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Chapter eight: Fasting
8.1.1.1 Pinnipeds
Almost all pinniped species fast, which is partly why most of our knowledge about marine
mammal fasting physiology comes from this group. Their fasting episodes also coincide
with periods when they are available on land for intensive study.
Importantly, most pinnipeds experience their first fasts during their initial year of
life, a time when physical growth is critical to survival, and their ability to forage is physically and behaviorally limited. After a period of near-constant energy intake via maternal
milk during the nursing period, the pups of many phocid species are abruptly weaned
and undergo a postweaning fast. This period often corresponds to their annual molt; in
northern elephant seals (Mirounga angustirostris) this postweaning fast lasts for 9–12 weeks
(Reiter et al. 1978). For the most part, pups remain on land during this period assumedly
to minimize the costs and tenure of the molt. Fasting in these pups is, therefore, a consequence of the environmental boundary imposed by the molt. Extended postweaning fasts
may also provide a window for the physiological development of diving capacity (Bennett
et al. 2010; Burns et al. 2014).
Some otariid pups also undergo postweaning fasts, also often associated with molts,
although many otariids wean later in life compared to phocids. Otariid pups may also
avoid complete fasting in the postweaning period due to greater experience and physiological capacity for independent foraging (Fowler et al. 2007). The more fundamental difference between phocid and otariids pups is that otariid pups characteristically contend
with regular fasts throughout their nursing period when their mothers regularly return
to sea to feed. The complex manner in which females devote time and energy to lactation while managing their own fast across the nursing period is covered in Chapter 10.
However, the nursing/foraging cycle of otariid females can be viewed as a balancing act
of maternal and offspring fasting episodes. The length of maternal foraging trips and the
resulting fasting episodes of pups can last from days to weeks, depending on species and
stage of lactation. These episodes can make up a significant portion of the total nursing
period for pups. For example, episodes of 4–5-day foraging bouts by Antarctic fur seal
mothers (Arctocephalus gazelle) mean that their pups are potentially fasting for 71% of the
total nursing period (Boyd 1991; Lunn et al. 1993). Conversely, the time that the mother is
attending her pup on land to provide adequate milk necessitates that the female is fasting
while expending tremendous levels of energy through lactation.
Pinniped males also fast during the breeding season. For many species, breeding
strategies rely upon continuously defending territories on beaches or breathing holes
underwater, often for months at a time. This requires them to expend large amounts of
energy fighting and mating while being precluded from feeding over the entire reproductive season. Male northern elephant seals have one of the highest sustained rates of
energy expenditure of any male pinniped (Crocker et al. 2012), losing about a third of their
starting body mass over the 3-month breeding period. As in many pinniped species with
territorial breeding systems, it has been observed that males are in very poor physical
shape by the end of the breeding season, and many are often not observed in subsequent
years (LeBoeuf 1974). These individuals are presumed to have died due to the high rates of
protein catabolism over the extended, expensive fast, although increased oxidative stress
may play a role (Sharick et al. 2014).
8.1.1.2 Cetaceans
Many large baleen whales travel great distances across oceans, alternating between summer feeding grounds in high-latitude cold waters, with warm-water, low-latitude winter
reproductive areas. Many whale species are observed to fast on these calving grounds.
Chapter eight: Fasting
8.1.1.1 Pinnipeds
Almost all pinniped species fast, which is partly why most of our knowledge about marine
mammal fasting physiology comes from this group. Their fasting episodes also coincide
with periods when they are available on land for intensive study.
Importantly, most pinnipeds experience their first fasts during their initial year of
life, a time when physical growth is critical to survival, and their ability to forage is physically and behaviorally limited. After a period of near-constant energy intake via maternal
milk during the nursing period, the pups of many phocid species are abruptly weaned
and undergo a postweaning fast. This period often corresponds to their annual molt; in
northern elephant seals (Mirounga angustirostris) this postweaning fast lasts for 9–12 weeks
(Reiter et al. 1978). For the most part, pups remain on land during this period assumedly
to minimize the costs and tenure of the molt. Fasting in these pups is, therefore, a consequence of the environmental boundary imposed by the molt. Extended postweaning fasts
may also provide a window for the physiological development of diving capacity (Bennett
et al. 2010; Burns et al. 2014).
Some otariid pups also undergo postweaning fasts, also often associated with molts,
although many otariids wean later in life compared to phocids. Otariid pups may also
avoid complete fasting in the postweaning period due to greater experience and physiological capacity for independent foraging (Fowler et al. 2007). The more fundamental difference between phocid and otariids pups is that otariid pups characteristically contend
with regular fasts throughout their nursing period when their mothers regularly return
to sea to feed. The complex manner in which females devote time and energy to lactation while managing their own fast across the nursing period is covered in Chapter 10.
However, the nursing/foraging cycle of otariid females can be viewed as a balancing act
of maternal and offspring fasting episodes. The length of maternal foraging trips and the
resulting fasting episodes of pups can last from days to weeks, depending on species and
stage of lactation. These episodes can make up a significant portion of the total nursing
period for pups. For example, episodes of 4–5-day foraging bouts by Antarctic fur seal
mothers (Arctocephalus gazelle) mean that their pups are potentially fasting for 71% of the
total nursing period (Boyd 1991; Lunn et al. 1993). Conversely, the time that the mother is
attending her pup on land to provide adequate milk necessitates that the female is fasting
while expending tremendous levels of energy through lactation.
Pinniped males also fast during the breeding season. For many species, breeding
strategies rely upon continuously defending territories on beaches or breathing holes
underwater, often for months at a time. This requires them to expend large amounts of
energy fighting and mating while being precluded from feeding over the entire reproductive season. Male northern elephant seals have one of the highest sustained rates of
energy expenditure of any male pinniped (Crocker et al. 2012), losing about a third of their
starting body mass over the 3-month breeding period. As in many pinniped species with
territorial breeding systems, it has been observed that males are in very poor physical
shape by the end of the breeding season, and many are often not observed in subsequent
years (LeBoeuf 1974). These individuals are presumed to have died due to the high rates of
protein catabolism over the extended, expensive fast, although increased oxidative stress
may play a role (Sharick et al. 2014).
8.1.1.2 Cetaceans
Many large baleen whales travel great distances across oceans, alternating between summer feeding grounds in high-latitude cold waters, with warm-water, low-latitude winter
reproductive areas. Many whale species are observed to fast on these calving grounds.