using measurements of egg length and breadth, digital image analysis, reflectance spectrophotometry and avian visual modelling, we examined Asian koel egg variation and potential mimicry in egg size and shape, and eggshell pattern and color in three sympatrically occurring host species in Bangladesh: the common myna (Acridotheres tristis), house crow, and long-tailed shrike (Lanius schach). We found some differences among Asian koel eggs laid in different host nests: a) Asian koel eggs in long-tailed shrike nests were larger than those laid in common myna and house crow nests, and b) Asian koel eggs in house crow nests were less elongated than those in common myna nests. However, these changes in Asian koel egg volume and shape were in the opposite direction with respect to their corresponding host egg characteristics. Thus, our study found no evidence for Asian koel host-specific egg mimicry in three sympatrically occurring host species.

Hosts elevate either within-clutch consistency or between-clutch distinctiveness of egg phenotypes in defence against brood parasites

In host-parasite arms races, hosts can evolve signatures of identity to enhance the detection of parasite mimics. In theory, signatures are most effective when within-individual variation is low ('consistency'), and between-individual variation is high ('distinctiveness'). However, empirical support for positive covariation in signature consistency and distinctiveness across species is mixed. Here, we attempt to resolve this puzzle by partitioning distinctiveness according to how it is achieved: (i) greater variation within each trait, contributing to elevated 'absolute distinctiveness' or (ii) combining phenotypic traits in unpredictable combinations ('combinatorial distinctiveness'). We tested how consistency covaries with each type of distinctiveness by measuring variation in egg colour and pattern in two African bird families (Cisticolidae and Ploceidae) that experience mimetic brood parasitism. Contrary to predictions, parasitized species, but not unparasitized species, exhibited a negative relationship between consistency and combinatorial distinctiveness. Moreover, regardless of parasitism status, consistency was negatively correlated with absolute distinctiveness across species. Together, these results suggest that (i) selection from parasites acts on how traits combine rather than absolute variation in traits, (ii) consistency and distinctiveness are alternative rather than complementary elements of signatures and (iii) mechanistic constraints may explain the negative relationship between consistency and absolute distinctiveness across species.

Interpopulation variation in sexual dichromatism in the Neotropical grasshopper Sphenarium purpurascens (Orthoptera: Pyrgomorphidae)

Cryptic coloration is an adaptative defensive mechanism against predators. Colour patterns appear cryptic through general background coloration matching or disruptive coloration. Disruptive coloration might evolve in visually heterogeneous microhabitats, whereas background matching could be favoured in chromatically homogeneous microhabitats. In this study, we used digital photography to explore the potential use of disruptive coloration and background matching in males and females of the Neotropical grasshopper Sphenarium purpurascens in different habitats. We found chromatic differences in three habitats and sexual dichromatism that might be explained by local adaptation. Although females and males were sexually dichromatic, interpopulation differences were found in the magnitude of the sexual dichromatism. In a highly contrasting environment, both males and females seemed to follow a disruptive strategy, whereas in a heterogeneous environments males and females followed different colour cryptic strategies, in which males were more disruptive than females, and females exhibited high background matching with fewer disruptive elements. Selective predation in different microhabitats and differences in mobility between the sexes might explain the colour pattern divergence between females and males.

Egg recognition: The importance of quantifying multiple repeatable features as visual identity signals

Brood parasitized and/or colonial birds use egg features as visual identity signals, which allow parents to recognize their own eggs and avoid paying fitness costs of misdirecting their care to others’ offspring. However, the mechanisms of egg recognition and discrimination are poorly understood. Most studies have put their focus on individual abilities to carry out these behavioural tasks, while less attention has been paid to the egg and how its signals may evolve to enhance its identification. We used 92 clutches (460 eggs) of the Eurasian coot Fulica atra to test whether eggs could be correctly classified into their corresponding clutches based only on their external appearance. Using SpotEgg, we characterized the eggs in 27 variables of colour, spottiness, shape and size from calibrated digital images. Then, we used these variables in a supervised machine learning algorithm for multi-class egg classification, where each egg was classified to the best matched clutch out of 92 studied clutches. The best model with all 27 explanatory variables assigned correctly 53.3% (CI = 42.6–63.7%) of eggs of the test-set, greatly exceeding the probability to classify the eggs by chance (1/92, 1.1%). This finding supports the hypothesis that eggs have visual identity signals in their phenotypes. Simplified models with fewer explanatory variables (10 or 15) showed lesser classification ability than full models, suggesting that birds may use multiple traits for egg recognition. Therefore, egg phenotypes should be assessed in their full complexity, including colour, patterning, shape and size. Most important variables for classification were those with the highest intraclutch correlation, demonstrating that individual recognition traits are repeatable. Algorithm classification performance improved by each extra training egg added to the model. Thus, repetition of egg design within a clutch would reinforce signals and would help females to create an internal template for true recognition of their own eggs. In conclusion, our novel approach based on machine learning provided important insights on how signallers broadcast their specific signature cues to enhance their recognisability.

Identifying female phenotypes that promote behavioral isolation in a sexually dimorphic species of fish Etheostoma zonale

In sexually dimorphic species characterized by exaggerated male ornamentation, behavioral isolation is often attributed to female preferences for conspecific male signals. Yet, in a number of sexually dimorphic species, male mate choice also results in behavioral isolation. In many of these cases, the female traits that mediate species boundaries are unclear. Females in sexually dimorphic species typically lack many of the elaborate traits that are present in males and that are often used for taxonomic classification of species. In a diverse and largely sexually dimorphic group of fishes called darters (Percidae: Etheostoma), male mate choice contributes to behavioral isolation between a number of species; however, studies addressing which female traits males prefer are lacking. In this study, we identified the dominant female pattern for two sympatric species, Etheostoma zonale and Etheostoma barrenense, using pattern energy analysis, and we used discriminate function analysis to identify which aspects of female patterning can reliably classify species. We then tested the role of female features in male mate choice for E. zonale, by measuring male preference for computer animations displaying the identified (species-specific) conspecific features. We found that the region above the lateral line is important in mediating male mate preferences, with males spending a significantly greater proportion of time with animations exhibiting conspecific female patterning in this region than with animations exhibiting heterospecific female patterning. Our results suggest that the aspects of female phenotypes that are the target of male mate choice are different from the conspicuous male phenotypes that traditionally characterize species.

Different ontogenetic trajectories of body colour, pattern and crypsis in two sympatric intertidal crab species

Animals frequently exhibit great variation in appearance, especially in heterogeneous habitats where individuals can be concealed differentially against backgrounds. Although background matching is a common anti-predator strategy, gaps exist in our understanding of within- and among-species variation. Specifically, the drivers of changes in appearance associated with habitat use and occurring through ontogeny are poorly understood. Using image analysis, we tested how individual appearance and camouflage in two intertidal crab species, the mud crab Panopeus americanus and the mottled crab Pachygrapsus transversus, relate to ontogeny and habitat use. We predicted that both species would change appearance with ontogeny, but that resident mud crabs would exhibit higher background similarity than generalist mottled crabs. Both species showed ontogenetic changes; the mud crabs became darker, whereas mottled crabs became more green. Small mud crabs were highly variable in colour and pattern, probably stemming from the use of camouflage in heterogeneous habitats during the most vulnerable life stage. Being habitat specialists, mud crabs were better concealed against all backgrounds than mottled crabs. Mottled crabs are motile and generalist, occupying macroalgae-covered rocks when adults, which explains why they are greener and why matches to specific habitats are less valuable. Differential habitat use in crabs can be associated with different coloration and camouflage strategies to avoid predation.

More than noise: context-dependent luminance contrast discrimination in a coral reef fish (Rhinecanthus aculeatus)

Achromatic (luminance) vision is used by animals to perceive motion, pattern, space and texture. Luminance contrast sensitivity thresholds are often poorly characterised for individual species and are applied across a diverse range of perceptual contexts using over-simplified assumptions of an animal's visual system. Such thresholds are often estimated using the receptor noise limited model (RNL). However, the suitability of the RNL model to describe luminance contrast perception remains poorly tested. Here, we investigated context-dependent luminance discrimination using triggerfish (Rhinecanthus aculeatus) presented with large achromatic stimuli (spots) against uniform achromatic backgrounds of varying absolute and relative contrasts. 'Dark' and 'bright' spots were presented against relatively dark and bright backgrounds. We found significant differences in luminance discrimination thresholds across treatments. When measured using Michelson contrast, thresholds for bright spots on a bright background were significantly higher than for other scenarios, and the lowest threshold was found when dark spots were presented on dark backgrounds. Thresholds expressed in Weber contrast revealed lower thresholds for spots darker than their backgrounds, which is consistent with the literature. The RNL model was unable to estimate threshold scaling across scenarios as predicted by the Weber-Fechner law, highlighting limitations in the current use of the RNL model to quantify luminance contrast perception. Our study confirms that luminance contrast discrimination thresholds are context dependent and should therefore be interpreted with caution.

Visual acuity and egg spatial chromatic contrast predict egg rejection behavior of American robins

Color and spatial vision is critical for recognition and discrimination tasks affecting fitness, including finding food and mates, and recognizing offspring. For example, as a counter defense to avoid the cost of raising the unrelated offspring of obligate interspecific avian brood parasites, many host species routinely view, recognize and remove the foreign egg(s) from their nests. Recent research has shown that host species visually attend to both chromatic and spatial pattern features of eggs; yet how hosts simultaneously integrate these features together when recognizing eggs remains an open question. Here, we tested egg rejection responses of American robins (Turdus migratorius) using a range of 3D-printed model eggs covered with blue and yellow checkered patterns differing in relative square sizes. We predicted that robins would reject a model egg if they could visually resolve the blue and yellow squares as separate features, or accept it if the squares blended together and appeared similar in color to the natural blue-green color of robin eggs as perceived by the avian visual system. As predicted, the probability of robins rejecting a model egg increased with greater sizes of its blue and yellow squares. Our results suggest that chromatic visual acuity and viewing distance have the potential to limit the ability of a bird to recognize a foreign egg in its nest, thus providing a limitation to host egg recognition that obligate interspecific avian brood parasites may exploit.

From micro- to macroevolution: brood parasitism as a driver of phenotypic diversity in birds

A fundamental question in biology is how diversity evolves and why some clades are more diverse than others. Phenotypic diversity has often been shown to result from morphological adaptation to different habitats. The role of behavioral interactions as a driver of broadscale phenotypic diversity has received comparatively less attention. Behavioral interactions, however, are a key agent of natural selection. Antagonistic behavioral interactions with predators or with parasites can have significant fitness consequences, and hence act as strong evolutionary forces on the phenotype of species, ultimately generating diversity between species of both victims and exploiters. Avian obligate brood parasites lay their eggs in the nests of other species, their hosts, and this behavioral interaction between hosts and parasites is often considered one of the best examples of coevolution in the natural world. In this review, we use the coevolution between brood parasites and their hosts to illustrate the potential of behavioral interactions to drive evolution of phenotypic diversity at different taxonomic scales. We provide a bridge between behavioral ecology and macroevolution by describing how this interaction has increased avian phenotypic diversity not only in the brood parasitic clades but also in their hosts.

Scattering of ultraviolet light by avian eggshells

Eggshells are essential for the reproduction of birds since the optical properties of shells may have an impact on biological functions such as heating and UV protection, recognition by parents or camouflage. Whereas ultraviolet reflection by some bird eggshells has been recently described, its physical origin remains poorly understood. In this study, we identified a porous structure in eggshells. Using Mie scattering modelling, we found it was most likely responsible for reflectance peaks (intensities of ca. 20-50%) observed in the near-UV range. These peaks were observed by spectrophotometric measurements from eggshells of several breeds of hen, one breed of duck and one breed of quail. This optical response was interpreted in terms of the distinct visual perception of hens and humans: eggshells appearing achromatic for humans proved to be chromatic for hens. Fluorescence emission from these eggs was also characterised and attributed to the presence of protoporphyrin IX and biliverdin IXα in the shells. Electron microscopy observations revealed the presence of pores within the so-called calcified shell part (i.e., at depths between ca. 20 μm and ca. 240 μm from the eggshell's outer surface). Mercury intrusion porosimetry allowed us to quantify the pore size distribution. Simulations of the UV response of this porous structure using Mie scattering theory as well as an effective approach accounting for multiple scattering indicate that these pores are responsible for the backscattering peaks observed in the UV range, in the case of beige hen eggshells. Due to the similarities between the pore size distributions observed for beige hen eggshells and other investigated poultry eggshells, we expect Mie backscattering to be the origin of the UV response of the eggshells of many other bird species.

Expanding the eggshell colour gamut: uroerythrin and bilirubin from tinamou (Tinamidae) eggshells

To date, only two pigments have been identified in avian eggshells: rusty-brown protoporphyrin IX and blue-green biliverdin IXα. Most avian eggshell colours can be produced by a mixture of these two tetrapyrrolic pigments. However, tinamou (Tinamidae) eggshells display colours not easily rationalised by combination of these two pigments alone, suggesting the presence of other pigments. Here, through extraction, derivatization, spectroscopy, chromatography, and mass spectrometry, we identify two novel eggshell pigments: yellow–brown tetrapyrrolic bilirubin from the guacamole-green eggshells of Eudromia elegans, and red–orange tripyrrolic uroerythrin from the purplish-brown eggshells of Nothura maculosa. Both pigments are known porphyrin catabolites and are found in the eggshells in conjunction with biliverdin IXα. A colour mixing model using the new pigments and biliverdin reproduces the respective eggshell colours. These discoveries expand our understanding of how eggshell colour diversity is achieved. We suggest that the ability of these pigments to photo-degrade may have an adaptive value for the tinamous.

Avian egg and nestling detection in the wild: should we rely on visual models or behavioural experiments?

The fields of avian egg and nestling colour detection have rapidly advanced owing to the application of visual models, which have allowed assessing of evolutionary questions considering receiver perception. Here, I first review the literature aiming to identify patterns of avian visual model usage. Second, I elaborate on limitations in the application of the receptor-noise limited perceptual (RNL hereafter) model. A systematic literature review revealed that the RNL model was the most used approach (81% of studies) in the field, and that most studies (76%) were concerned with classic evolutionary questions in avian brood parasitism. Some known limitations of the RNL model deal with model assumptions and parameterization, or, a poor consideration of post-detection neural processes. Others, however, are specific of the fields of egg and nestling discrimination and deal with the highly variable nature of ambient light at the nests, the complex colour design of eggs and nestlings, the multi-dimensional nature of perception, and the possible implication of learning. I, therefore, conclude that visual models should be used with caution to establish inference about egg and nestling discrimination, and rather be used to provide reasonable hypotheses which need to be validated with behavioural experiments. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Signal detection and optimal acceptance thresholds in avian brood parasite-host systems: implications for egg rejection

Signal detection theory addresses the challenge of successfully identifying informative signals in noisy information contexts, allowing optimal behavioural decisions in diverse ecological contexts. The optimal acceptance threshold hypothesis proposed by Reeve (Reeve 1989 Am. Nat. 133, 407-435. (doi:10.1086/284926)) is an elegant theoretical model to predict the flexibility of acceptance thresholds for conspecific discrimination. This model has provided a robust framework used to explore recognition systems in a broad range of contexts such as animal communication, nest-mate discrimination or anti-parasitic host responses. In this review, we discuss key concepts related to the optimal acceptance threshold hypothesis applied to egg rejection decisions in avian brood parasite-host interactions. We explore those factors determining signal detectability in parasitized nests and how hosts adjust their rejection decisions to both the risk of parasitism and the potential costs associated with egg rejection. In addition, we discuss recent results that challenge some traditional assumptions of the optimal acceptance threshold hypothesis and provide a novel perspective to explore rejection decisions, such as the existence of single-threshold decision rules or acceptance decisions. An integrative view combining current evidence with traditional theory is needed to further advance the comprehension of optimal acceptance thresholds. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Egg patterns as identity signals in colonial seabirds: a comparison of four alcid species

The ability to recognize mates, kin, offspring and neighbors by their individually distinctive traits-individual recognition (IR)-is widespread in animals. Much work has investigated IR from the perspective of the recognizer, but less is known about the extent to which signals have evolved to facilitate IR. To explore this, one approach is to compare putative identity signals among species that differ in life history and extent of IR. In Common Murres (Uria aalge), a colonially breeding seabird, the eggs of individual females are remarkably variable in terms of color and pattern (maculation). Common Murres also appear to recognize their own eggs, leading to the hypothesis that variable egg phenotypes evolved to promote recognizability. However, we lack a quantitative assessment of the egg pattern information in Common Murres and their close relatives. Here, we analyzed images of eggs laid by four alcid species: Common Murres, Thick-billed Murres (Uria lomvia), Razorbills (Alca torda) and Dovekies (Alle alle). We extracted pattern measures believed to be relevant to bird vision and calculated Beecher's information statistic (H_s ), which allowed us to compare the amount of identity information contained in each species' egg patterns. Murres, which nest in dense colonies and can recognize their own eggs, have egg patterns with a relatively large amount of identity information compared to Razorbills and Dovekies. Egg recognition has not been demonstrated in Razorbills and Dovekies, whose colonies are less dense. Our results are consistent with the hypothesis that complex patterns of Murre eggs may have evolved to increase individual recognizability.

Camouflage accuracy in Sahara-Sahel desert rodents

Camouflage helps animals to hide from predators and is therefore key to survival. Although widespread convergence of animal phenotypes to their natural environment is well-established, there is a lack of knowledge about how species compromise camouflage accuracy across different background types in their habitat. Here we tested how background matching has responded to top-down selection by avian and mammalian predators using Sahara-Sahel desert rodents in North Africa. We show that the fur colouration of several species has become an accurate match to different types of desert habitats. This is supported by a correlation analysis of colour and pattern metrics, investigation of animal-to-background similarities at different spatial scales and is confirmed by modelling of two predator vision systems. The background match was closest across large (or global) spatial scales, suggesting a generalist camouflage tactic for many background types. Some species, may have a better match to the background over small (or focal) spatial scales, which could be the result of habitat choices or differential predation. Nevertheless, predicted discrimination distances of fur colouration were virtually indistinguishable for mammalian and low for avian vision model, which implies effective camouflage. Our study provides one of the best documented cases of multilevel camouflage accuracy in geographically widespread taxa. We conclude that background matching has become an effective and common adaptation against predatory threat in Sahara-Sahelian desert rodents.

Convergent evolution of reduced eggshell conductance in avian brood parasites

Brood parasitism has evolved independently in several bird lineages, giving rise to strikingly similar behavioural adaptations that suggest convergent evolution. By comparison, convergence of physiological traits that optimize this breeding strategy has received much less attention, yet these species share many similar physiological traits that optimize this breeding strategy. Eggshell structure is important for embryonic development as it controls the flux of metabolic gases, such as O₂, CO₂ and H₂O, into and out of the egg; in particular, water vapour conductance ( G_H2O) is an essential process for optimal development of the embryo. Previous work has shown that common cuckoos ( Cuculus canorus) have a lower than expected eggshell G_H2O compared with their hosts. Here, we sought to test whether this is a trait found in other independently evolved avian brood parasites, and therefore reflects a general adaptation to a parasitic lifestyle. We analysed G_H2O for seven species of brood parasites from four unique lineages as well as for their hosts, and combined this with species from the literature. We found lower than expected G_H2O among all our observed brood parasites both compared with hosts (except for brown-headed cowbirds ( Molothrus ater)) and compared with the expected rates given their phylogenetic positions. These findings suggest that a lowered G_H2O may be a general adaptation for brood parasitism, perhaps helping the parasite nestling to develop greater aerobic fitness. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Higher-level pattern features provide additional information to birds when recognizing and rejecting parasitic eggs

Despite a recent explosion of research on pattern recognition, in both neuroscience and computer vision, we lack a basic understanding of how most animals perceive and respond to patterns in the wild. Avian brood parasites and their hosts provide an ideal study system for investigating the mechanisms of pattern recognition. The cuckoo finch, Anomalospiza imberbis, and its host the tawny-flanked prinia, Prinia subflava, lay highly polymorphic eggs with a great deal of variation in colour and patterning, with the cuckoo finch capable of close egg mimicry. Behavioural experiments in Zambia have previously shown that prinias use colour and multiple 'low-level' (occurring in early stages of visual processing) pattern attributes, derived from spatial frequency analysis, when rejecting foreign eggs. Here, we explore the extent to which host birds might also use 'higher-level' pattern attributes, derived from a feature detection algorithm, to make rejection decisions. Using a SIFT-based pattern recognition algorithm, NaturePatternMatch, we show that hosts are more likely to reject a foreign egg if its higher-level pattern features-which capture information about the shape and orientation of markings-differ from those of the host eggs. A revised statistical model explains about 37% variance in egg rejection behaviour, and differences in colour, low-level and higher-level pattern features all predict rejection, accounting for 42, 44 and 14% of the explained variance, respectively. Thus, higher-level pattern features provide a small but measurable improvement to the original model and may be especially useful when colour and low-level pattern features provide hosts with little information. Understanding the relative importance of low- and higher-level pattern features is a valuable goal for future work on animal coloration, especially in the contexts of mimicry, camouflage and individual recognition. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Egg retrieval versus egg rejection in cuckoo hosts

Before complex nests evolved, birds laid eggs on the ground, and egg retrieval evolved as an adaptation against accidental displacement of eggs outside the nest. Therefore, egg retrieval is an ancient, and likely ancestral, widespread behaviour in birds. However, it has received little attention in studies of avian brood parasitism, perhaps because most parasitism occurs in species with complex nests, a context in which egg retrieval seems irrelevant. However, for cavity-nesting hosts of avian brood parasites, egg retrieval may still play an important role in the coevolutionary interactions between obligate brood parasites and hosts, because egg retrieval can be considered to be antagonistic to egg rejection behaviour in hosts, yet both may involve cognition to recognize eggs. We hypothesized that (1) cavity-nesting hosts should retrieve misplaced eggs from outside the nest cup, (2) brood parasitism has modulated egg retrieval behaviour in cavity-nesting hosts and (3) hosts use the same visual cues for decision-making during egg recognition in both egg retrieval and egg rejection actions. To test these hypotheses, we performed a series of experiments in a cavity-nesting host, the green-backed tit ( Parus monticolus). Foreign eggs with different levels of mimicry were placed within or outside nest cups of hosts to test their responses. We found that host decisions about whether to retrieve or reject an egg both depended on the degree of mimicry. However, hosts sometimes first retrieved poorly mimetic foreign eggs and then rejected them. Alternatively, hosts sometimes failed to retrieve highly mimetic conspecific eggs. We suggest that egg retrieval in hosts is likely to be a result of the interaction between ancient retrieval behaviour and subsequent adaptation against brood parasitism. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Background matching, disruptive coloration, and differential use of microhabitats in two neotropical grasshoppers with sexual dichromatism

Cryptic coloration is an adaptative defensive mechanism against predators. Color patterns can become cryptic through background coloration-matching and disruptive coloration. Disruptive coloration may evolve in visually heterogeneous microhabitats, whereas background matching could be favored in chromatically homogeneous microhabitats. In this work, we used digital photography to explore the potential use of disruptive coloration and background matching in males and females of two grasshopper species of the Sphenarium genus in different habitats. We found chromatic differences in the two grasshopper species that may be explained by local adaptation. We also found that the females and males of both species are dichromatic and seem to follow different color cryptic strategies, males are more disruptive than females, whereas females have a high background matching with less disruptive elements. The selective pressures of the predators in different microhabitats and the differences in mobility between sexes may explain the color pattern divergence between females and males. Nevertheless, more field experiments are needed in order to understand the relative importance of disruptive and background matching coloration in the evolution of sexual dichromatism in these grasshoppers.

Differences in signal contrast and camouflage among different colour variations of a stomatopod crustacean, Neogonodactylus oerstedii

Animal colouration is often a trade-off between background matching for camouflage from predators, and conspicuousness for communication with con- or heterospecifics. Stomatopods are marine crustaceans known to use colour signals during courtship and contests, while their overall body colouration may provide camouflage. However, we have little understanding of how stomatopods perceive these signals in their environment or whether overall body coloration does provide camouflage from predators. Neogonodactylus oerstedii assess meral spot colour during contests, and meral spot colour varies depending on local habitat. By calculating quantum catch for N. oerstedii's 12 photoreceptors associated with chromatic vision, we found that variation in meral spot total reflectance does not function to increase signal contrast in the local habitat. Neogonodactylus oerstedii also show between-habitat variation in dorsal body colouration. We used visual models to predict a trichromatic fish predator's perception of these colour variations. Our results suggest that sandy and green stomatopods are camouflaged from a typical fish predator in rubble fields and seagrass beds, respectively. To our knowledge, this is the first study to investigate signal contrast and camouflage in a stomatopod. These results provide new insight into the function and evolution of colouration in a species with a complex visual system.

Ultraviolet reflectance and pattern properties in leopard geckos (Eublepharis macularius)

Complex visual signaling through various combinations of colors and patterns has been well documented in a number of diurnal reptiles. However, there are many nocturnal species with highly sensitive vision, being able to discriminate colors in night conditions, as was shown in geckos. Because of their sensitivity to chromatic signals, including UV (ultraviolet), they may have potential hidden features in their coloration, which may play role in intraspecific communication (e.g. mate choice) or interspecific signals (e.g. antipredatory function). We explored this hypothesis in nocturnal Leopard geckos (Eublepharis macularius), a species using visual signals in both antipredation defense and courtship, having ontogenetic color change accompanied by a shift in behavior. We used UV photography and visual modeling in order to compare various aspects of their coloration (luminance, contrast, color proportions) between sexes, age groups and populations. We found that Leopard geckos have considerable UV reflectance in white patches on their tails (and on the head in juveniles). Though, no prominent differences were detected in their coloration between various groups. We hypothesize that the limitation of UV reflectance to the head and tail, which are both actively displayed during defense, especially in juveniles, might potentially boost the effect of antipredation signaling.

Interacting Effects of Eyespot Number and Ultraviolet Reflectivity on Predation Risk in Bicyclus anynana (Lepidoptera: Nymphalidae)

Small marginal eyespots on lepidopteran wings are conspicuous elements that attract a predator's attention to deflect attacks away from the body, but the role of ultraviolet (UV) reflectivity at the center of these patterns and variation in eyespot number in altering the function of eyespots remains unclear. Here, we performed a field-based predation experiment with artificial prey items based on the appearance of squinting bush brown butterflies Bicyclus anynana (Butler, 1879). We tested how two visual properties of the wing pattern affect predation risk: i) the number of eyespots on the ventral forewing surface-two or four; and ii) the UV reflectivity of eyespot centers-normal (where the UV reflectivity of the centers contrasts strongly with that of the darker surrounding ring) or blocked (where this contrast is reduced). In total, 807 prey items were deployed at two sites. We found a significant interaction between the number of ventral forewing eyespots and UV reflectivity in the eyespot centers: in items with fewer eyespots, blocking UV resulted in increased predation risk whereas in items with more eyespots, blocking UV resulted in decreased predation risk. If higher predation of paper models can be equated with higher levels of wing margin/eyespot conspicuity, these results demonstrate that UV reflectivity is an important factor in making eyespots more conspicuous to predators and suggest that the fitness of particular butterfly eyespot number variants may depend on the presence or absence of UV in their centers and on the ability of local predator guilds to detect UV.

Complete mitochondrial genome and the phylogenetic position of the common cuckoo, Cuculus canorus bakeri (Aves: Cuculiformes)

The complete mitochondrial genome of Common Cuckoo (Cuculus canorus bakeri) is determined in this study and was deposited in Genbank with accession number MN067867. The length of the mitogenome is 17,457 base pairs (bp) including 13 protein-coding genes (PCGs), 22 transfer (t RNA) RNA genes, two ribosomal RNA (r RNA) genes, one control region (CR) and one short pseudo-control region. The structure is similar to mitochondrial genome of other Cuculiforme species. Phylogenetic tree shows that C. canorus bakeri is closely related to C. poliocephalus. The study has provided useful information for further studies on the evolution, genetic diversity, and phylogenetic relationships in Common Cuckoo.

Anti-parasitic egg rejection by great reed warblers (Acrocephalus arundinaceus) tracks differences along an eggshell color gradient

One of the most effective defenses against avian brood parasitism is the rejection of the foreign egg from the host's nest. Until recently, most studies have tested whether hosts discriminate between own and foreign eggs based on the absolute differences in avian-perceivable eggshell coloration and maculation. However, recent studies suggest that hosts may instead contrast egg appearances across a directional eggshell color gradient. We assessed which discrimination rule best explained egg rejection by great reed warblers Acrocephalus arundinaceus, a frequent host to an egg-mimetic race of common cuckoos Cuculus canorus. We deployed 3D-printed model eggs varying in blue-green to brown coloration and in the presence of maculation. Using visual modeling, we calculated the absolute chromatic and achromatic just-noticeable differences (JNDs), as well as directional JNDs across a blue-green to brown egg color gradient, between host and model eggs. While most model eggs were rejected by great reed warblers, browner eggs were rejected with higher probability than more blue-green eggs, and the rejection probability did not depend on maculation. Directional egg color discrimination shown here and in a suite of recent studies on other host species may shape the cognitive decision rules that hosts use to recognize foreign eggs and affect the course of evolution in parasitic egg mimicry.

Background matching and disruptive coloration as habitat-specific strategies for camouflage

Camouflage is a key defence across taxa and frequently critical to survival. A common strategy is background matching, resembling the colour and pattern of the environment. This approach, however, may be ineffective in complex habitats where matching one patch may lead to increased visibility in other patches. In contrast, disruptive coloration, which disguises body outlines, may be effective against complex backgrounds. These ideas have rarely been tested and previous work focuses on artificial systems. Here, we test the camouflage strategies of the shore crab (Carcinus maenas) in two habitats, being a species that is highly variable, capable of plastic changes in appearance, and lives in multiple environments. Using predator (bird and fish) vision modelling and image analysis, we quantified background matching and disruption in crabs from rock pools and mudflats, predicting that disruption would dominate in visually complex rock pools but background matching in more uniform mudflats. As expected, rock pool individuals had significantly higher edge disruption than mudflat crabs, whereas mudflat crabs more closely matched the substrate than rock pool crabs for colour, luminance, and pattern. Our study demonstrates facultative expression of camouflage strategies dependent on the visual environment, with implications for the evolution and interrelatedness of defensive strategies.