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Marine Mammal Physiology: Requisites for Ocean Living
8.1.3.1 Pinnipeds
8.1.3.1.1 Phocids Several studies have examined changes in RMR during the
postweaning fast of phocid pups. Collectively, these illustrate apparent species-specific
differences in patterns of changes in metabolism during the fast. Fasting gray seal pups
demonstrate the expected rapid decrease in mass-specific RMR, which reaches a stable
level 45% below initial levels by day 10 and continues largely unchanged through day
47 of the fast (Nordøy et al. 1990). A subsequent study confirmed the constancy of postweaning mass-specific metabolism in both gray seals and harp seals fasting for 8 weeks in
air, although they also found an increase in RMR over the fast for individuals fasting (but
not tested) in water, potentially due to differences in blubber and mass loss incurred due
to higher thermal costs (Worthy and Lavigne 1987). Such changes in RMR seem to conform
to the standard fasting model.
However, a study on northern elephant seals revealed a different pattern. During
the course of their 10-week molting fast, metabolism of pups gradually declined by 35%
(Rea and Costa 1992). This may explain why fasting pups lose mass at a faster rate during the first 4 weeks of the fast than during the following 4-week period. Metabolism
also notably increased during the final week of the fast, just prior to when they set off
to sea to forage.
A few studies have been conducted on fasting in older phocids. Both experimentally fasted juvenile and adult harbor seals and gray seals display rapid (within 24 h)
decreases in mass-specific metabolism (Markussen et al. 1992; Boily and Lavigne
1995; Markussen 1995), which is indicative of a rapid entrance into an adaptive “fasting state.”
8.1.3.1.2 Otariids Many otariids pups are minimally active during the fasting
period. For example, subantarctic fur seal pups spend more time sleeping than other otariid pups, contributing to lower daily rate of energy expenditure and lower rates of mass
loss during their nursing fasts (Beauplet et al. 2003).
Several studies have examined changes in the RMR of otariid pups during the
nursing period. Mass-specific RMR decreased significantly while Antarctic fur seal
pups were fasting over 5 days (normal for maternal foraging trips) (Arnould et al.
2001). However, it is unclear whether part of the large drop might have been due to a
large increase in metabolism attributable to digestion of the milk (i.e., the heat increment of feeding) in their stomachs at the start of the fasting period, pointing out a
difficulty in measuring and interpreting resting metabolism in intermittent feeders.
Subantarctic fur seal pups show increasing fasting abilities, including a decrease in
mass-specific metabolism during their onshore fasts, which themselves increase from
5 to 30 days over the course of their extended period of maternal investment (Verrier
et al. 2011).
However, the metabolic response of mammals during life-history stages or seasons
when food shortages normally occur may be different from the response during periods
when food shortages are unexpected. Young Steller sea lion pups (6–14 weeks old) significantly decrease metabolism in response to 48 h fasts. However, 6–24-month-old northern
fur seals (Callorhinus ursinus) showed no changes in RMR when subject to a similar fast
(Rosen et al. 2014). The difference might be attributable to the fact that, at this age, Steller
sea lions would still be undergoing natural fasts during the nursing stage, while northern
fur seals would already be weaned. By contrast, juvenile Steller sea lions outside their
natural fasting period showed a 31% decrease in RMR over a 14-day experimental fast
(Rosen and Trites 2002).
Marine Mammal Physiology: Requisites for Ocean Living
8.1.3.1 Pinnipeds
8.1.3.1.1 Phocids Several studies have examined changes in RMR during the
postweaning fast of phocid pups. Collectively, these illustrate apparent species-specific
differences in patterns of changes in metabolism during the fast. Fasting gray seal pups
demonstrate the expected rapid decrease in mass-specific RMR, which reaches a stable
level 45% below initial levels by day 10 and continues largely unchanged through day
47 of the fast (Nordøy et al. 1990). A subsequent study confirmed the constancy of postweaning mass-specific metabolism in both gray seals and harp seals fasting for 8 weeks in
air, although they also found an increase in RMR over the fast for individuals fasting (but
not tested) in water, potentially due to differences in blubber and mass loss incurred due
to higher thermal costs (Worthy and Lavigne 1987). Such changes in RMR seem to conform
to the standard fasting model.
However, a study on northern elephant seals revealed a different pattern. During
the course of their 10-week molting fast, metabolism of pups gradually declined by 35%
(Rea and Costa 1992). This may explain why fasting pups lose mass at a faster rate during the first 4 weeks of the fast than during the following 4-week period. Metabolism
also notably increased during the final week of the fast, just prior to when they set off
to sea to forage.
A few studies have been conducted on fasting in older phocids. Both experimentally fasted juvenile and adult harbor seals and gray seals display rapid (within 24 h)
decreases in mass-specific metabolism (Markussen et al. 1992; Boily and Lavigne
1995; Markussen 1995), which is indicative of a rapid entrance into an adaptive “fasting state.”
8.1.3.1.2 Otariids Many otariids pups are minimally active during the fasting
period. For example, subantarctic fur seal pups spend more time sleeping than other otariid pups, contributing to lower daily rate of energy expenditure and lower rates of mass
loss during their nursing fasts (Beauplet et al. 2003).
Several studies have examined changes in the RMR of otariid pups during the
nursing period. Mass-specific RMR decreased significantly while Antarctic fur seal
pups were fasting over 5 days (normal for maternal foraging trips) (Arnould et al.
2001). However, it is unclear whether part of the large drop might have been due to a
large increase in metabolism attributable to digestion of the milk (i.e., the heat increment of feeding) in their stomachs at the start of the fasting period, pointing out a
difficulty in measuring and interpreting resting metabolism in intermittent feeders.
Subantarctic fur seal pups show increasing fasting abilities, including a decrease in
mass-specific metabolism during their onshore fasts, which themselves increase from
5 to 30 days over the course of their extended period of maternal investment (Verrier
et al. 2011).
However, the metabolic response of mammals during life-history stages or seasons
when food shortages normally occur may be different from the response during periods
when food shortages are unexpected. Young Steller sea lion pups (6–14 weeks old) significantly decrease metabolism in response to 48 h fasts. However, 6–24-month-old northern
fur seals (Callorhinus ursinus) showed no changes in RMR when subject to a similar fast
(Rosen et al. 2014). The difference might be attributable to the fact that, at this age, Steller
sea lions would still be undergoing natural fasts during the nursing stage, while northern
fur seals would already be weaned. By contrast, juvenile Steller sea lions outside their
natural fasting period showed a 31% decrease in RMR over a 14-day experimental fast
(Rosen and Trites 2002).