Hold the Hot Sauce!

Most of us are familiar with the extra heat generated by our bodies when we digest a large meal. Most people feel warm, and some may even break into a sweat. Scientifically, this is known as the heat increment of feeding. A substantial proportion of the energy contained in a meal can be ‘lost’ through the process of breaking down and assimilating the food we eat. Some animals can use the heat generated through digestion to offset the cost of thermoregulation (keeping warm). It would make sense for Steller sea lions to use this energy-saving mechanism, given the long periods they spend in the frigid Alaskan waters.

The question is important for determining the energy requirements of sea lions in the wild. Consortium Researchers Dr. David Rosen and Dr. Andrew Trites conducted studies on four captive Steller sea lions to test this theory. The energy used by the animals was measured prior to and after eating a meal of herring. The tests were conducted in a swim tank, where water temperature was maintained between 2 and 8 degrees C.

Rosen and Trites found no evidence that heat generated from digestion is used by sea lions to stay warm. “This was quite surprising to us”, said Dr. Rosen, “but it is consistent with the results from other studies. It appears that many of the animals that you would expect to be most inclined to use thermal substitution – baby terns (a type of bird), moles, and muskrats when in water – do not appear to do so.”

Why Steller sea lions do not use the heat they generate from digestion is unknown. This question requires further study considering that numerous animals make use of this mechanism, and that it would, in theory, reduce the amount of food sea lions would have to eat. Dr. Rosen and Dr. Trites are trying to determine whether certain animals have this ability over others, or whether certain conditions trigger any mammal to make use of this energy saving strategy.

Publication:

No evidence for bioenergetic interaction between digestion and thermoregulation in Steller sea lions, Eumetopias jubatus. Rosen, D.A.S. and A.W. Trites. 2003. Physiological and Biochemical Zoology 76(6):899-906. abstract The increase in metabolism during digestion—the heat increment of feeding—is often regarded as an energetic waste product. However, it has been suggested that this energy could offset thermoregulatory costs in cold environments. We investigated this possibility by measuring the rate of oxygen consumption of four juvenile Steller sea lions (Eumetopias jubatus) before and after they ingested a meal in water temperatures of 2-8 degrees C. Rates of oxygen consumption of fasted and fed animals increased in parallel with decreasing water temperature, such that the apparent heat increment of feeding did not change with water temperature. These results suggest that Steller sea lions did not use the heat released during digestion to offset thermoregulatory costs.

7 April 2004