177
Chapter eight: Fasting
that species which normally fast in water are more adapted to preserving their lipid
layer for thermoregulation than species which fast on land (Worthy and Lavigne 1987).
Experimentally, gray seal pups (which normally fast on land) had thinner blubber layers
after a 10-week fast in water compared those fasted on land, whereas harp seals (which
normally fast partly in water) showed no differences in body composition whether fasting
on land or in water (Worthy and Lavigne 1987).
Maintaining stable circulating glucose levels can be particularly challenging for pups,
and initial drops are sometimes observed early in the fast, before production catches up to
utilization. Harp seal pups maintain high, constant plasma levels of glucose over 32-day
weaning fasts, except for an early, insignificant decline (Worthy and Lavigne 1982; Nordøy
et al. 1993). In contrast, gray seal pups exhibit a constant 20% decline in glucose levels that
reflects decreasing replacement rates over their 52-day fasts (Nordøy et al. 1990). Plasma
glucose concentrations in northern elephant seal pups are quite variable. While they may
sometimes decline slightly during the fast, they generally remain at consistent high levels,
with no obvious changes related to either the start of weaning or the decrease in circulating ketones that is thought to signal the end of weaning (Costa and Ortiz 1982; Castellini
and Costa 1990; Ortiz et al. 2001).
Breeding male phocids also rely on lipid metabolism during their extensive fasts. Male
northern elephant seals meet only 7% of energy expenditure through protein breakdown
(Crocker et al. 2012). Protein use declines with initial proportion of body fat (so thinner
males use more protein catabolism), emphasizing the importance of adequate pre-fast lipid
stores. Circulating metabolites in these fasting males differ from expectations for fasting
mammals relying on lipolysis and also differ from fasting females and pups (Castellini
and Costa 1990; Houser et al. 2007). β-OHB levels are very low and only increase slightly
during fasting in contrast to females and pups, suggesting ketone regulation differs with
life history. Further, BUN levels (and serum FFA) are consistent over the fasting and are
unrelated to protein catabolism (Crocker et al. 2012).
Fasting may also occur outside of the breeding season, and some experiments have
attempted to mimic these “unpredicted” but natural fasting episodes. Juvenile harbor
seals (Phoca vitulina) experimentally fasted for 2 weeks derived most (~75%) of their energy
from fat, showed a marked (20%) metabolic decrease (Markussen 1995; Markussen et al.
1992), and a linear decline in body mass typical of lipid-based metabolism.
8.1.2.1.2 Otariids Fasts endured by otariid pups during maternal foraging trips
can last from days to weeks. Antarctic fur seal pups transition to a lipid-based metabolism within 2–3 days of their 5-day nursing fasts (Arnould et al. 2001). Protein turnover
accounts for only 5.4% of total energy expenditure over the course of their fast, during
which they exhibit decreases in plasma BUN, triglyceride concentrations, and circulating
glucose levels and increases in β-OHB.
Subantarctic fur seal pups (Arctocephalus tropicalis) undergo relatively prolonged
nursing fasts compared to most other otariids, which initially last 10 days but extend
up to 3–4 weeks just prior to weaning (Georges and Guinet 2000). On average, mass loss
during these fasts comprised 56% lipids and 10% protein (the remainder was water),
suggesting that 93% of their energy requirements derived from lipids (Beauplet et al.
2003). Curiously, protein catabolism was twice as high in female pups, despite their
greater lipid reserves.
Based on blood chemistry, 6-week-old Steller sea lion pups (Eumetopias jubatus) make
a rapid metabolic transition to fasting within 16 h, with rapidly decreased plasma BUN
Chapter eight: Fasting
that species which normally fast in water are more adapted to preserving their lipid
layer for thermoregulation than species which fast on land (Worthy and Lavigne 1987).
Experimentally, gray seal pups (which normally fast on land) had thinner blubber layers
after a 10-week fast in water compared those fasted on land, whereas harp seals (which
normally fast partly in water) showed no differences in body composition whether fasting
on land or in water (Worthy and Lavigne 1987).
Maintaining stable circulating glucose levels can be particularly challenging for pups,
and initial drops are sometimes observed early in the fast, before production catches up to
utilization. Harp seal pups maintain high, constant plasma levels of glucose over 32-day
weaning fasts, except for an early, insignificant decline (Worthy and Lavigne 1982; Nordøy
et al. 1993). In contrast, gray seal pups exhibit a constant 20% decline in glucose levels that
reflects decreasing replacement rates over their 52-day fasts (Nordøy et al. 1990). Plasma
glucose concentrations in northern elephant seal pups are quite variable. While they may
sometimes decline slightly during the fast, they generally remain at consistent high levels,
with no obvious changes related to either the start of weaning or the decrease in circulating ketones that is thought to signal the end of weaning (Costa and Ortiz 1982; Castellini
and Costa 1990; Ortiz et al. 2001).
Breeding male phocids also rely on lipid metabolism during their extensive fasts. Male
northern elephant seals meet only 7% of energy expenditure through protein breakdown
(Crocker et al. 2012). Protein use declines with initial proportion of body fat (so thinner
males use more protein catabolism), emphasizing the importance of adequate pre-fast lipid
stores. Circulating metabolites in these fasting males differ from expectations for fasting
mammals relying on lipolysis and also differ from fasting females and pups (Castellini
and Costa 1990; Houser et al. 2007). β-OHB levels are very low and only increase slightly
during fasting in contrast to females and pups, suggesting ketone regulation differs with
life history. Further, BUN levels (and serum FFA) are consistent over the fasting and are
unrelated to protein catabolism (Crocker et al. 2012).
Fasting may also occur outside of the breeding season, and some experiments have
attempted to mimic these “unpredicted” but natural fasting episodes. Juvenile harbor
seals (Phoca vitulina) experimentally fasted for 2 weeks derived most (~75%) of their energy
from fat, showed a marked (20%) metabolic decrease (Markussen 1995; Markussen et al.
1992), and a linear decline in body mass typical of lipid-based metabolism.
8.1.2.1.2 Otariids Fasts endured by otariid pups during maternal foraging trips
can last from days to weeks. Antarctic fur seal pups transition to a lipid-based metabolism within 2–3 days of their 5-day nursing fasts (Arnould et al. 2001). Protein turnover
accounts for only 5.4% of total energy expenditure over the course of their fast, during
which they exhibit decreases in plasma BUN, triglyceride concentrations, and circulating
glucose levels and increases in β-OHB.
Subantarctic fur seal pups (Arctocephalus tropicalis) undergo relatively prolonged
nursing fasts compared to most other otariids, which initially last 10 days but extend
up to 3–4 weeks just prior to weaning (Georges and Guinet 2000). On average, mass loss
during these fasts comprised 56% lipids and 10% protein (the remainder was water),
suggesting that 93% of their energy requirements derived from lipids (Beauplet et al.
2003). Curiously, protein catabolism was twice as high in female pups, despite their
greater lipid reserves.
Based on blood chemistry, 6-week-old Steller sea lion pups (Eumetopias jubatus) make
a rapid metabolic transition to fasting within 16 h, with rapidly decreased plasma BUN