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Marine Mammal Physiology: Requisites for Ocean Living
8.1.2.1 Pinnipeds
8.1.2.1.1 Phocids One of the best-studied fasting scenarios is the postweaning fast
of phocid seals, which can last up to 10 weeks. For harp seals (Phoca groenlandica), 86%–97%
of energy is derived from lipids in the first 8 weeks of their fast (Worthy and Lavigne
1983; Nordøy et al. 1990, 1993), and 72%–80% of this energy comes from blubber lipids
(Siverston 1941; Stewart and Lavigne 1980; Worthy and Lavigne 1983; Kovacs and Lavigne
1985). Extended fasting episodes (>8 weeks) are associated with increased protein catabolism (but still only up to 15.7%) (Worthy and Lavigne 1983). In postweaning gray seal pups
(Halichoerus grypus), the energetic contribution from lipids can reach 94%–97% (Nordøy
and Blix 1985; Øritsland et al. 1985; Reilly 1991). Similarly, northern elephant seal pups may
derive up to 98% of their energetic requirements from lipids over their 10-week fast (Pernia
et al. 1980; Houser and Costa 2001).
While phocid seals rely extensively on lipid metabolism during the bulk of their postweaning fast, the underlying metabolic shift is not instantaneous. Many appear to rely
initially on protein stores (up to 1–3 weeks) (Worthy and Lavigne 1983, 1987; Rea 1995). For
example, during the initial 3 days of the very short hooded seal (Cystophora cristata) pup
fasts, 16% of the total mass loss was from fat while 28% was from protein (the remainder
was water) (Lydersen et al. 1997). The pups also have high initial rates of mass loss, which
rapidly decrease to lower, stable rates by day 5 (Bowen et al. 1987). Harp seal pups similarly utilize greater levels of protein in the initial 2 weeks of their fast before switching
almost entirely to lipid catabolism (Worthy and Lavigne 1983, 1987). This protein to lipid
switch is partly reflected in changes in their blood chemistry (Nordøy et al. 1993). While
gray seal pups exhibit uniformly low levels of protein oxidation starting at day 4 of their
postweaning fast (Nordøy et al. 1990) their rates of mass loss, which continue to drop over
the first 10 days of fasting, appear to be partly disconnected from changes in metabolic
substrate use.
Plasma FFA increase continuously across the fasts of harp and gray seal pups (Nordøy
and Blix 1991), reaching levels even higher than seen in northern elephant seals (Castellini
et al. 1987). These increased FFA are accompanied by linearly increasing β-OHB in harp
and gray seal pups after day 8 (Nordøy and Blix 1991). On the other hand, northern elephant seal pups produce ketone bodies almost immediately upon the onset of fasting
(Castellini and Costa 1990). These levels increase until ~day 55, before declining sharply,
soon followed by the pup’s departure to sea. It is interesting that northern elephant seal
pups likely have the longest postweaning fast and seem able to switch to a total reliance
on lipid metabolism almost immediately (Castellini and Costa 1990; Adams and Costa
1993). Protein catabolism indicative of “end-stage” fasting is scarce among phocid seals, as
most begin foraging before lipid reserves are overly depleted (Castellini and Rea 1992). An
exception are “starveling” post-molt northern elephant seal pups, which are not indicative
of the general population (Houser and Costa 2003).
As previously noted, depletion of the blubber layer can have thermoregulatory consequences, and many studies have examined the thermal effects of changes in blubber
reserves during fasting in phocid seal pups (e.g., Irving and Hart 1957; Worthy 1991;
Muelbert and Bowen 1993). Northern elephant seal pups exhibit differing energy usage
patterns depending on body mass and body composition at the end of nursing. Fatter
pups catabolize more lipids and spare proportionally more protein than do leaner pups.
As a consequence, leaner pups, catabolizing relatively more protein versus lipid, conclude
their postweaning fast with sufficient remaining lipid stores for thermoregulation during their first foraging trip (Noren et al. 2003; Noren and Mangel 2004). There is evidence
Marine Mammal Physiology: Requisites for Ocean Living
8.1.2.1 Pinnipeds
8.1.2.1.1 Phocids One of the best-studied fasting scenarios is the postweaning fast
of phocid seals, which can last up to 10 weeks. For harp seals (Phoca groenlandica), 86%–97%
of energy is derived from lipids in the first 8 weeks of their fast (Worthy and Lavigne
1983; Nordøy et al. 1990, 1993), and 72%–80% of this energy comes from blubber lipids
(Siverston 1941; Stewart and Lavigne 1980; Worthy and Lavigne 1983; Kovacs and Lavigne
1985). Extended fasting episodes (>8 weeks) are associated with increased protein catabolism (but still only up to 15.7%) (Worthy and Lavigne 1983). In postweaning gray seal pups
(Halichoerus grypus), the energetic contribution from lipids can reach 94%–97% (Nordøy
and Blix 1985; Øritsland et al. 1985; Reilly 1991). Similarly, northern elephant seal pups may
derive up to 98% of their energetic requirements from lipids over their 10-week fast (Pernia
et al. 1980; Houser and Costa 2001).
While phocid seals rely extensively on lipid metabolism during the bulk of their postweaning fast, the underlying metabolic shift is not instantaneous. Many appear to rely
initially on protein stores (up to 1–3 weeks) (Worthy and Lavigne 1983, 1987; Rea 1995). For
example, during the initial 3 days of the very short hooded seal (Cystophora cristata) pup
fasts, 16% of the total mass loss was from fat while 28% was from protein (the remainder
was water) (Lydersen et al. 1997). The pups also have high initial rates of mass loss, which
rapidly decrease to lower, stable rates by day 5 (Bowen et al. 1987). Harp seal pups similarly utilize greater levels of protein in the initial 2 weeks of their fast before switching
almost entirely to lipid catabolism (Worthy and Lavigne 1983, 1987). This protein to lipid
switch is partly reflected in changes in their blood chemistry (Nordøy et al. 1993). While
gray seal pups exhibit uniformly low levels of protein oxidation starting at day 4 of their
postweaning fast (Nordøy et al. 1990) their rates of mass loss, which continue to drop over
the first 10 days of fasting, appear to be partly disconnected from changes in metabolic
substrate use.
Plasma FFA increase continuously across the fasts of harp and gray seal pups (Nordøy
and Blix 1991), reaching levels even higher than seen in northern elephant seals (Castellini
et al. 1987). These increased FFA are accompanied by linearly increasing β-OHB in harp
and gray seal pups after day 8 (Nordøy and Blix 1991). On the other hand, northern elephant seal pups produce ketone bodies almost immediately upon the onset of fasting
(Castellini and Costa 1990). These levels increase until ~day 55, before declining sharply,
soon followed by the pup’s departure to sea. It is interesting that northern elephant seal
pups likely have the longest postweaning fast and seem able to switch to a total reliance
on lipid metabolism almost immediately (Castellini and Costa 1990; Adams and Costa
1993). Protein catabolism indicative of “end-stage” fasting is scarce among phocid seals, as
most begin foraging before lipid reserves are overly depleted (Castellini and Rea 1992). An
exception are “starveling” post-molt northern elephant seal pups, which are not indicative
of the general population (Houser and Costa 2003).
As previously noted, depletion of the blubber layer can have thermoregulatory consequences, and many studies have examined the thermal effects of changes in blubber
reserves during fasting in phocid seal pups (e.g., Irving and Hart 1957; Worthy 1991;
Muelbert and Bowen 1993). Northern elephant seal pups exhibit differing energy usage
patterns depending on body mass and body composition at the end of nursing. Fatter
pups catabolize more lipids and spare proportionally more protein than do leaner pups.
As a consequence, leaner pups, catabolizing relatively more protein versus lipid, conclude
their postweaning fast with sufficient remaining lipid stores for thermoregulation during their first foraging trip (Noren et al. 2003; Noren and Mangel 2004). There is evidence