The Need for Salt Glands

tatus) is well known for its unusual diet and foraging mode. Feeding on subtidal/intertidal algae, it incurs a high load of salts (primarily sodium chloride, with some potassium) from its food (Dunson, 1969; Shoemaker and Nagy, 1984). To cope with this high-salt diet, the marine iguana uses large cranial salt glands that excrete most of the sodium, potassium, and chloride ingested; forceful expulsion of the secreted fluid is the cause of the dramatic snorting and sneezing observed in these animals (SchmidtNielsen and Fange, 1958). Other lizards also possess these extrarenal osmoregulatory organs (reviews in Peaker and Linzell, 1975; Dunson, 1976; Minnich, 1979, 1982). They are typically found in herbivorous or omnivorous lizards feeding on potassium-rich plants and secrete predominantly potassium chloride. Some lizards are found in marine or intertidal habitats and feed on sodium-rich foods; as in the marine iguana, their glands secrete primarily sodium chloride (Hillman and Pough, 1976; Hazard et al., 1998). An underlying, sometimes unstated, assumption of studies on marine species has been that their glands are adapted for sodium excretion, whereas the glands of terrestrial herbivores are adapted for potassium secretion. However, it is possible that there is little or no difference in the actual secretory capabilities of the glands of marine and terrestrial species, and that the sodium or potassium secretion observed in the field is simply dictated by dietary ion content. This chapter summarizes the distribution of salt glands among lizard taxa and evaluates aspects of salt secretion that might be important for the evolution of marine lizards. There are three major descriptive characteristics of salt secretion: secretion rate, composition of the secreted fluid, and concentration of the secreted fluid. As there are few data on secretion concentrations in lizards, this chapter focuses on the other two characteristics.