Hot Spots: New Approach Measures Heat Flux in Steller Sea Lions

Staying warm in winter is a constant challenge for most warm-blooded Arctic animals, and marine mammals such as Steller sea lions are no exception. They depend on a diet of fatty fish to fuel their metabolism and maintain their insulating blubber layer, which minimizes heat flux, or the amount of energy they lose to their surroundings per unit area.

For scientists studying the decline of Steller sea lion populations in Alaska, the most accurate way to measure heat flux is to attach a heat flux sensor (HFS) directly to the skin of a sea lion. This approach has been successful in measuring heat flux for short periods of up to several minutes, but long-term field studies have all met with the same obstacle: there has been no good way to attach an HFS to a wild sea lion for long periods of time—until now.

In a study recently published in The Journal of Experimental Marine Biology and Ecology, researcher Kate Willis, from the Laboratory for Applied Biotelemetry & Biotechnology of Texas A&M University, tested a new design for a long-term HFS attachment to pinnipeds. Her design successfully measured heat flux on both seals and sea lions, in water and on land, for periods ranging from four minutes to seven days. Willis also found that the temperature and speed of the water flowing past the HFS unit, along with the unit itself, affected the heat flux reading, and she suggested a correction factor that accounts for these variables.

Once Willis had validated the design of her HFS attachment, she began a second study that measured heat flux on two stationary Steller sea lions at the Vancouver Aquarium Marine Science Centre and on two swimming Steller sea lions at the Alaska SeaLife Center in Seward, Alaska . She measured heat flux at four locations along the body trunk and compared her results with thermal images that she had taken of the animals resting on land.

This second study showed that both swimming and stationary animals lost more heat from the shoulders and hips than from the mid-trunk and axillary areas. A sea lion lost more body heat overall while swimming with a drag-inducing harness, but maintained the same pattern of heat loss as it had when swimming normally. Willis' results indicate that certain areas of Steller sea lions’ bodies may be preferentially used to offload excess heat. This is likely due to the varying thickness and distribution of blubber under the skin, as well as physiological changes in the way Steller sea lions transport heat through different parts of their body.

Related Publications:
A novel approach to measuring heat flux in swimming animals. Willis, K. , Horning, M. 2005. Journal of Experimental Marine Biology and Ecology 315:147-162. abstract We present a design for long-term or removable attachment of heat flux sensors (HFSs) to stationary or swimming animals in water that enables collection of heat flux data on both captive and free-ranging pinnipeds. HFSs were modified to allow for independent, continuous, and long-term or removable attachment to study animals. The design was tested for effects of HFSs and the attachment mechanism on resultant heat flux. Effects were insulative and consistent across water temperatures and flow speeds, resulting in a correction factor of 3.42. This correction factor was applied to all measurements of heat flux from animal experiments to account for the thermal resistance of HFSs and insulative effects of the attachment mechanism. Heat flux and skin temperature data were collected from two captive Steller sea lions (Eumetopias jubatus) as they swam in a large habitat tank over time periods ranging from approximately 4 to 9 min. Of the 72 HFSs deployed using the attachm! ent mechanism, data were successfully retrieved from 70. The HFS attachment mechanism was also used on two wild free-ranging Weddell seals (Leptonychotes weddellii) off Ross Island, Antarctica, for up to 7 days. Heat flux data were retrieved from all eight sensors deployed. These results, along with those from Steller sea lions, suggest that HFSs can be deployed with success on captive and wild animals using the designed attachment mechanism.
Spatial variation of heat flux in Steller sea lions: evidence for consistent avenues of heat exchange along the body trunk. Willis, K., M. Horning, D.A.S. Rosen and A.W. Trites. 2005. Journal of Experimental Marine Biology and Ecology 315:163-175. abstract Maintaining insulative fat stores is vital for homeothermic marine mammals foraging in cold polar waters. To accomplish this, animals must balance acquisition and expenditure of energy. If this balance is shifted, body condition can decrease, challenging thermal homeostasis and further affecting energy balance. Prior studies of temperature regulation in sea lions have neither quantified basic all-inclusive heat flux values for animals swimming in cold water, nor determined whether they exhibit consistent spatial patterns of heat flux. Heat flux and skin temperature data were thus collected from four captive Steller sea lions using heat flux sensors (HFSs) with embedded thermistors. Optimal sensor placement was established using infrared thermography to locate the major areas of heat flux along the surface of the animals. Experiments were conducted on swimming animals in a large habitat tank with and without a drag harness, and on stationary animals in a temperature- and current controlled swim flume. All heat flux measurements were corrected by a previously determined correction factor of 3.42 to account for insulative effects of the HFSs and attachment mechanism. Heat flux from shoulders and hips was consistently greater than from mid-trunk and axillary areas in both swimming and stationary animals, suggesting that certain areas of the body are preferentially used to offload excess heat. Mean heat flux for animals swimming with a drag harness was significantly greater than for unencumbered animals, indicating a likely increase in heat production beyond minimum heat loss. Thus, thermal stress does not appear to constitute significant costs for Steller sea lions swimming under conditions of increased drag at speeds of approximately 1 m/s in water temperatures of approximately 8.0 °C.