Dining Intertidally: Diet, Energetics, and Osmotic Relations of Two Shoreline-Foraging Tropidurid Lizard Species

Abstract. Because marine environments present potentially stressful hyperosmotic and hypersaline situations for reptiles, those species that live in or near seawater are of special interest regarding their feeding and drinking behavior and their osmotic and ionic regulatory physiology. We determined diets, field metabolic rates (FMRs), rates of water influx and efflux, and aspects of osmoregulation in intertidally foraging populations of two congeneric species of tropidurid lizards: Microlophus albemarlensis in the Galápagos Islands and M. atacamensis in the Atacama Desert of northern Chile. Male M. albemarlensis ate only crabs, specializing on the legs of small Sally Lightfoot crabs, while the much smaller females favored insects but ate whole small crabs as well. In contrast, M. atacamensis males and (the also smaller) females both ate mainly seaweed, along with some crabs and insects. Adults of both species were maintaining constant body masses on these hyperosmotic diets, although subadult M. atacamensis lost mass during our study. FMRs of both species, which were hypothesized to be high due to presumed extra energy costs of osmoregulation, were actually comparatively low, averaging 65% (M. albemarlensis) and 78% (M. atacamensis) of expected FMRs for lizards of their body mass. However, water flux rates were typical of free-living reptiles, averaging 79 and 116%, respectively, of predicted rates. Osmotic concentrations in blood plasma and urine of M. albemarlensis were typical of lizards in general, but plasma of M. atacamensis was about 50% higher (498 ± 42 mOsm), perhaps reflecting an unavoidable consequence of, or even an adaptation to, their higher dietary osmolyte content (1,680 mOsm in “sap” of dietary seaweed). Other possible adaptations that facilitate exploitation of unusual diet items and ingestion of seawater are discussed. In particular, the roles of nasal salt glands and urinary precipitates in osmoregulation and ion homeostasis of both species are likely important but are currently unknown and should be studied.