Thermoregulation in Steller sea lions: an experimental approach

Abstract

The availability of baseline thermal flux data is important for testing hypotheses of endangered species such as the Steller sea lion (Eumetopias jubatus), whose numbers have declined since the 1960s by more than 80% in portions of their range. The overall objective of this study was to quantify heat flow in Steller sea lions under controlled conditions at various water temperatures, flow speeds, and for stationary versus foraging animals. Heat flux and skin temperature data were collected from seven captive Steller sea lions using heat flux sensors (HFSs) with embedded thermistors. Optimal sensor placement was determined using infrared thermography to measure the major pathways of heat flow along the surface of the animals. Experiments were conducted on swimming animals in a large habitat tank with and without a harness to test for effects of increased hydrodynamic drag, and on stationary animals in a temperature controlled swim flume. In the latter, measurements were taken at three water temperatures (9.0 ± 0.2 °C, n=5; 7.0 ± 0.2 °C, n=5; and 4.0 ± 0.0 °C, n=5) and five current settings (0, 0.2, 0.4, 0.8, and 1.0 m/s). The novel sensor attachment technique designed and utilized in this study allowed for the collection of heat flux data independent of human manipulation during actual measurements. Results from infrared thermography and HFSs revealed consistent spatial patterns in heat loss for foraging animals, suggesting that the body trunk plays an important role in temperature regulation. Mean heat flux for animals swimming with a harness was significantly greater than mean heat flux for unencumbered animals, and heat flux increased with decreasing water temperatures. These data provide insight into the thermoregulatory responses of Steller sea lions, and show that although animals lose more heat in colder water, they also unload rather than conserve excess heat when swimming with drag. These data represent valuable new direct measurements of heat flux for Steller sea lions in water using animal-borne heat flux sensors, and may be useful for comparison with other swimming otariids.