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Marine Mammal Physiology: Requisites for Ocean Living
content in some species. However, in contrast to most terrestrial mammals, the seals are
able to avoid the protein wasting effects of cortisol. Protein sparing is efficient in lactating
seals, with protein oxidation providing a maximum of 4%–8% of energy expenditure for
various species as measured from the production of urea or changes in body composition
(Crocker et al. 1998). Some investigations have suggested that elevations in cortisol may
serve as a re-feeding signal in otariids and help initiate the return to sea to forage (Guinet
et al. 2004). Interestingly growth hormone, the primary regulator of lipolysis, milk output,
and fat content in dairy cattle, appears to play a more minor role in seals.
Several enzymes are important in the process of lipolysis (release of fatty acids from
triglycerides). Direct measurements of lipolysis rates during lactation in elephant seals
have shown that they are uncoupled from the levels of circulating fatty acids during
lactation (Houser et al. 2007). The enzymes that allow adipose tissue to reuptake fatty
acids for storage, fatty acid translocase (CD36) and fatty acid transport protein 1 (FATP1),
decrease with fasting in seals (Viscarra and Ortiz 2013). This suggests that reducing
the reuptake of fatty acids by adipose tissue is an important feature that keeps fatty
acid levels high to supply the mammary gland as fasting seals deplete their fat reserves
while lactating. In most mammals, the primary enzyme responsible for lipolysis of fats
is hormone-sensitive lipase (HSL). Surprisingly, HSL levels are very low in the blubber
of lactating elephant seals. Instead, the enzyme adipocyte triglyceride lipase (ATGL) that
removes one fatty acid from a triglyceride molecule has an increased importance when
compared to other mammals (Fowler et al. 2015). In some species, like gray and harbor
seals, the levels of an enzyme bound to mammary gland tissues called lipoprotein lipase
(LPL) is important to allowing the mammary gland to uptake fatty acids from triglycerides in circulation. In these species, LPL levels increase in parallel with milk fat content
(Iverson et al. 1995). In other species, like elephant seals, this enzyme plays a minor role
(McDonald and Crocker 2006). These differences suggest that despite similar reproductive strategies, the various seal lineages may have evolved important metabolic differences in lactation physiology under strong evolutionary pressure for efficient lactation
while fasting.
Since blubber is the source of fatty acids for milk synthesis, its composition and the
way individual fatty acids are mobilized and used for metabolism can potentially influence the composition of milk. It has been reported in many different species of marine
mammals that blubber layers are stratified from inner to outer layers (Strandberg et al.
2008). External layers have a higher proportion of medium chain (≤18 C) monounsaturated fatty acids (MUFA), possibly as a homeoviscous adaptation for the purpose of
maintaining membrane fluidity at the low temperatures encountered at depth. Interior
layers in phocid blubber are highly enriched in saturated fatty acids (SFA) and longchain (≥20 C) MUFA. This inner layer is heavily metabolized during fasting and lactation (Fowler et al. 2014). In phocids, the mobilization of specific fatty acids from blubber,
and their incorporation into milk, conforms to biochemical predictions based on the
number of carbons and saturated bonds. Long-chain (>20 C) MUFA are the least mobilized and polyunsaturated fatty acids (PUFA) and SFA are more highly mobilized from
the blubber. In the mammary gland of terrestrial mammals, fatty acid synthesis from
glucose and ketones results in short and medium chain fatty acids containing fewer
than 12 carbons. However, these short- and medium-length fatty acids are usually not
detected in seal milk, suggesting that there is little de novo lipid synthesis in the mammary gland. In other words, plasma fatty acid delivery and uptake by the mammary
gland is responsible for milk fat content. PUFA availability to the developing pup’s
muscle tissue may contribute to the development of oxidative capabilities for diving
Marine Mammal Physiology: Requisites for Ocean Living
content in some species. However, in contrast to most terrestrial mammals, the seals are
able to avoid the protein wasting effects of cortisol. Protein sparing is efficient in lactating
seals, with protein oxidation providing a maximum of 4%–8% of energy expenditure for
various species as measured from the production of urea or changes in body composition
(Crocker et al. 1998). Some investigations have suggested that elevations in cortisol may
serve as a re-feeding signal in otariids and help initiate the return to sea to forage (Guinet
et al. 2004). Interestingly growth hormone, the primary regulator of lipolysis, milk output,
and fat content in dairy cattle, appears to play a more minor role in seals.
Several enzymes are important in the process of lipolysis (release of fatty acids from
triglycerides). Direct measurements of lipolysis rates during lactation in elephant seals
have shown that they are uncoupled from the levels of circulating fatty acids during
lactation (Houser et al. 2007). The enzymes that allow adipose tissue to reuptake fatty
acids for storage, fatty acid translocase (CD36) and fatty acid transport protein 1 (FATP1),
decrease with fasting in seals (Viscarra and Ortiz 2013). This suggests that reducing
the reuptake of fatty acids by adipose tissue is an important feature that keeps fatty
acid levels high to supply the mammary gland as fasting seals deplete their fat reserves
while lactating. In most mammals, the primary enzyme responsible for lipolysis of fats
is hormone-sensitive lipase (HSL). Surprisingly, HSL levels are very low in the blubber
of lactating elephant seals. Instead, the enzyme adipocyte triglyceride lipase (ATGL) that
removes one fatty acid from a triglyceride molecule has an increased importance when
compared to other mammals (Fowler et al. 2015). In some species, like gray and harbor
seals, the levels of an enzyme bound to mammary gland tissues called lipoprotein lipase
(LPL) is important to allowing the mammary gland to uptake fatty acids from triglycerides in circulation. In these species, LPL levels increase in parallel with milk fat content
(Iverson et al. 1995). In other species, like elephant seals, this enzyme plays a minor role
(McDonald and Crocker 2006). These differences suggest that despite similar reproductive strategies, the various seal lineages may have evolved important metabolic differences in lactation physiology under strong evolutionary pressure for efficient lactation
while fasting.
Since blubber is the source of fatty acids for milk synthesis, its composition and the
way individual fatty acids are mobilized and used for metabolism can potentially influence the composition of milk. It has been reported in many different species of marine
mammals that blubber layers are stratified from inner to outer layers (Strandberg et al.
2008). External layers have a higher proportion of medium chain (≤18 C) monounsaturated fatty acids (MUFA), possibly as a homeoviscous adaptation for the purpose of
maintaining membrane fluidity at the low temperatures encountered at depth. Interior
layers in phocid blubber are highly enriched in saturated fatty acids (SFA) and longchain (≥20 C) MUFA. This inner layer is heavily metabolized during fasting and lactation (Fowler et al. 2014). In phocids, the mobilization of specific fatty acids from blubber,
and their incorporation into milk, conforms to biochemical predictions based on the
number of carbons and saturated bonds. Long-chain (>20 C) MUFA are the least mobilized and polyunsaturated fatty acids (PUFA) and SFA are more highly mobilized from
the blubber. In the mammary gland of terrestrial mammals, fatty acid synthesis from
glucose and ketones results in short and medium chain fatty acids containing fewer
than 12 carbons. However, these short- and medium-length fatty acids are usually not
detected in seal milk, suggesting that there is little de novo lipid synthesis in the mammary gland. In other words, plasma fatty acid delivery and uptake by the mammary
gland is responsible for milk fat content. PUFA availability to the developing pup’s
muscle tissue may contribute to the development of oxidative capabilities for diving