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The toco toucan (Ramphastos toco), the largest member of the toucan family, possesses the largest beak relative to body size of all birds. This exaggerated feature has received various interpretations, from serving as a sexual ornament to being a refined adaptation for feeding. However, it is also a significant surface area for heat exchange. Here we show the remarkable capacity of the toco toucan to regulate heat distribution by modifying blood flow, using the bill as a transient thermal radiator. Our results indicate that the toucan's bill is, relative to its size, one of the largest thermal windows in the animal kingdom, rivaling elephants' ears in its ability to radiate body heat.

Terrestrial animals often use evaporative cooling to lower body temperature. Evaporation can occur from humid body surfaces or from fluids interfaced to the environment through a number of different mechanisms, such as sweating or panting. In Diptera, some flies move tidally a droplet of fluid out and then back in the buccopharyngeal cavity for a repeated number of cycles before eventually ingesting it. This is referred to as the bubbling behaviour. The droplet fluid consists of a mix of liquids from the ingested food, enzymes from the salivary glands, and antimicrobials, associated to the crop organ system, with evidence pointing to a role in liquid meal dehydration. Herein, we demonstrate that the bubbling behaviour also serves as an effective thermoregulatory mechanism to lower body temperature by means of evaporative cooling. In the blowfly, Chrysomya megacephala , infrared imaging revealed that as the droplet is extruded, evaporation lowers the fluid´s temperature, which, upon its re-ingestion, lowers the blowfly’s body temperature. This effect is most prominent at the cephalic region, less in the thorax, and then in the abdomen. Bubbling frequency increases with ambient temperature, while its cooling efficiency decreases at high air humidities. Heat transfer calculations show that droplet cooling depends on a special heat-exchange dynamic, which result in the exponential activation of the cooling effect.

Amphibians balance their thermal and water budgets depending on their physiological state and the physical environment, with both factors capable of constraining activity. Most mechanistic assessments emphasize thermal over water constraints, potentially missing important aspects of amphibian ecophysiological patterns. Here, we evaluate the potential role of thermal and hydric constraints on the activity time of three Neotropical frogs (Leptodactylus fuscus, L. mystacinus, and L. macrosternum) across their geographic distribution. We inferred environmental suitability based on heat and mass transfer principles through a mechanistic modeling procedure anchored to empirically obtained laboratory and field data. We integrated species‐specific thermal, hydric, and performance attributes with their immediate physical environment (ground‐level microclimate) under nocturnal conditions, while allowing for the interactive response of retreating into shelter when facing physiological heat or water stress. Our results demonstrate the desiccation‐prone role of smaller body sizes in increasing hydric restrictions and inhibiting activity, even under thermally adequate conditions, as well as the role of shelters as thermal and hydric refugia. More strikingly, thermal‐induced restrictions in activity were linked to low temperatures rather than warmer conditions, indicating that their engagement in activity is mostly driven by the lower thermal bounds of their functional organismal performance. These findings provide a broader picture of climatic constraints on anuran activity and distribution, as well as insights into how species may respond to changing climatic conditions.

Due to their highly permeable skin and ectothermy, terrestrial amphibians are challenged by compromises between water balance and body temperature regulation. The way in which such compromises are accommodated, under a range of temperatures and dehydration levels, impacts importantly the behavior and ecology of amphibians. Thus, using the terrestrial toad Rhinella schneideri as a model organism, the goals of this study were twofold. First, we determined how the thermal sensitivity of a centrally relevant trait—locomotion—was affected by dehydration. Secondly, we examined the effects of the same levels of dehydration on thermal preference and thermal tolerance. As dehydration becomes more severe, the optimal temperature for locomotor performance was lowered and performance breadth narrower. Similarly, dehydration was accompanied by a decrease in the thermal tolerance range. Such a decrease was caused by both an increase in the critical minimal temperature and a decrease in the thermal maximal temperature, with the latter changing more markedly. In general, our results show that the negative effects of dehydration on behavioral performance and thermal tolerance are, at least partially, counteracted by concurrent adjustments in thermal preference. We discuss some of the potential implications of this observation for the conservation of anuran amphibians. Using the terrestrial toad Rhinella schneideri as a model organism, we evaluated the effect of the hydro thermal compromise affecting ecological relevant traits, such as locomotion, thermal tolerance, and behavioral thermoregulation. We showed the negative effects of dehydration on behavioral performance and thermal tolerance are, at least partially, counteracted by concurrent adjustments in thermal preference. These observations have important implications for the conservation of terrestrial anurans.

The cardiovascular system of all animals is affected by gravitational pressure gradients, the intensity of which varies according to organismic features, behavior, and habitat occupied. A previous nonphylogenetic analysis of heart position in snakes—which often assume vertical postures—found the heart located 15%–25% of total body length from the head in terrestrial and arboreal species but 25%–45% in aquatic species. It was hypothesized that a more anterior heart in arboreal species served to reduce the hydrostatic blood pressure when these animals adopt vertical postures during climbing, whereas an anterior heart position would not be needed in aquatic habitats, where the effects of gravity are less pronounced. We analyzed a new data set of 155 species from five major families of Alethinophidia (one of the two major branches of snakes, the other being blind snakes, Scolecophidia) using both conventional and phylogenetically based statistical methods. General linear models regressing log10snout‐heart position on log10snout‐vent length (SVL), as well as dummy variables coding for habitat and/or clade, were compared using likelihood ratio tests and the Akaike Information Criterion. Heart distance to the tip of the snout scaled isometrically with SVL. In all instances, phylogenetic models that incorporated transformation of the branch lengths under an Ornstein‐Uhlenbeck model of evolution (to mimic stabilizing selection) better fit the data as compared with their nonphylogenetic counterparts. The best‐fit model predicting snake heart position included aspects of both habitat and clade and indicated that arboreal snakes in our study tend to have hearts placed more posteriorly, opposite the trend identified in previous studies. Phylogenetic signal in relative heart position was apparent both within and among clades. Our results suggest that overcoming gravitational pressure gradients in snakes most likely involves the combined action of several cardiovascular and behavioral adaptations in addition to alterations in relative heart location.

Because of ectothermy and the high permeability of their skin, the thermal biology and water balance of anuran amphibians are highly influenced by variations in environmental temperature and water availability. Seasonal variation in these climatic variables are often followed by concurrent adjustments in frogs' thermal biology and water balance. We investigated seasonal variation in the thermal biology and water balance of snouted treefrogs, Scinax fuscovarius (Anura, Hylidae), a midsize nocturnal treefrog with continuous activity throughout the year. We measured body temperature when active in the field (Tfield), thermal preference (Tpref) in a thermal gradient in the laboratory, and the minimum and maximum critical temperatures (CTmin and CTmax, respectively). Regarding water balance, we measured rates of evaporative water loss (EWL) and the skin resistance (Rs) to evaporation at three temperatures (15, 25, and 358C), dehydration tolerance, and rates of water uptake (WU), both at 258C. Measurements were carried out in the middle of the hot and wet spring–summer and the cold and dry autumn–winter. We found that Tfield, Tpref, dehydration tolerance, and WU were higher in the hot–wet compared to the cold–dry season. Evaporative water loss rates increased with experimental temperature but did not vary seasonally. There was no seasonal variation in thermal tolerance for either CTmax or CTmin. Our data show that S. fuscovarius experience seasonal adjustments in some parameters of their thermal biology and water balance, but not in others. This variation may reflect different levels of plasticity among the sampled variables and quite likely functional compromises between body temperature regulation and water balance.

Aggression is an important component of behavior in many animals and may be crucial to providing individuals with a competitive advantage when resources are limited. Although much is known about the effects of catecholamines and hormones on aggression, relatively few studies have examined the effects of physical performance on aggression. Here we use a large, sexually dimorphic teiid lizard to test whether individuals that show high levels of physical performance (bite force) are also more aggressive toward a potential threat (i.e., a human approaching the lizard). Our results show that independent of their sex, larger individuals with higher bite forces were indeed more aggressive. Moreover, our data show that individuals with higher bite forces tend to show decreased escape responses and are slower, providing evidence for a trade-off between fight and flight abilities. As bite force increased dramatically with body size, we suggest that large body size and bite force may reduce the threshold for an individual to engage in an aggressive encounter, allowing it to potentially gain or maintain resources and fight off predators while minimizing the risk of injury.

Geographical gradients of body size express climate-driven constraints on animals, but whether they exist and what causes them in ectotherms remains contentious. For amphibians, the water conservation hypothesis posits that larger bodies reduce evaporative water loss (EWL) along dehydrating gradients. To address this hypothesis mechanistically, we build on well-established biophysical equations of water exchange in anurans to propose a state-transition model that predicts an increase of either body size or resistance to EWL as alternative specialization along dehydrating gradients. The model predicts that species whose water economy is more sensitive to variation in body size than to variation in resistance to EWL should increase in size in response to increasing potential evapotranspiration (PET). To evaluate the model predictions, we combine physiological measurements of resistance to EWL with geographic data of body size for four different anuran species. Only one species, Dendropsophus minutus, was predicted to exhibit a positive body size–PET relationship. Results were as predicted for all cases, with one species—Boana faber—showing a negative relationship. Based on an empirically verified mathematical model, we show that clines of body size among anurans depend on the current values of those traits and emerge as an advantage for water conservation. Our model offers a mechanistic and compelling explanation for the cause and variation of gradients of body size in anurans.

The general pattern of sexual size dimorphism (SSD) in amphibians is characterized by females being larger than males. However, the reverse pattern, a male-biased SSD, may be widespread within some particular groups, as seems to be the case for the Neotropical treefrog genus Bokermannohyla. Although SSD is commonly associated with factors influential to breeding success, the evolutionary determinants of SSD remain controversial. Thus, the study of SSD, particularly on lesser-known species, remains critical to advance our understanding of the evolution of body size and other traits associated with the reproductive biology of amphibians. Herein, we examined the occurrence of sexual dimorphism in body size and other morphometric attributes in Bokermannohyla alvarengai and provided a detailed account of its reproductive biology. This species is endemic to a threatened habitat, occurs at low densities, and has a poorly documented natural history. We found that, at maturity, males of B. alvarengai were larger than females, with hypertrophied forelimbs and larger prepollex spines. This set of features, common to other Bokermannohyla species, is often attributed to the occurrence of male territorial defense and aggression between males. We found that B. alvarengai has a prolonged breeding season that extends from the middle of the dry season to the middle of the rainy season. Adult males did not form breeding aggregations and seemed to establish and defend territories; these males often had skin scars indicative of male–male combat. However, alternative interpretations for differences in body size and secondary sexual characters cannot be ruled out and are discussed.

We examined the effects of meal size on the postprandial metabolic response of the lancehead Bothrops alternatus (Duméril, Bibron & Duméril, 1894), fed mice equaling to 5, 10, 20, and 40% of the snake's body mass. The maximum O2 consumption rates measured during digestion increased with meal size, reaching levels up to 2.8-7.8-fold higher than the metabolic rate measured during fasting. Specific Dynamic Action (SDA) duration also increased with meal size, lasting from 54 to 212 hours to complete. Under our experimental conditions, 30ºC, the majority of our snakes failed to completely digest prey with a relative size of 40%. The SDA coefficient ranged from 17 to 27% of the energy content of the meal and was not affected by meal size. [PUBLICATION ABSTRACT]