ABSTRACT
Salt glands are used by some vertebrates to excrete hyperosmotic NaCl or KCl solutions in response to dietary salt loads. Control of secretion varies across taxa; some secrete in response to osmotic challenges while others secrete in response to specific dietary ions. We hypothesized that differences in control could be related to different diet-related selective pressures on herbivorous, marine, and insectivorous species. We studied control of secretion and flexibility of cation (sodium or potassium) and anion (chloride or bicarbonate) secretion in two insectivorous lizard species, Schneider's skinks (Eumeces schneideri, Scincidae) and green anoles (Anolis carolinensis, Polychrotidae). Lizards were injected daily for four days with combinations of cations (potassium, sodium, and histidine control) and anions (chloride and acetate control), isoosmotic saline, or sham injection. Secretions were collected daily and analyzed for sodium, potassium, and chloride. Both species secreted only in response to chloride; sodium appeared to have a slight inhibitory effect. Regardless of cation load, skinks secreted a combination of potassium and sodium, while anoles secreted solely potassium. In both species, total cation secretion was matched closely by chloride; very little bicarbonate was secreted. As predicted, secretion in insectivorous lizards was initiated by the dietary ion ecologically most important for these species, chloride, which otherwise cannot be excreted without significant water loss (unlike the cations, which may be excreted as insoluble urate salts). This gives further support to the hypothesis that ecological factors drive the evolution of control mechanisms in lizard salt glands.
