Variation of thermal parameters in two different color morphs of a diurnal poison toad, Melanophryniscus rubriventris (Anura: Bufonidae)

Differential effect of dehydration on the voluntary activity of a diurnal toad

Anuran amphibians' ability to maintain their activity at high temperatures or low humidity depends on their capacity to face dehydration, especially when they display diurnal and terrestrial life habits. Melanophryniscus rubriventris is a diurnal and terrestrial toad from humid Yungas Andean forests that breeds in temporary ponds. It is exposed to the recurrent risk of dehydration because of pond desiccations during the breeding season. Here, we study how M. rubriventris males behaviorally respond to dehydration by measuring their voluntary activity under an ex-situ experiment. Toads with different hydration levels were exposed to a circular track for voluntary activity measurements. Dehydrated males of M. rubriventris toads did not adopt a water-conserving posture staying active during the test and increasing walking under severe dehydration. Certain tolerance to dehydration would allow performing daily activities under challenging diurnal conditions. The increased walking under severe dehydration suggests water or shelter-seeking behavior that would be crucial for diurnal and terrestrial toads to overcome the unpredictable hydric environment during the breeding season.

Some like it hotter: Differential thermal preferences among lizard color morphs

Temperature rules the lives of ectotherms. To perform basic biological functions, ectotherms must make behavioral adjustments to keep their body temperatures near a preferred temperature (Tpref). Many color polymorphic lizards are active thermoregulators and exhibit morph differences in traits related to thermoregulation, such as color, body size, and microhabitat use. The Aegean wall lizard, Podarcis erhardii, is a heliothermic lizard with orange, white, and yellow color morphs that differ in size, behavior, and microhabitat use. Here, we tested whether P. erhardii color morphs from the same population from Naxos island, Greece, differ in Tpref. We hypothesized that orange morphs would prefer lower temperatures than white and yellow morphs because orange morphs are often found on cooler substrates and in microhabitats with more vegetation cover. We obtained Tpref for 95 individuals using laboratory thermal gradient experiments of wild-caught lizards and found that orange morphs do, indeed, prefer cooler temperatures. Average orange morph Tpref was 2.85 °C lower than average white and yellow morph Tpref. Our results add support to the idea that P. erhardii color morphs have multivariate alternative phenotypes and present the possibility that thermally heterogeneous environments play a role in the maintenance of color polymorphism in this species.

TRPM8 thermosensation in poikilotherms mediates both skin colour and locomotor performance responses to cold temperature

Thermoregulation is a homeostatic process to maintain an organism's internal temperature within a physiological range compatible with life. In poikilotherms, body temperature fluctuates with that of the environment, with both physiological and behavioral responses employed to modify body temperature. Changing skin colour/reflectance and locomotor activity are both well-recognized temperature regulatory mechanisms, but little is known of the participating thermosensor/s. We find that Xenopus laevis tadpoles put in the cold exhibit a temperature-dependent, systemic, and rapid melanosome aggregation in melanophores, which lightens the skin. Cooling also induces a reduction in the locomotor performance. To identify the cold-sensor, we focus on transient receptor potential (trp) channel genes from a Trpm family. mRNAs for several Trpms are present in Xenopus tails, and Trpm8 protein is present in skin melanophores. Temperature-induced melanosome aggregation is mimicked by the Trpm8 agonist menthol (WS12) and blocked by a Trpm8 antagonist. The degree of skin lightening induced by cooling is correlated with locomotor performance, and both responses are rapidly regulated in a dose-dependent and correlated manner by the WS12 Trpm8 agonist. We propose that TRPM8 serves as a cool thermosensor in poikilotherms that helps coordinate skin lightening and behavioural locomotor performance as adaptive thermoregulatory responses to cold.

A comprehensive database of amphibian heat tolerance

Rising temperatures represent a significant threat to the survival of ectothermic animals. As such, upper thermal limits represent an important trait to assess the vulnerability of ectotherms to changing temperatures. For instance, one may use upper thermal limits to estimate current and future thermal safety margins (i.e., the proximity of upper thermal limits to experienced temperatures), use this trait together with other physiological traits in species distribution models, or investigate the plasticity and evolvability of these limits for buffering the impacts of changing temperatures. While datasets on thermal tolerance limits have been previously compiled, they sometimes report single estimates for a given species, do not present measures of data dispersion, and are biased towards certain parts of the globe. To overcome these limitations, we systematically searched the literature in seven languages to produce the most comprehensive dataset to date on amphibian upper thermal limits, spanning 3,095 estimates across 616 species. This resource will represent a useful tool to evaluate the vulnerability of amphibians, and ectotherms more generally, to changing temperatures.

Artificial light at night and captivity induces differential effects on leukocyte profile, body condition, and erythrocyte size of a diurnal toad

Light pollution or artificial lighting at night (ALAN) is an emerging threat to biodiversity that can disrupt physiological processes and behaviors. Because ALAN stressful effects are little studied in diurnal amphibian species, we investigated if chronic ALAN exposure affects the leukocyte profile, body condition, and blood cell sizes of a diurnal toad. We hand-captured male toads of Melanophryniscus rubriventris in Angosto de Jaire (Jujuy, Argentina). We prepared blood smears from three groups of toads: "field" (toads processed in the field immediately after capture), "natural light" (toads kept in the laboratory under captivity with natural photoperiod), and "constant light" (toads kept in the laboratory under captivity with constant photoperiod/ALAN). We significantly observed higher neutrophil proportions and neutrophils to lymphocytes ratio in toads under constant light treatment. In addition, we observed significantly better body condition and higher erythrocyte size in field toads compared with captive toads. In summary, ALAN can trigger a leukocyte response to stress in males of the diurnal toad M. rubriventris. In addition, captivity can affect the body condition and erythrocyte size of these toads.

Avian predation intensity as a driver of clinal variation in colour morph frequency

Phenotypic variation provides the framework for natural selection to work upon, enabling adaptive evolution. One of the most discernible manifestations of phenotypic variability is colour variation. When this variation is discrete, genetically based colour pattern morphs occur simultaneously within a population. Why and how colour polymorphisms are maintained is an evolutionary puzzle. Several evolutionary drivers have been hypothesized as influencing clinal patterns of morph frequency, with spatial variation in climate and predation being considered especially important. Despite this, no study has examined both of their roles simultaneously. The aims of this study were to: (a) examine the covariation of physiology, environmental variables and colouration at a local scale; and (b) determine if these factors and their interplay explain broad clinal variation in morph frequency. We used the lizard Liopholis whitii as a model system, as this species displays a discrete, heritable polymorphism for colour pattern (plain-backed, patterned morphs) whose morph frequency varies latitudinally. We measured reflectance, field activity temperatures and microhabitat structure to test for differences in crypsis, thermal biology and microhabitat selection of patterned and plain-backed morphs within a single population where colour morphs occur sympatrically. We then used data from the literature to perform a broad-scale analysis to identify whether these factors also explained the latitudinal variation of morph frequency in this species. At the local scale, plain-backed morphs were found to be less cryptic than patterned morphs while no other differences were detected in terms of thermal biology, dorsal reflectance and microhabitat use. At a broader scale, predation was the most influential factor mediating morph frequency across latitudes. However, the observed pattern of morph frequency is opposite to what the modelling results suggest in that the incidence of the least cryptic morph is highest where predation pressure is most severe. Clinal variation in the level of background matching between morphs or the potential reproductive advantage by the plain-backed morph may, instead, be driving the observed morph frequency. Together, these results provide key insights into the evolution of local adaptation as well as the ecological forces involved in driving the dynamics of colour polymorphism.

Diversity in warning coloration: selective paradox or the norm?

Aposematic theory has historically predicted that predators should select for warning signals to converge on a single form, as a result of frequency‐dependent learning. However, widespread variation in warning signals is observed across closely related species, populations and, most problematically for evolutionary biologists, among individuals in the same population. Recent research has yielded an increased awareness of this diversity, challenging the paradigm of signal monomorphy in aposematic animals. Here we provide a comprehensive synthesis of these disparate lines of investigation, identifying within them three broad classes of explanation for variation in aposematic warning signals: genetic mechanisms, differences among predators and predator behaviour, and alternative selection pressures upon the signal. The mechanisms producing warning coloration are also important. Detailed studies of the genetic basis of warning signals in some species, most notably Heliconius butterflies, are beginning to shed light on the genetic architecture facilitating or limiting key processes such as the evolution and maintenance of polymorphisms, hybridisation, and speciation. Work on predator behaviour is changing our perception of the predator community as a single homogenous selective agent, emphasising the dynamic nature of predator–prey interactions. Predator variability in a range of factors (e.g. perceptual abilities, tolerance to chemical defences, and individual motivation), suggests that the role of predators is more complicated than previously appreciated. With complex selection regimes at work, polytypisms and polymorphisms may even occur in Müllerian mimicry systems. Meanwhile, phenotypes are often multifunctional, and thus subject to additional biotic and abiotic selection pressures. Some of these selective pressures, primarily sexual selection and thermoregulation, have received considerable attention, while others, such as disease risk and parental effects, offer promising avenues to explore. As well as reviewing the existing evidence from both empirical studies and theoretical modelling, we highlight hypotheses that could benefit from further investigation in aposematic species. Finally by collating known instances of variation in warning signals, we provide a valuable resource for understanding the taxonomic spread of diversity in aposematic signalling and with which to direct future research. A greater appreciation of the extent of variation in aposematic species, and of the selective pressures and constraints which contribute to this once‐paradoxical phenomenon, yields a new perspective for the field of aposematic signalling.

Phytogeographic patterns and cryptic diversity in an aposematic toad from NW Argentina

The Yungas Redbelly Toad, Melanophryniscus rubriventris, is patchily distributed in Argentina, confined to the upland portion (1000-2000m above sea level) of the montane forests of northern and central regions of Salta, and in central-eastern and south-eastern Jujuy. This species is known for its striking aposematic color variation across its geographic distribution, and was once treated as a complex of three subspecies based on distinctive color patterns. Here we assess the geographical genetic variation within M. rubriventris and quantify divergence in color and pattern among individuals sampled from Northwestern Argentina. We compare multi-gene phylogeography of M. rubriventris to patterns of dorsal and ventral coloration to test whether evolutionary affinities predict variation in warning color. Our results reveal two well-supported species lineages: one confined to the extreme northern portion of our sampling area, and the other extending over most of the Argentine portion of the species' range, within which there are two populations. However, these well-supported evolutionary relationships do not mirror the marked variation in warning coloration. This discordance between DNA genealogy and warning color variation may reflect selection brought about by differences in local predation pressures, potentially coupled with effects of sexual selection and thermoregulation.

The effect of skin reflectance on thermal traits in a small heliothermic ectotherm

Variation in colour patterning is prevalent among and within species. A number of theories have been proposed in explaining its evolution. Because solar radiation interacts with the pigmentation of the integument causing light to either be reflected or absorbed into the body, thermoregulation has been considered to be a primary selective agent, particularly among ectotherms. Accordingly, the colour-mediated thermoregulatory hypothesis states that darker individuals will heat faster and reach higher thermal equilibria while paler individuals will have the opposite traits. It was further predicted that dark colouration would promote slower cooling rates and higher thermal performance temperatures. To test these hypotheses we quantified the reflectance, selected body temperatures, performance optima, as well as heating and cooling rates of an ectothermic vertebrate, Lampropholis delicata. Our results indicated that colour had no influence on thermal physiology, as all thermal traits were uncorrelated with reflectance. We suggest that crypsis may instead be the stronger selective agent as it may have a more direct impact on fitness. Our study has improved our knowledge of the functional differences among individuals with different colour patterns, and the evolutionary significance of morphological variation within species.

Diurnal? Calling activity patterns reveal nocturnal habits in the aposematic toad Melanophryniscus rubriventris

In diurnal species with short breeding seasons, an extension of diel activity to the night hours could be favoured to maximize mating opportunities, but individuals must deal with physiological and behavioural constraints. We tested this hypothesis in the Yungas Red-belly Toad (Melanophryniscus rubriventris (Vellard, 1947)). We registered the diel pattern of male calling activity in two localities using automated recording systems, and related it to abiotic factors such as temperature, relative air humidity, and precipitation. The diel pattern of vocalization was mainly diurnal. Interestingly though, nocturnal calling activity was a common feature, representing between 40% and 43% of call records in both localities. Vocal activity was significantly influenced by time of the day and presence of rainfall. Calling males showed high plasticity, with activity in the entire environmental range of relative air humidity and temperature. Nocturnal calling seems to play an important role in the mating strategy of males, and it is probably more frequent in the genus Melanophryniscus than currently assumed. We discuss the implications of our findings in relation to different aspects of ecology of the species and suggest that bright colouration in M. rubriventris might result from a compromise between several nonconflicting functions (e.g., aposematism and thermoregulation).

Dependencia térmica de la salamandra endémica de Colombia Bolitoglossa ramosi (Caudata, Plethodontidae)

RESUMEN La temperatura ambiental es un factor determinante en los procesos fisiológicos y comportamentales de los anfibios ya que son ectotérmos y consecuentemente dependen de una fuente de calor externa para alcanzar su temperatura corporal óptima. El objetivo de este trabajo fue determinar la dependencia térmica de la salamandra endémica de Colombia Bolitoglossa ramosi Brame & Wake, 1972 con respecto a las temperaturas del aire y del sustrato. Para esto se realizaron diferentes muestreos en el municipio de Líbano, Tolima, Colombia, entre las 18:00 y las 24:00 horas, en Abril y Mayo de 2015. Allí se capturaron 34 individuos a los cuales se les registró directamente en campo: la temperatura corporal, la masa corporal y la longitud corporal. También, se midió la temperatura del sustrato y la temperatura del aire en el lugar donde fue encontrado el animal. La temperatura corporal de los individuos tuvo una media de 18.3±0.55°C, mostrando una relación positiva y significativa con la temperatura del sustrato y la temperatura del aire, lo que demuestra que la especie es termoconformadora. Además, la temperatura corporal mostró una dependencia térmica mayor con la temperatura del sustrato que con la del aire, indicando que B. ramosi presenta una regulación tigmotérmica. Finalmente, la temperatura corporal no se relacionó con la longitud corporal ni con la masa corporal. Esta información es importante para el conocimiento de la biología térmica de la especie y las posibles respuestas fisiológicas ante el incremento de las temperaturas ambientales.