Physiological Ecology and Salt Loading of California Gulls at an Alkaline, Hypersaline Lake

Abstract

Behavioral and physiological research were combined to determine how California gulls (Larus californicus) are able to survive and breed at Mono Lake, California, a lake that is two and one-half times more concentrated (2,400 mOsm kg⁻¹) than seawater and contains high concentrations of sulfates and carbonates as well as sodium and chloride. Adults avoided salt loading by (1) taking in very little lake water along with brine shrimp (and brine flies) they obtained from the lake and (2) visiting freshwater sources along the shore. Their stomach contents averaged 466 mOsm kg⁻¹, slightly more dilute than the salt concentration of brine shrimp (~500 mOsm kg⁻¹). Stomach contents of chicks and fledglings averaged 636 and 651 mOsm kg⁻¹, respectively. Hematocrits, serum osmolality, serum sodium and potassium, and body-water content of adults, chicks, and fledglings were similar to those of other birds and indicated neither salt loading nor dehydration. Salt glands of adults and chicks were not hypertrophied. Experiments with captive juveniles showed that (1) gulls dislike the taste of even 10% Mono Lake water and (2) flightless juveniles can survive at least 3 days on brine shrimp in 125% Mono Lake water. Free-living juveniles can evidently survive much longer without access to fresh water. It is likely that gulls that are adapted to hypersaline lakes do not require fresh water so long as they are feeding on prey with dilute body fluids and high body-water content. However, during the transition to independence when chicks must learn to feed for themselves-a period of several days-salt loading and dehydration may pose a problem.