Cutaneous adaptations to water balance in amphibians

Abstract

1. 1. The integument of the amphibians is, in general terms, not adapted to a strictly terrestrial life, in that it does not serve as a barrier to a bidirectional water flow. 2. 2. One of the major problems which thus faces the terrestrial amphibians is dehydration. 3. 3. To reduce water loss, or to absorb water through the skin, some amphibians have developed morphological and behavioural adaptations capable of acting together to maintain the hydric balance. 4. 4. Skin texture and sculpturing seem to be involved with water absorption. A granular ventral skin absorbs more efficiently than a smooth skin; the occurrence of cutaneous grooves in the ventral and lateral regions of the body in the salamander and the bufonids permits the transfer of water to other parts of the body by capillarity. 5. 5. The arrangement of the superficial epidermal cells, sometimes united, sometimes separated by ample intercellular spaces, and also, in some cases, the concentration of cutaneous lipids, influence the permeability of the skin to water. 6. 6. Some amphibian species form a cocoon composed of various layers of stratum corneum; this covers the body and reduces water loss considerably during the dry season. 7. 7. It has been observed that the hydration capacity in some anurans is related to the cutaneous vascularization in the ventral pelvic region, and that, in general, species of terrestrial habit have a greater degree of vascularization than those of aquatic habit. 8. 8. A number of substances, including hormones such as vasotocin, oxytocin, thyroxin and prolactin, have some influence on the permeability of the skin to water. 9. 9. The cutaneous mucus produced by the amphibians has great functional diversity. In some anuran species it has a role in thermoregulation and water economy. 10. 10. The anuran genera Phyllomedusa and Litoria wipe the skin with the legs so as to spread the lipid secretion from the cutaneous glands over the body surface. This secretion causes a marked reduction in the loss of body water. 11. 11. The hydric balance in some anuran species is probably associated with the secretions of the granular glands, and in particular with the glycosaminoglycans found in granular alveoli in the parotoid and paracnemid glands of bufonids.