Toads Taste Sodium With Their Skin: Sensory Function in a Transporting Epithelium

Abstract

Amphibians do not normally ingest water orally. Rather, they obtain water by osmotic flow across their skin (Bentley and Yorio, 1979). Previous studies of the desert toad, Bufo punctatus, showed that dehydrated animals placed on a surface moistened with pure water adopted a particular type of behavior, termed the water absorption response (WR), in which toads splayed their hindlimbs and oscillated their body to maximize the area of skin in contact with the surface (Brekke et al. 1991). If the surface was moistened with a hyperosmotic urea solution, the animals moved over the surface as if to search for osmotically available water and soon jumped off the surface without initiating WR behavior. Since the skin was the only part of the animal in contact with the surface, it was concluded that receptors in the skin enabled the animals to avoid WR behavior on surfaces where water was not osmotically available. Similarly, toads did not initiate WR behavior when NaCl was used as the solute. These observations suggested that toads have either cutaneous receptors for detecting osmolality, per se, or cutaneous receptors for specific solutes.