Metabolic Depression in Steller sea lions It is obvious that an animal will have difficulty making ends meet if it eats less than normal. However, some animals can make physiological and behavioral changes to limit the effect of decreased food intake, such as lowering their resting metabolic rate. This is called 'metabolic depression', and is part of an animal's overall 'fasting response'. It is analogous to lowering the idling speed of a car engine to conserve gas (energy).

Resting Steller sea lions

A recently published study by Dr. David Rosen and Dr. Andrew Trites investigated the circumstances and degree to which Steller sea lions use metabolic depression to limit their energy expenditures. They did this by experimentally decreasing the energy intake of 4 captive Steller sea lions in 4 different ways.

The first experiment found that the sea lions significantly lowered their resting metabolic rates while fasting for 9-14 days (as would occur during the breeding season). In the next two experiments the animals were fed low-energy squid and pollock, and displayed a similar physiological response. In other words, the sea lions consuming squid and pollock lost weight and lowered their resting metabolic rates, as they did while fasting.

However, a totally different physiological response occurred over the month-long fourth experiment when the sea lions were fed 50% of their normal ration of herring. During these trials, the sea lions did not exhibit metabolic depression or show a fasting response. Although the animals also lost weight, their resting metabolism either did not change or was higher than normal. Such a result is typical of a 'foraging response', which reflects a case where an animal perceives that food is available, and temporarily increases its energy output in order to forage.

Consortium scientists are now trying to understand what cues animals use to decide between a foraging and a fasting response. They suspect that a combination of physical and energetic satiation, in tandem with the sea lions' expectations of food availability, likely play a role. Such research has direct bearing on the nutritional stress hypothesis, and the role that shifts in diet may have played in the decline of Steller sea lions.

You can read more about these findings in a recently published paper by Rosen and Trites (2002).

13 Nov. 2002