Life in the Fasting Lane

Is age and season the secret to surviving lean times?

In human circles, periodic fasting is often associated with physical or spiritual purification. But fasting also occurs naturally – and frequently – in wildlife. Among Steller sea lions in the North Pacific, adult males fast for weeks at a time during the summer breeding season. Reproducing females, who may be as young as three years, fast for up to two weeks directly after giving birth. Even newborn pups fast at an early age while their nursing mothers
forage at sea.

A natural tolerance for periodic fasting – likely tied to an ability to slow metabolism and expend less energy – helps to equip Steller sea lions for the uncertainties of life in the North Pacific. But the metabolic cost of fasting is still unclear. How is weight loss minimized? How is body fat stored and metabolized? How do age and season affect a sea lion’s tolerance for periodic fasting?

To answer these and other questions, a team of scientists recently studied nine captive Steller sea lions housed at the Vancouver Aquarium. The study, authored by Dr. Lorrie Rea of the Alaska Department of Fish and Game (and formerly with the National Marine Mammal Laboratory), and Drs. David Rosen and Andrew Trites of the Marine Mammal Research Unit at the University of British Columbia, was recently published in the Canadian Journal of Zoology.

Seasonal Variation
The researchers monitored changes in the sea lions’ body masses and body conditions (measured by percent total body lipid content, or %TBL) during fasts of 7-14 days. Four of the sea lions were juveniles between 1.75 and 2.5 years of age; the remaining five were subadults between 3.5 and 6 years of age. The trials were repeated during both the summer breeding season and the nonbreeding season, to identify any seasonal influences in their ability to adapt to limited food.

As expected, all animals experienced some loss in body mass during fasting. Juveniles lost more mass per day than subadults during the breeding season, but there were no significant age-related differences during the nonbreeding season. Juveniles lost mass at a significantly higher rate during the breeding season than during the nonbreeding season, suggesting that the impact of fasting is greatest among juveniles during the breeding
season.

In all trials during the nonbreeding season, subadults reached the maximum allowable mass loss before completing the 14-day trial. Juveniles were even more poorly adapted to conserve resources during the nonbreeding season, showing significant increases in rate of mass loss by the end of the 7-day trial.

“If these fasting experiments were not terminated early to ensure the continued good health of the captive sea lions,” the authors write, “one would expect that the rate of mass loss would have increased further as animals exceeded the limit of their ability to spare protein and would have likely reverted to catabolizing predominantly protein tissue to meet their energetic needs, potentially damaging vital organs.”

Shifting Ocean Climate
In the wild, juveniles and subadults typically fast for shorter periods than adults. The researchers point out that the ability to withstand prolonged fasting may simply be physiologically limited in younger sea lions until they become adults, when it is needed (e.g., during active breeding season). Juveniles may also have difficulty depressing their metabolism while fasting to conserve body mass because they have higher metabolic demands to grow and develop.

Natural periods of fasting in the wild may be complicated by changing ocean conditions, such as a shift in ocean climate that limits the availability of key prey. Such a change happened in the late 1970s in Alaska, and has contributed to a dramatic decrease in Steller sea lion populations in Western Alaska.

The results of the current study suggest that juvenile sea lions would be more affected by a food shortage during the breeding season than would subadults, who are more capable of adapting to fasting during the breeding season by slowing their metabolism to prevent undue weight loss. During the nonbreeding season, both juveniles and subadults would be equally affected by food limitation.

Based on this understanding, the authors conclude that a sea lion’s body condition prior to fasting likely plays a significant role in determining its ability to adapt to fasting. As the results of the current study indicate, lean animals will likely use more protein reserves for energy during fasting, while animals with a higher %TBL will likely preferentially metabolize body lipids before depleting tissue and other proteins.

The study provides valuable insight into the physiological effects of food limitation across age groups and seasons. Equipped with this research, scientists are closer to understanding how wild Steller sea lions respond to the demands of their ever-changing ocean environment.

7 May 2007

Publication:
Utilization of stored energy reserves during fasting varies by age and season in Steller sea lions. Rea, L.D., D.A.S. Rosen and A.W Trites. 2007. Canadian Journal of Zoology 85:190-200. abstract Nine captive Steller sea lions (Eumetopias jubatus (Schreber, 1776), 1.75–6 years of age) were fasted for 7–14 d to test the effect of short-term fasting on changes in body mass and body condition. Trials were repeated during both the summer breeding season and the nonbreeding season in seven animals to elucidate whether there was a seasonal component to the ability of Steller sea lions to adapt to limited food resources. Mean percent mass loss per day was higher during the breeding season in juveniles (1.8% ± 0.2%·d–1) than in subadults (1.2% ± 0.1%·d–1), but there were no significant age-related differences during the nonbreeding season (juveniles, 1.5% ± 0.3%·d–1; subadults, 1.7% ± 0.3%·d–1). A decrease in the rate of mass loss occurred after the first 3 d of fasting only in subadults during the breeding season. Percent total body lipid ranged from 11% to 28% of total body mass at the initiation of fasting trials. Animals with lower initial percent total body lipid exhibited higher subsequent rates of mass loss and a lower percentage of tissue catabolism derived from lipid reserves. There was no evidence of metabolic adaptation to fasting in juveniles, which suggests that juvenile sea lions would be more negatively impacted by food limitation during the breeding season than would subadults.