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Role of the skin in thermoregulation in amphibians
S29
s20-5
S20-7
Role of the skin in thermoregulation in amphibians. Shoemaker VH Department of Biology, University of California Riverside, Riverside, California, USA 92521
Dermal lipids in the skin of Florida hylid frogs
Amphibians typically have high cutaneous evaporative water loss (EWL) and body temperature (Tb) is significantly cooler than environmental temperature (Te). Some, including the anuran genera Chiromantis and Phyllomedusa, have very low EWL at moderate Te and Tb matches Te. However, at Te above about 38 C these frogs maintain a constant body temperature over a wide range of Te, wind speed, and humidity. They precisely regulate Tb by controlling secretion rates from integumentary glands that are under sympathetic control and governed in part by thermosensitive elements in the brain. Thus the mechanism is similar to mammalian sweating. The regulated temperature is lower in frogs with water reserves in the bladder than in slightly dehydrated frogs, reflecting the trade off between thermoregulation and the conservation of water. Hydration condition also has a strong influence on the preferred body temperatures of these frogs in a thermal gradient. Chiromantis xerampelina undergoes a dramatic color change from cryptic mottled brown or gray at low Tb and radiant intensities to conspicuous brilliant white at high Tb and radiant input. This high reflectance of solar energy minimizes heat gain and the need to expend water for thermoregulation.
S20-6
Evolution of the cutaneous water barrier in amphibians. Lillvwhite HB Dept. of Zoology, Univ. of Florida, Gainesville 32611, USA The integument of extant amphibians is generally characterized by having limited mechanical protection, high water content and secretory activity, few epidermal cell layers, and high rates of water and ion exchange. Water balance is achieved by combinations of physiological, morphological and behavioral traits. Protection of terrestrial species against cutaneous water losses (CWL) involves numerous characters having variable effectiveness. As in most other terrestrial organisms, lipids provide an effective waterproofing mechanism, although resistance to CWL is variable and not well understood. Superficial lipids secreted onto skin surfaces and distributed or “layered” by wiping behaviors appear to provide the more effective of documented barriers to CWL. Such secreted barriers are transiently formed and can be circumvented by secretions of mucus which increase CWL during periods of heat stress. Observations suggest that intraepidermal lipids might play a role in resisting CWL in some species. However, lamellar granules and layered lipid-keratin complexes derived from them, as occur in reptiles and mammals, are absent from amphibian skin. The pliable nature of amphibian integument and its limited potential for keratinization appear to have constrained the possible evolutionary pathways for structuring lipids as barriers to CWL in active amphibians.
Barbeau. T.R. and Lillywhite, H.B. Department of Zoology, University of Florida, Gainesville FL, U.S. Amphibian skin typically exhibits a rate of evaporative water loss similar to a free water surface. Several arboreal frogs, however, display considerably lower rates of evaporation from their skin. In this study, one possible mechanism allowing reduced cutaneous water loss in Florida hylid frogs was investigated using histological techniques. The dorsal cutaneous glands and skin secretions of seven hylid species were examined for the presence of mucopolysaccharides and lipids. Mucopolysaccharides, secreted from cutaneous glands, formed a continuous layer over the dorsal integument. Lipids, although found within the cutaneous glands of two species, were not detected in secretions from the dorsum. Lipids were associated with the iridophore layer in all species examined. Alcohol-based extraction procedures were unsuccesstil in removing these lipids Corn the iridophore layer, indicating that these lipids are not secreted onto the epidermis but are bound to structural components or are structural in nature. The presence of iridophoreassociated lipids may allow hylid frogs to exploit terrestrial environments unavailable to amphibians lacking these structures.
S20-8
Role of the cocoon as a barrier to evaporative water loss in Australian frogs (Neobatrachus and Cyclorana). Withers PC Department of Zoology, University of Western Australia Nedlands, Western Australia, Australia 6907. Measurements of Australian frogs (Neobatrachus and Cyclorana spp) indicate that cocooning frogs have an exponential reduction in EWL and fairly linear increase in R over time, corresponding to the temporal addition of layers to the cocoon. The evaporative water loss (EWL) of cocooned frogs is reduced about 50-200 fold and resistance (R) is about 50-200 set cm-‘. The biophysical properties of cocoon are generally similar for various species, although there is some variation in both resistance per thickness (S-20 lo4 set cm-‘) and diffusion coefficient (0.4-2.4 10m5cm* sec.‘). The hygroscopic property of frog cocoon resembles that of mammalian stratum comeum, hair and wool, and muco-polysaccharides; there is a slight increase in water content of cocoon over a wide range of humidities but a very steep increase in water content and substantial hydration and swelling at >96% RH. This extreme hygroscopic behaviour of frog cocoon at very high RH may reflect low polymer cross-linking in frog cocoon, related to its high digestibility for these dermatophagous frogs. The prevention of over-hydration and mechanical breakdown of frog cocoon in vivo may be attributed to the restriction of high water content to only very high RH (>96%).
s20-5
S20-7
Role of the skin in thermoregulation in amphibians. Shoemaker VH Department of Biology, University of California Riverside, Riverside, California, USA 92521
Dermal lipids in the skin of Florida hylid frogs
Amphibians typically have high cutaneous evaporative water loss (EWL) and body temperature (Tb) is significantly cooler than environmental temperature (Te). Some, including the anuran genera Chiromantis and Phyllomedusa, have very low EWL at moderate Te and Tb matches Te. However, at Te above about 38 C these frogs maintain a constant body temperature over a wide range of Te, wind speed, and humidity. They precisely regulate Tb by controlling secretion rates from integumentary glands that are under sympathetic control and governed in part by thermosensitive elements in the brain. Thus the mechanism is similar to mammalian sweating. The regulated temperature is lower in frogs with water reserves in the bladder than in slightly dehydrated frogs, reflecting the trade off between thermoregulation and the conservation of water. Hydration condition also has a strong influence on the preferred body temperatures of these frogs in a thermal gradient. Chiromantis xerampelina undergoes a dramatic color change from cryptic mottled brown or gray at low Tb and radiant intensities to conspicuous brilliant white at high Tb and radiant input. This high reflectance of solar energy minimizes heat gain and the need to expend water for thermoregulation.
S20-6
Evolution of the cutaneous water barrier in amphibians. Lillvwhite HB Dept. of Zoology, Univ. of Florida, Gainesville 32611, USA The integument of extant amphibians is generally characterized by having limited mechanical protection, high water content and secretory activity, few epidermal cell layers, and high rates of water and ion exchange. Water balance is achieved by combinations of physiological, morphological and behavioral traits. Protection of terrestrial species against cutaneous water losses (CWL) involves numerous characters having variable effectiveness. As in most other terrestrial organisms, lipids provide an effective waterproofing mechanism, although resistance to CWL is variable and not well understood. Superficial lipids secreted onto skin surfaces and distributed or “layered” by wiping behaviors appear to provide the more effective of documented barriers to CWL. Such secreted barriers are transiently formed and can be circumvented by secretions of mucus which increase CWL during periods of heat stress. Observations suggest that intraepidermal lipids might play a role in resisting CWL in some species. However, lamellar granules and layered lipid-keratin complexes derived from them, as occur in reptiles and mammals, are absent from amphibian skin. The pliable nature of amphibian integument and its limited potential for keratinization appear to have constrained the possible evolutionary pathways for structuring lipids as barriers to CWL in active amphibians.
Barbeau. T.R. and Lillywhite, H.B. Department of Zoology, University of Florida, Gainesville FL, U.S. Amphibian skin typically exhibits a rate of evaporative water loss similar to a free water surface. Several arboreal frogs, however, display considerably lower rates of evaporation from their skin. In this study, one possible mechanism allowing reduced cutaneous water loss in Florida hylid frogs was investigated using histological techniques. The dorsal cutaneous glands and skin secretions of seven hylid species were examined for the presence of mucopolysaccharides and lipids. Mucopolysaccharides, secreted from cutaneous glands, formed a continuous layer over the dorsal integument. Lipids, although found within the cutaneous glands of two species, were not detected in secretions from the dorsum. Lipids were associated with the iridophore layer in all species examined. Alcohol-based extraction procedures were unsuccesstil in removing these lipids Corn the iridophore layer, indicating that these lipids are not secreted onto the epidermis but are bound to structural components or are structural in nature. The presence of iridophoreassociated lipids may allow hylid frogs to exploit terrestrial environments unavailable to amphibians lacking these structures.
S20-8
Role of the cocoon as a barrier to evaporative water loss in Australian frogs (Neobatrachus and Cyclorana). Withers PC Department of Zoology, University of Western Australia Nedlands, Western Australia, Australia 6907. Measurements of Australian frogs (Neobatrachus and Cyclorana spp) indicate that cocooning frogs have an exponential reduction in EWL and fairly linear increase in R over time, corresponding to the temporal addition of layers to the cocoon. The evaporative water loss (EWL) of cocooned frogs is reduced about 50-200 fold and resistance (R) is about 50-200 set cm-‘. The biophysical properties of cocoon are generally similar for various species, although there is some variation in both resistance per thickness (S-20 lo4 set cm-‘) and diffusion coefficient (0.4-2.4 10m5cm* sec.‘). The hygroscopic property of frog cocoon resembles that of mammalian stratum comeum, hair and wool, and muco-polysaccharides; there is a slight increase in water content of cocoon over a wide range of humidities but a very steep increase in water content and substantial hydration and swelling at >96% RH. This extreme hygroscopic behaviour of frog cocoon at very high RH may reflect low polymer cross-linking in frog cocoon, related to its high digestibility for these dermatophagous frogs. The prevention of over-hydration and mechanical breakdown of frog cocoon in vivo may be attributed to the restriction of high water content to only very high RH (>96%).