Thermal limits in young northern fur seals, Callorhinus ursinus

The thermoregulatory abilities of northern fur seals (Callorhinus ursinus) during their first two years in the frigid waters of the North Pacific Ocean may limit their geographic distribution and alter the costs for exploiting different species of prey. We determined the thermoneutral zone of six young northern fur seals by measuring their metabolism in ambient air and controlled water temperatures (0 degrees C-12 degrees C) from ages 8 to 24 mo. We found that the ambient air temperatures within our study (overall 1.5 degrees C-23.9 degrees C) did not affect resting metabolic rates. Calculated lower critical temperatures in water varied between 3.9 degrees C and 8.0 degrees C, while an upper critical temperature in water was only discernible during a single set of trials. These thermal responses provide insight into the possible physiological constraints on foraging ecology in young northern fur seals, as well as the potential energetic consequences of ocean climate change and altered prey distributions.