Control of thermoregulatory evaporation in the waterproof treefrogChiromantis xerampelina

Background colour matching in a wild population of Alytes obstetricans

The capacity for physiological colour change has long been described in anuran amphibians. Camouflage against predators seems to be the most relevant function of dynamic changes in skin colour of frogs, but key aspects such as the rate at which these changes occur, or the specific colour components involved are not completely clear. Whereas most research on the topic has been reported on tree frogs in laboratory conditions, studies in other anurans or in the field are much scarcer. Here we show a potentially plastic, adaptive response in coloration of common midwife toads, Alytes obstetricans, from a population of central Portugal, whose pigmentation varied with their natural backgrounds. Using quantitative image analysis, we compared hue, saturation and brightness of dorsal skin coloration of toads and the colour of the area of ground immediately around them. We found a positive correlation between coloration of toads and background colour for the three components of the colour. As well as other anuran species, A. obstetricans might adjust skin coloration to match the surrounding environment, thus benefitting from short-term reversible crypsis strategies against predators. A less supported hypothesis would be that toads accurately select matching backgrounds to improve concealment as an antipredatory strategy.

Effects of water vapor density on cutaneous resistance to evaporative water loss and body temperature in green tree frogs (Hyla cinerea)

Increased cutaneous resistance to evaporative water loss (Rc) in tree frogs results in decreased water loss rate and increased body temperature. We examined sensitivity of Rc to water vapor density (WVD) in Hyla cinerea by exposing individual frogs and agar models to four different WVD environments and measuring cutaneous evaporative water loss rate and body temperature simultaneously using a gravimetric wind tunnel measuring system. We found that water loss rate varied inversely and body temperature directly with WVD but that models were affected to a greater extent than were animals. Mean Rc was significantly different between the highest WVD environment and each of the three drier environments but did not differ among the drier environments, indicating that Rc initially increases and then reaches a plateau in response to decreasing WVD. Rc was equivalent when calculated using either WVD difference or WVD deficit as the driving force for evaporation. We also directly observed secretions from cutaneous glands while measuring body temperature and tested secretions and skin samples for the presence of lipids. We found that irregular transient body temperature depressions observed during wind tunnel trials occur due to evaporative cooling from intermittent skin secretions containing lipids, although we were unable to identify lipid-secreting glands.

Body temperature and resistance to evaporative water loss in tropical Australian frogs

Although the skin of most amphibians measured to date offers no resistance to evaporative water loss (EWL), some species, primarily arboreal frogs, produce skin secretions that increase resistance to EWL. At high air temperatures, it may be advantageous for amphibians to increase EWL as a means to decrease body temperature. In Australian hylid frogs, most species do not decrease their resistance at high air temperature, but some species with moderate resistance (at moderate air temperatures) gradually decrease resistance with increasing air temperature, and some species with high resistance (at moderate air temperatures) abruptly decrease resistance at high air temperatures. Lower skin resistance at high air temperatures decreases the time to desiccation, but the lower body temperatures allow the species to avoid their critical thermal maximum (CT(Max)) body temperatures. The body temperatures of species with low to moderate resistances to EWL that do not adjust resistance at high air temperatures do not warm to their CT(Max), although for some species, this is because they have high CT(Max) values. As has been reported previously for resistance to EWL generally, the response pattern of change of EWL at high air temperatures has apparently evolved independently among Australian hylids. The mechanisms involved in causing resistance and changes in resistance are unknown.

Wiping Behavior, Skin Resistance, and the Metabolic Response to Dehydration in the Arboreal FrogPhyllomedusa hypochondrialis

Several species of arboreal frogs secrete lipids from cutaneous glands and wipe these secretions over the body surfaces to reduce evaporative water losses. Following wiping, frogs become immobile in water-conserving postures, and some have suggested they are torpid. Here we report wiping behaviors and the physiological correlates of immobile postures in the arboreal monkey frog Phyllomedusa hypochondrialis. Skin resistance to water loss was comparatively high, and rates of evaporation were as low as 4% of that from a free water surface. Standard rates of metabolism (SMR) varied from 89 microL O2 h(-1) at 18 degrees C to 316 microL O2 h(-1) at 34 degrees C and were sensitive to both temperature (T) and body mass (W; mL O2 h(-1) = 0.016W0.642 x 10(0.030T)). The mean SMR did not change significantly during four consecutive days of dehydration when animals lost 19%-34% of body mass. Therefore, it appears these frogs do not routinely depress metabolic rates following wiping. However, some individuals that lost higher percentages of body water exhibited trends of decreasing oxygen consumption, suggesting that suppression of metabolic rates might occur at greater levels of body water deficit or perhaps during a slower course of dehydration than imposed by our experiments (e.g., individuals that are secluded during periods of drought).

Tribute to R. G. Boutilier: Skin colour and body temperature changes in baskingBokermannohyla alvarengai(Bokermann 1956)

In amphibians solar basking far from water sources is relatively uncommon since the highly permeable amphibian skin does not represent a significant barrier to the accompanying risk of losing water by evaporation. A South American frog, Bokermannohyla alvarengai (Bokermann 1956), however,spends a significant amount of the day exposed to full sun and relatively high temperatures. The means by which this frog copes with potentially high rates of evaporative water loss and high body temperatures are unknown. Thus, in this study, skin colour changes, body surface temperature, and evaporative water loss rates were examined under a mixture of field and laboratory conditions to ascertain whether changes in skin reflectivity play an important role in this animal's thermal and hydric balance. Field data demonstrated a tight correlation between the lightness of skin colour and frog temperature,with lighter frogs being captured possessing higher body temperatures. Laboratory experiments supported this relationship, revealing that frogs kept in the dark or at lower temperatures (20°C) had darker skin colours,whereas frogs kept in the light or higher temperatures (30°C) had skin colours of a lighter hue. Light exhibited a stronger influence on skin colour than temperature alone, suggesting that colour change is triggered by the increase in incident solar energy and in anticipation of changes in body temperature. This conclusion is corroborated by the observation that cold,darkly coloured frogs placed in the sun rapidly became lighter in colour during the initial warming up period (over the first 5 min), after which they warmed up more slowly and underwent a further, albeit slower, lightening of skin colour. Surprisingly, despite its natural disposition to bask in the sun,this species does not possess a `waterproof' skin, since its rates of evaporative water loss were not dissimilar from many hylid species that live in arboreal or semi-aquatic environments. The natural history of B. alvarengai is largely unknown and, therefore, it is likely that the herein reported colour change and basking behaviour represent a complex interaction between thermoregulation and water balance with other ecologically relevant functions, such as crypsis.

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.