Experimental evidence of aposematic signal in black tadpoles

Aposematism is a widespread defense mechanism in animals. Anuran amphibians are known for their extensive repertoire of defenses, especially in the adult stage. Color-based mechanisms are particularly common in this group, although their use during the larval stage is still understudied. The hypothesis of aposematism in tadpoles has been suggested for several species that present conspicuous coloration and unpalatability. Tadpoles of some bufonid toads have been considered aposematic for decades, as they have a conspicuous black color, are unpalatable, and form aggregations that make them even more visible. However, few empirical studies have tested aposematism in these larvae against visually oriented predators, such as birds. In this paper, we tested whether the uniform black coloration acts as an aposematic signal in anuran larvae. The research was conducted through field predation experiments using artificial replicas of tadpoles of different colors (brown, gray, and black). The survival of black replicas was significantly higher than that of control groups, supporting the aposematism hypothesis. Given that the replicas varied only in color, we conclude that the differential attacks among treatments were carried out by visually oriented predators, probably birds, that had learned to recognize and avoid black tadpoles.

The genomics of mimicry: Gene expression throughout development provides insights into convergent and divergent phenotypes in a Müllerian mimicry system

A common goal in evolutionary biology is to discern the mechanisms that produce the astounding diversity of morphologies seen across the tree of life. Aposematic species, those with a conspicuous phenotype coupled with some form of defence, are excellent models to understand the link between vivid colour pattern variations, the natural selection shaping it, and the underlying genetic mechanisms underpinning this variation. Mimicry systems in which species share a conspicuous phenotype can provide an even better model for understanding the mechanisms of colour production in aposematic species, especially if comimics have divergent evolutionary histories. Here we investigate the genetic mechanisms by which mimicry is produced in poison frogs. We assembled a 6.02-Gbp genome with a contig N50 of 310 Kbp, a scaffold N50 of 390 Kbp and 85% of expected tetrapod genes. We leveraged this genome to conduct gene expression analyses throughout development of four colour morphs of Ranitomeya imitator and two colour morphs from both R. fantastica and R. variabilis which R. imitator mimics. We identified a large number of pigmentation and patterning genes differentially expressed throughout development, many of them related to melanophores/melanin, iridophore development and guanine synthesis. We also identify the pteridine synthesis pathway (including genes such as qdpr and xdh) as a key driver of the variation in colour between morphs of these species, and identify several plausible candidates for colouration in vertebrates (e.g. cd36, ep-cadherin and perlwapin). Finally, we hypothesise that keratin genes (e.g. krt8) are important for producing different structural colours within these frogs.

Evolution of personality and locomotory performance traits during a late Pleistocene island colonization in a tree frog

Recent empirical and theoretical studies suggest that personality and locomotory performance traits linked to dispersal abilities are crucial components of the dispersal syndromes, and that they can evolve during range expansions and colonization processes. Island colonization is one of the best characterized processes in dispersal biogeography, and its implication in the evolution of phenotypic traits has been investigated over a wide range of temporal scales. However, the effect of island colonization on personality and performance traits of natural populations, and how these traits could drive island colonization, has been little explored. Noteworthy, no studies have addressed these processes in the context of late Pleistocene range expansions. Here, we investigated the contribution of island colonization triggered by postglacial range expansions to intraspecific variation in personality and locomotory performance traits. We compared boldness, exploration, jumping performance, and stickiness abilities in populations from 3 equidistant areas of the Tyrrhenian tree frog Hyla sarda, 2 from the main island (Corsica Island), and 1 from the recently colonized island of Elba. Individuals from Elba were significantly bolder than individuals from Corsica, as they emerged sooner from a shelter (P = 0.028), while individuals from Corsica showed markedly higher jumping and stickiness performance (both P < 0.001), resulting as more performing than those of Elba. We discuss these results in the context of the major microevolutionary processes at play during range expansion, including selection, spatial sorting, founder effects, and their possible interaction with local adaptation processes.

Color polymorphism in the Cuban endemic livebearing fish Limia vittata (Teloestei, Poeciliidae): Potential roles of sexual and natural selection

Color polymorphism can be maintained in natural populations by natural selection or sexual selection. In this study, we use two different approaches to test which of these evolutionary mechanisms may explain the presence of color polymorphism in the Cuban Limia (Limia vittata), an endemic livebearing fish from Cuba. First, we investigate the role of sexual selection using traditional binary choice tests looking at both female and male preferences relative to varying degrees of black spotting in stimulus mates. Second, we assess the role of natural selection by analyzing the frequency and geographic distribution of black-spotted and nonspotted morphs of L. vittata in natural populations from Cuba. The frequency of black-spotted morphs is significantly higher in brackish and saltwater environments compared with freshwater habitats, which could be related to higher predation pressure in coastal ecosystems compared with purely freshwater environments. Our results suggest that habitat variation is the most important factor in maintaining color polymorphism in L. vittata. Salinity levels could be indirectly responsible for maintaining different color morphs in this species, likely due to the regulatory effect of saline gradients on predation regimes.

How to behave when marooned: the behavioural component of the island syndrome remains underexplored

Animals on islands typically depart from their mainland relatives in assorted aspects of their biology. Because they seem to occur in concert, and to some extent evolve convergently in disparate taxa, these changes are referred to as the 'island syndrome'. While morphological, physiological and life-history components of the island syndrome have received considerable attention, much less is known about how insularity affects behaviour. In this paper, we argue why changes in personality traits and cognitive abilities can be expected to form part of the island syndrome. We provide an overview of studies that have compared personality traits and cognitive abilities between island and mainland populations, or among islands. Overall, the pickings are remarkably slim. There is evidence that animals on islands tend to be bolder than on the mainland, but effects on other personality traits go either way. The evidence for effects of insularity on cognitive abilities or style is highly circumstantial and very mixed. Finally, we consider the ecological drivers that may induce such changes, and the mechanisms through which they might occur. We conclude that our knowledge of the behavioural and cognitive responses to island environments remains limited, and we encourage behavioural biologists to make more use of these 'natural laboratories for evolution'.

The relationships between toad behaviour, antipredator defences, and spatial and sexual variation in predation pressure

BACKGROUND

Animal behaviour is under strong selection. Selection on behaviour, however, might not act in isolation from other fitness-related traits. Since predators represent outstanding selective forces, animal behaviour could covary with antipredator defences, such that individuals better suited against predators could afford facing the costs of riskier behaviours. Moreover, not all individuals undergo equivalent degrees of predation pressure, which can vary across sexes or habitats. Individuals under lower predation pressure might also exhibit riskier behaviours.

METHODS

In this work, I tested these hypotheses on natterjack toads (Epidalea calamita). Specifically, I gauged activity time, exploratory behaviour and boldness in standard laboratory conditions, and assessed whether they correlated with body size and antipredator strategies, namely sprint speed, parotoid gland area and parotoid gland colour contrast. Additionally, I compared these traits between sexes and individuals from an agrosystem and pine grove, since there is evidence that males and agrosystem individuals are subjected to greater predation pressure.

RESULTS

Sprint speed as well as parotoid gland contrast and size appeared unrelated to the behavioural traits studied. In turn, body mass was negatively related to activity time, boldness and exploration. This trend is consistent with the fact that larger toads could be more detectable to their predators, which are mostly gape unconstrained and could easily consume them. As predicted, females exhibited riskier behaviours. Nonetheless, agrosystem toads did not differ from pine grove toads in the behavioural traits measured, despite being under stronger predation pressure.

Evidence for coloration plasticity in the yellow-bellied toad, Bombina variegata

Phenotypic adaptation in terms of background color matching to the local habitat is an important mechanism for survival in prey species. Thus, intraspecific variation in cryptic coloration is expected among localities with dissimilar habitat features (e.g., soil, vegetation). Yellow-bellied toads (Bombina variegata) display a dark dorsal coloration that varies between populations, assumed to convey crypsis. In this study, we explored I) geographic variation in dorsal coloration and II) coloration plasticity in B. variegata from three localities differing in substrate coloration. Using avian visual modeling, we found that the brightness contrasts of the cryptic dorsa were significantly lower on the local substrates than substrates of other localities. In experiments, individuals from one population were able to quickly change the dorsal coloration to match a lighter substrate. We conclude that the environment mediates an adaptation in cryptic dorsal coloration. We suggest further studies to test the mechanisms by which the color change occurs and explore the adaptive potential of coloration plasticity on substrates of varying brightness in B. variegata and other species.

Habitat ephemerality affects the evolution of contrasting growth strategies and cannibalism in anuran larvae

Ephemeral streams are challenging environments for tadpoles; thus, adaptive features that increase the survival of these larvae should be favored by natural selection. In this study, we compared the adaptive growth strategies of Bombina orientalis (the oriental fire-bellied toad) tadpoles from ephemeral streams with those of such tadpoles from non-ephemeral streams. Using a common garden experiment, we tested the interactive effects of location (ephemeral vs. non-ephemeral), food availability, and growing density on larval period, weight at metamorphosis, and cannibalism. We found that tadpoles from ephemeral streams underwent a shorter larval period compared with those from non-ephemeral streams but that this difference was contingent on food availability. The observed faster growth is likely to be an adaptive response because tadpoles in ephemeral streams experience more biotic/abiotic stressors, such as desiccation risk and limited resources, compared with those in non-ephemeral streams, with their earlier metamorphosis potentially resulting in survival benefits. As a trade-off for their faster growth, tadpoles from ephemeral streams generally had a lower body weight at metamorphosis compared with those from non-ephemeral streams. We also found lower cannibalism rates among tadpoles from ephemeral streams, which can be attributed to the indirect fitness costs of cannibalizing their kin. Our study demonstrates how ephemeral habitats have affected the evolutionary change in cannibalistic behaviors in anurans and provides additional evidence that natural selection has mediated the evolution of growth strategies of tadpoles in ephemeral streams.

The genomics of mimicry: Gene expression throughout development provides insights into convergent and divergent phenotypes in a Müllerian mimicry system

A common goal in evolutionary biology is to discern the mechanisms that produce the astounding diversity of morphologies seen across the tree of life. Aposematic species, those with a conspicuous phenotype coupled with some form of defence, are excellent models to understand the link between vivid colour pattern variations, the natural selection shaping it, and the underlying genetic mechanisms underpinning this variation. Mimicry systems in which multiple species share the same conspicuous phenotype can provide an even better model for understanding the mechanisms of colour production in aposematic species, especially if comimics have divergent evolutionary histories. Here we investigate the genetic mechanisms by which vivid colour and pattern are produced in a Müllerian mimicry complex of poison frogs. We did this by first assembling a high-quality de novo genome assembly for the mimic poison frog Ranitomeya imitator. This assembled genome is 6.8 Gbp in size, with a contig N50 of 300 Kbp R. imitator and two colour morphs from both Ranitomeya fantastica and R. variabilis which R. imitator mimics. We identified a large number of pigmentation and patterning genes that are differentially expressed throughout development, many of them related to melanocyte development, melanin synthesis, iridophore development and guanine synthesis. Polytypic differences within species may be the result of differences in expression and/or timing of expression, whereas convergence for colour pattern between species does not appear to be due to the same changes in gene expression. In addition, we identify the pteridine synthesis pathway (including genes such as qdpr and xdh) as a key driver of the variation in colour between morphs of these species. Finally, we hypothesize that genes in the keratin family are important for producing different structural colours within these frogs.

The island rule explains consistent patterns of body size evolution in terrestrial vertebrates

Island faunas can be characterized by gigantism in small animals and dwarfism in large animals, but the extent to which this so-called 'island rule' provides a general explanation for evolutionary trajectories on islands remains contentious. Here we use a phylogenetic meta-analysis to assess patterns and drivers of body size evolution across a global sample of paired island-mainland populations of terrestrial vertebrates. We show that 'island rule' effects are widespread in mammals, birds and reptiles, but less evident in amphibians, which mostly tend towards gigantism. We also found that the magnitude of insular dwarfism and gigantism is mediated by climate as well as island size and isolation, with more pronounced effects in smaller, more remote islands for mammals and reptiles. We conclude that the island rule is pervasive across vertebrates, but that the implications for body size evolution are nuanced and depend on an array of context-dependent ecological pressures and environmental conditions.

Disentangling the Impacts of Speciation, Sympatry and the Island Effect on the Morphology of Seven Hynobius sp. Salamanders

Closely related individuals from different areas can see their morphologies change based on differences between clades, but also ecological variables such as the island effect or sympatry. This is the case of salamanders, which have adapted to a broad range of ecological niches, ranging from underground dwellers in xeric landscape to tropical arboreal habitats. On the Korean Peninsula, salamanders from the Hynobius clade are widespread on the mainland and islands, with several partially sympatric clades and candidate species. Currently, seven lineages have been identified based on mtDNA, four of them matching named species and three others for which the species status remains untested. While the morphology of Korean Hynobius is known to be variable between genetically segregated clades, we hypothesise that (1) the candidate species are morphologically different, and that (2) the island effect and (3) the sympatric status have significant impacts on the morphology of individuals within the genus. Here we measured 329 Hynobius salamanders from all seven clades, in areas of sympatry and allopatry, and on islands and on the mainland (Graphical Abstract A). We determined that the island effect had a significant impact on the morphology of the genus, with mainland individuals generally displaying a broader range of morphology than islandic individuals (Graphical Abstract B). We also determined that sympatry had an impact on morphology, with the sizes of individuals from clades in sympatric areas diverging from each other (Graphical Abstract C). Finally, we demonstrated that all seven clades have significantly different morphologies, and we described the three candidate species that had already been isolated based on mtDNA and microsatellite data: Hynobius notialis sp. nov., Hynobius geojeensis sp. nov. and Hynobius perplicatus sp. nov. We conclude that looking at morphology alone would be misleading about the true diversity of Hynobius species, and species in general, because of the island and patry effects.

The evolution of polymorphism in the warning coloration of the Amazonian poison frog Adelphobates galactonotus

While intraspecific variation in aposematic signals can be selected for by different predatory responses, their evolution is also contingent on other processes shaping genetic variation. We evaluate the relative contributions of selection, geographic isolation, and random genetic drift to the evolution of aposematic color polymorphism in the poison frog Adelphobates galactonotus, distributed throughout eastern Brazilian Amazonia. Dorsal coloration was measured for 111 individuals and genetic data were obtained from 220 individuals at two mitochondrial genes (mtDNA) and 7963 Single Nucleotide Polymorphisms (SNPs). Four color categories were described (brown, blue, yellow, orange) and our models of frog and bird visual systems indicated that each color was distinguishable for these taxa. Using outlier and correlative analyses we found no compelling genetic evidence for color being under divergent selection. A time-calibrated mtDNA tree suggests that the present distribution of dorsal coloration resulted from processes occurring during the Pleistocene. Separate phylogenies based on SNPs and mtDNA resolved the same well supported clades, each containing different colored populations. Ancestral character state analysis provided some evidence for evolutionary transitions in color type. Genetic structure was more strongly associated with geographic features, than color category, suggesting that the distribution of color is explained by localized processes. Evidence for geographic isolation together with estimates of low effective population size implicates drift as playing a key role in color diversification. Our results highlight the relevance of considering the neutral processes involved with the evolution of traits with important fitness consequences.

Phenotypic and genetic divergence in reed frogs across a mosaic hybrid zone on São Tomé Island

Although naturally heterogeneous environments can lead to mosaic hybrid zones, human-induced habitat fragmentation can also lead to environmental heterogeneity and hybridization. Here we quantify phenotypic and molecular divergence across a reed frog mosaic hybrid zone on São Tomé Island as a first step towards understanding the consequences of hybridization across this heterogeneous landscape. The São Tomé giant reed frog (Hyperolius thomensis) is strongly tied to cool, wet, forest habitats whereas the distribution of Moller’s reed frog (H. molleri) spans cool, wet, forests to warm, dry, disturbed habitats. Correspondingly, hybridization is concentrated in the more forested, cool, wet sites relative to non-forested, warmer, drier habitats. Four of six sites with hybrid frogs are artificial water bodies near the forest edge, indicating that both breeding habitat and broader scale environmental variation are probably important for understanding interspecific interactions and the extent of hybridization in this system. Phenotypic variation (body size and ventral coloration) largely tracks genetic and environmental variation across the hybrid zone with larger and more pigmented frogs occurring in forested, cool, wet habitats. Understanding whether human-induced changes in habitat break down reproductive barriers will be essential for conservation management of the less abundant, forest-associated H. thomensis in the face of rampant hybridization.