Balancing hypoxia and hypothermia in cold-submerged frogs.

Many animals respond to hypoxic stress by selecting cooler environments, the so-called 'behavioural hypothermia' response. Amphibians overwintering in ice-covered ponds and lakes offer an ecologically relevant test of this response since they must choose between the confounding metabolic effects of profound hypothermia or hypoxia; thermal and chemical conditions can vary from 0 degrees C and normoxic at the ice-water interface to 4 degrees C and markedly hypoxic at depths of 2-4 m. To mimic such environmental conditions, we constructed an experimental chamber that enabled continuous electronic surveillance of an animal's movement along a thermal gradient. When Rana temporaria pre-acclimated to 3.5 degrees C were placed in a normoxic thermal gradient ranging from 0.8 to 8 degrees C, they invariably favoured the warmer end of the chamber. Upon exposure to hypoxia, however, their preferred temperature shifted from a median of 6.8 degrees C (P02 = 158 mmHg; 1 mmHg = 0.133 kPa) to 1.9 degrees C (P02 = 25 mmHg). Metabolic rate measurements from animals exposed simultaneously to acute changes in water temperature and PO2 suggest that movement to colder conditions in hypoxia effects the greatest metabolic savings and prolongs the onset of a plasma lactacidosis.