Excessive drinking (polydipsia) in a galápagos mockingbird ( Nesomimus)

Abstract

1. 1. A species of Galápagos mockingbird Nesomimus parvulus on a diet of insects, goat meat and bread drank between 14 and 30 per cent of its body weight/day in distilled water. As the NaCl concentration of the drinking water was increased to 300 mM, the amount drunk increased. Lesser amounts of 400 and 500 mM NaCl were drunk. The maximum cloacal fluid to plasma chloride concentration ratio was 3·2–3·6. 2. 2. Another Galápagos mockingbird species, Nesomimus macdonaldi, on the same diet drank 52 to 89 per cent of its body weight/day in distilled water, an excessive consumption of water. As the salinity of the drinking water was increased in 100 mM steps, the amount drunk remained constant until a precipitous decline occurred on 400 mM NaCl. The maximum cloacal fluid to plasma chloride concentration ratio was estimated as 3·4–4.·1. On a dry diet (chicken mash), drinking varied between 54 and 119 per cent body weight/day. This species feeds partly in the intertidal zone on marine invertebrates and probably has no access to fresh water for long periods of time. The cause of the pronounced polydipsia observed and its relevance to adaptation of this species to arid conditions deserve futher study.