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Marine Mammal Physiology: Requisites for Ocean Living
concentrations and increased β-OHB (Rea et al. 2000). Subsequent increases in plasma
BUN implies that the pups reverted to protein catabolism after only 2.5 days of fasting.
Older Steller sea lions undergoing 7–14 days of experimental fasts (Rea et al. 2009) also
demonstrated both age- and season-specific responses. Similar to fasting pups, BUN
decreased rapidly in both juveniles and subadults. This decline was more rapid for juveniles during the non-breeding season, when animals had slightly higher pre-fasting lipid
reserves compared to during the breeding season. Significantly increased BUN concentrations observed at the end of the non-breeding season fasts suggest that subadult Steller
sea lions are unable to maintain a protein-sparing metabolism for a full 14 days during
this season. Subadult and juvenile sea lions also exhibited lower circulating ketone body
concentrations compared to pups, suggesting age-related differences in substrate use.
Breeding male otariids also undergo substantial fasts, but almost nothing is known about
their physiology during this period.
8.1.2.2 Other marine mammals
We know virtually nothing about the metabolic basis of fasting physiology in any other
marine mammal species. Sea otters are very intolerant to fasting, becoming rapidly
hypoglycemic after even overnight fasts. Presumably, this is due to their relative small
lipid stores coupled with a very high metabolic rate. Cetaceans presumably support their
fasting from massive lipid reserves contained in their blubber. Working backward from
the sodium (eliminating saltwater ingestion) and urea concentrations (eliminating protein catabolism) measured in the urine of migrating humpback whales, the metabolic
water derived from lipids was determined after a 3-month fast (Bentley 1963). This single
study estimated that 97% of catabolized on-board energy reserves (by tissue weight)
came from fat and 3% came from protein. Polar bears are also extremely tolerant to fasting; summer land-based bears display extremely elevated serum FFA levels, for example
(Nelson et al. 1983). As Arctic seal predators, they rely heavily on seasonal sea ice to
access productive feeding grounds, and can experience extended periods where marine
food is inaccessible. They are also known to recycle nitrogen from urea during fasting
to limit protein loss.
8.1.3 Energetic conservation during fasting
The impact of fasting depends on the accrued energetic deficit, which is a direct consequence of an animal’s energy expenditure. Therefore, it is not surprising that animals may
make behavioral and physiological adjustments to minimize their energy requirements
during a fast. What is surprising is that energetic conservation efforts often co-occur
with energetically expensive, critical life-history requirements that constrain the degree
and potential avenues of energy conservation. The most energetically costly concurrent
demands are likely aspects of reproduction (in females: lactation and mating; in males:
defense of breeding territories).
The molting period is another inflexible life-history requirement that imposes additional energy requirements but is associated with fasts for many marine mammals. Molting
has required direct costs (skin and hair replacement) and potential indirect costs (thermoregulation). The latter is reduced in pinnipeds by hauling out on land, given the decreased
thermal conductivity of air versus water. Some species, such as elephant seals, have a more
condensed and drastic molt than others; the shorter molting period allows them to haul
out and fast during the entire process. Killer whales migrate to warmer waters to molt,
and there is evidence that they do not forage during these trips (Durban and Pitman 2011).
Marine Mammal Physiology: Requisites for Ocean Living
concentrations and increased β-OHB (Rea et al. 2000). Subsequent increases in plasma
BUN implies that the pups reverted to protein catabolism after only 2.5 days of fasting.
Older Steller sea lions undergoing 7–14 days of experimental fasts (Rea et al. 2009) also
demonstrated both age- and season-specific responses. Similar to fasting pups, BUN
decreased rapidly in both juveniles and subadults. This decline was more rapid for juveniles during the non-breeding season, when animals had slightly higher pre-fasting lipid
reserves compared to during the breeding season. Significantly increased BUN concentrations observed at the end of the non-breeding season fasts suggest that subadult Steller
sea lions are unable to maintain a protein-sparing metabolism for a full 14 days during
this season. Subadult and juvenile sea lions also exhibited lower circulating ketone body
concentrations compared to pups, suggesting age-related differences in substrate use.
Breeding male otariids also undergo substantial fasts, but almost nothing is known about
their physiology during this period.
8.1.2.2 Other marine mammals
We know virtually nothing about the metabolic basis of fasting physiology in any other
marine mammal species. Sea otters are very intolerant to fasting, becoming rapidly
hypoglycemic after even overnight fasts. Presumably, this is due to their relative small
lipid stores coupled with a very high metabolic rate. Cetaceans presumably support their
fasting from massive lipid reserves contained in their blubber. Working backward from
the sodium (eliminating saltwater ingestion) and urea concentrations (eliminating protein catabolism) measured in the urine of migrating humpback whales, the metabolic
water derived from lipids was determined after a 3-month fast (Bentley 1963). This single
study estimated that 97% of catabolized on-board energy reserves (by tissue weight)
came from fat and 3% came from protein. Polar bears are also extremely tolerant to fasting; summer land-based bears display extremely elevated serum FFA levels, for example
(Nelson et al. 1983). As Arctic seal predators, they rely heavily on seasonal sea ice to
access productive feeding grounds, and can experience extended periods where marine
food is inaccessible. They are also known to recycle nitrogen from urea during fasting
to limit protein loss.
8.1.3 Energetic conservation during fasting
The impact of fasting depends on the accrued energetic deficit, which is a direct consequence of an animal’s energy expenditure. Therefore, it is not surprising that animals may
make behavioral and physiological adjustments to minimize their energy requirements
during a fast. What is surprising is that energetic conservation efforts often co-occur
with energetically expensive, critical life-history requirements that constrain the degree
and potential avenues of energy conservation. The most energetically costly concurrent
demands are likely aspects of reproduction (in females: lactation and mating; in males:
defense of breeding territories).
The molting period is another inflexible life-history requirement that imposes additional energy requirements but is associated with fasts for many marine mammals. Molting
has required direct costs (skin and hair replacement) and potential indirect costs (thermoregulation). The latter is reduced in pinnipeds by hauling out on land, given the decreased
thermal conductivity of air versus water. Some species, such as elephant seals, have a more
condensed and drastic molt than others; the shorter molting period allows them to haul
out and fast during the entire process. Killer whales migrate to warmer waters to molt,
and there is evidence that they do not forage during these trips (Durban and Pitman 2011).