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Marine Mammal Physiology: Requisites for Ocean Living
fecundity or lower breeding success in the following year if they are not able to recover
those reserves from foraging.
The different lactation strategies exhibited by phocids and otariids result in different
energetic investment in their offspring. The larger body size of phocids allows them to
store more energy as fat, enabling longer fasting durations and more rapid rates of milk
energy delivery. Most phocid species that fast during lactation lose 30%–40% of body mass
during breeding. Because of the abbreviated nature of phocid lactation, pup maintenance
costs are low and the bulk of the energy given in milk can be used for growth (>80% in
elephant seals and gray seals). Additionally, the short lactation period results in a lower
maternal metabolic overhead. Maternal metabolic overhead is the proportion of a female’s
energy expenditure that goes toward meeting her own metabolic needs. These costs can
include maintenance metabolism but also the energy costs of milk synthesis and maternal behavior. By compressing the period of parental investment, phocid seals increase
the efficiency of lactation, increasing the proportion of energy expenditure given to their
pups. In addition, the rapid energy delivery means that the pups can store more of the
energy obtained as body reserves. In contrast, as a result of the intermittent feeding strategy exhibited by otariids, only 20%–50% of ingested milk energy is available for growth
(McDonald et al. 2012). Additionally, the longer lactation period requires otariid mothers
to expend more energy to meet her and her pup’s maintenance metabolism requirements
prior to nutritional independence. Thus, despite smaller body size at weaning, otariid
pups may receive greater absolute maternal investment when compared to the mother’s
body size. For this reason, offspring behavior can also have important impacts on how
maternal energy is allocated in otariids (McDonald et al. 2012).
Pinnipeds have the most energy dense milk of all mammals, allowing for rapid transfer of energy to the pups (Table 10.2). Their milk is high in fat and protein content (as high
as 61% fat in hooded seals and 14% protein in northern fur seals—Callorhinus ursinus) but
very low in carbohydrate content (<1%). By comparison, typical whole milk from a dairy
cow is ~4% fat, ~3% protein, and ~5% sugars. Species with shorter lactation durations produce milk with higher fat and energy contents in order to provision the pup more quickly.
In many species, milk composition changes dramatically over lactation, increasing in fat
content with time onshore. There is a strong negative association between milk fat content and the time spent onshore with the pup in both phocids and otariids (Costa 1991).
Additionally, otariid milk has increased lipid content following longer foraging trips.
In order to produce such energy dense milk while fasting, pinnipeds must be able
to rapidly mobilize stored body fat and protein from muscle and vital body organs and
deliver it to the mammary gland, while preventing the mobilized protein from being
burned as fuel for metabolism or used to make sugars (see Chapter 8). Pinniped milk also
contains significant amounts of water. Although the high energy content reduces the water
needed for milk production, the bulk of this water must be provided through metabolic
water production from maternal metabolism. This water is produced largely from the oxidation of fats for energy. The metabolic adaptations for extended fasting are covered in detail
in Chapter 8. Here, we consider the role that nutrient mobilization and fasting plays in
determining the ability of pinnipeds to produce such energy dense milk.
The onshore accessibility of pinnipeds for study has allowed a greater level of scientific investigation of lactation physiology than is possible in free-ranging cetaceans. In
addition to numerous studies on milk composition and production, recent work has examined the endocrine and cellular processes associated with nutrient mobilization for lactation. The primary determinant of nutrient delivery to the mammary gland is mobilization
of fatty acids from stored triglycerides in adipose tissue, or lipolysis. Fasting and lactating
Marine Mammal Physiology: Requisites for Ocean Living
fecundity or lower breeding success in the following year if they are not able to recover
those reserves from foraging.
The different lactation strategies exhibited by phocids and otariids result in different
energetic investment in their offspring. The larger body size of phocids allows them to
store more energy as fat, enabling longer fasting durations and more rapid rates of milk
energy delivery. Most phocid species that fast during lactation lose 30%–40% of body mass
during breeding. Because of the abbreviated nature of phocid lactation, pup maintenance
costs are low and the bulk of the energy given in milk can be used for growth (>80% in
elephant seals and gray seals). Additionally, the short lactation period results in a lower
maternal metabolic overhead. Maternal metabolic overhead is the proportion of a female’s
energy expenditure that goes toward meeting her own metabolic needs. These costs can
include maintenance metabolism but also the energy costs of milk synthesis and maternal behavior. By compressing the period of parental investment, phocid seals increase
the efficiency of lactation, increasing the proportion of energy expenditure given to their
pups. In addition, the rapid energy delivery means that the pups can store more of the
energy obtained as body reserves. In contrast, as a result of the intermittent feeding strategy exhibited by otariids, only 20%–50% of ingested milk energy is available for growth
(McDonald et al. 2012). Additionally, the longer lactation period requires otariid mothers
to expend more energy to meet her and her pup’s maintenance metabolism requirements
prior to nutritional independence. Thus, despite smaller body size at weaning, otariid
pups may receive greater absolute maternal investment when compared to the mother’s
body size. For this reason, offspring behavior can also have important impacts on how
maternal energy is allocated in otariids (McDonald et al. 2012).
Pinnipeds have the most energy dense milk of all mammals, allowing for rapid transfer of energy to the pups (Table 10.2). Their milk is high in fat and protein content (as high
as 61% fat in hooded seals and 14% protein in northern fur seals—Callorhinus ursinus) but
very low in carbohydrate content (<1%). By comparison, typical whole milk from a dairy
cow is ~4% fat, ~3% protein, and ~5% sugars. Species with shorter lactation durations produce milk with higher fat and energy contents in order to provision the pup more quickly.
In many species, milk composition changes dramatically over lactation, increasing in fat
content with time onshore. There is a strong negative association between milk fat content and the time spent onshore with the pup in both phocids and otariids (Costa 1991).
Additionally, otariid milk has increased lipid content following longer foraging trips.
In order to produce such energy dense milk while fasting, pinnipeds must be able
to rapidly mobilize stored body fat and protein from muscle and vital body organs and
deliver it to the mammary gland, while preventing the mobilized protein from being
burned as fuel for metabolism or used to make sugars (see Chapter 8). Pinniped milk also
contains significant amounts of water. Although the high energy content reduces the water
needed for milk production, the bulk of this water must be provided through metabolic
water production from maternal metabolism. This water is produced largely from the oxidation of fats for energy. The metabolic adaptations for extended fasting are covered in detail
in Chapter 8. Here, we consider the role that nutrient mobilization and fasting plays in
determining the ability of pinnipeds to produce such energy dense milk.
The onshore accessibility of pinnipeds for study has allowed a greater level of scientific investigation of lactation physiology than is possible in free-ranging cetaceans. In
addition to numerous studies on milk composition and production, recent work has examined the endocrine and cellular processes associated with nutrient mobilization for lactation. The primary determinant of nutrient delivery to the mammary gland is mobilization
of fatty acids from stored triglycerides in adipose tissue, or lipolysis. Fasting and lactating