232
Marine Mammal Physiology: Requisites for Ocean Living
Pregnant females enter dens on land or, more rarely, on stable ice, and fast through the
period of gestation and for the first several months of lactation. After a short 4-month gestation period, polar bears give birth to a small (~0.5 kg) cub while in their dens. The combined
fasting period can be up to 8 months and females lose over 40% of the body mass during
denning. Like other fasting adapted marine mammals, the bulk of energy reserves used are
stored fat (Atkinson and Ramsay 1995). Fatter females produce larger cubs at the time of emergence. Like many pinnipeds, body fat content has direct impacts on the ability to spare protein
from vital body organs while fasting (Atkinson et al. 1996). Despite these similarities, there are
several important differences between polar bear blubber and that of other marine mammals.
Polar bear adipose tissue does not exhibit the vertical stratification of fatty acid composition
found in some cetaceans and pinnipeds, and does not undergo changes in fatty acid composition across lactation (Thiemann et al. 2006). In contrast to several pinnipeds, polar bear
blubber undergoes dramatic changes in total lipid content as stored fats are used for milk
synthesis. For example, a polar bear female’s blubber declines from 78% to 62% lipid across
the denning period. In contrast, an elephant seal undergoes significant loss of blubber during
lactation but the remaining adipose tissue has a relatively constant lipid content of ~90%.
During denning, polar bear females have low metabolic rates that are less that resting
metabolism, despite the costs of milk synthesis. This shows that denning is a good strategy
for maximizing lactation efficiency. After emergence, mother and cub may stay in the denning area for a while before they make the trek to the sea ice to hunt seals. Polar bears have
an extremely long lactation period ranging from 1.5 to 2.5 years, depending on environmental characteristics and the ability of the cub to hunt independently. Peak lactation is thought
to occur during the denning period or soon after bears emerge from the den. Polar bear milk
composition changes throughout denning and feeding, declining in milk fat content from as
high as 36% fat and increasing in protein content after females begin to feed (Derocher et al.
1993) (Table 10.1). Unlike other marine mammals, polar bear milk has substantial carbohydrate content (as high as 5%), that declines across the lactation period (Arnould and Ramsay
1994). As the cubs grow, milk energy production lessens and females with older cubs may
cease lactation during seasonal periods of fasting. Unlike many pinnipeds, the blubber of
female polar bears has a fairly consistent fatty acid composition across lactation.
10.3 Toolbox
The study of lactation and post-partum reproduction in marine mammals has focused
largely on pinnipeds because they are accessible on land while nursing their young. These
studies have investigated how females invest energy in their offspring, how the offspring
allocate the acquired energy toward growth and development, and the metabolic features
that enable production of high energy density milk while fasting. These studies use a combination of techniques that allow estimation of energy expenditure and energy intake,
ranging from determining mass and body condition, to using data loggers to measure
movement and behavior. As technology advances, we will continue to increase our knowledge about post-partum physiology, particularly in the groups such as cetaceans, where
data are lacking.
10.3.1 Mass and body condition
In the 1970s and 1980s, it was recognized that phocids provide an ideal study system in
which to investigate reproductive effort and the potential cost of reproduction because the
physiology of lactation in phocids is significantly impacted by constraints resulting from
Marine Mammal Physiology: Requisites for Ocean Living
Pregnant females enter dens on land or, more rarely, on stable ice, and fast through the
period of gestation and for the first several months of lactation. After a short 4-month gestation period, polar bears give birth to a small (~0.5 kg) cub while in their dens. The combined
fasting period can be up to 8 months and females lose over 40% of the body mass during
denning. Like other fasting adapted marine mammals, the bulk of energy reserves used are
stored fat (Atkinson and Ramsay 1995). Fatter females produce larger cubs at the time of emergence. Like many pinnipeds, body fat content has direct impacts on the ability to spare protein
from vital body organs while fasting (Atkinson et al. 1996). Despite these similarities, there are
several important differences between polar bear blubber and that of other marine mammals.
Polar bear adipose tissue does not exhibit the vertical stratification of fatty acid composition
found in some cetaceans and pinnipeds, and does not undergo changes in fatty acid composition across lactation (Thiemann et al. 2006). In contrast to several pinnipeds, polar bear
blubber undergoes dramatic changes in total lipid content as stored fats are used for milk
synthesis. For example, a polar bear female’s blubber declines from 78% to 62% lipid across
the denning period. In contrast, an elephant seal undergoes significant loss of blubber during
lactation but the remaining adipose tissue has a relatively constant lipid content of ~90%.
During denning, polar bear females have low metabolic rates that are less that resting
metabolism, despite the costs of milk synthesis. This shows that denning is a good strategy
for maximizing lactation efficiency. After emergence, mother and cub may stay in the denning area for a while before they make the trek to the sea ice to hunt seals. Polar bears have
an extremely long lactation period ranging from 1.5 to 2.5 years, depending on environmental characteristics and the ability of the cub to hunt independently. Peak lactation is thought
to occur during the denning period or soon after bears emerge from the den. Polar bear milk
composition changes throughout denning and feeding, declining in milk fat content from as
high as 36% fat and increasing in protein content after females begin to feed (Derocher et al.
1993) (Table 10.1). Unlike other marine mammals, polar bear milk has substantial carbohydrate content (as high as 5%), that declines across the lactation period (Arnould and Ramsay
1994). As the cubs grow, milk energy production lessens and females with older cubs may
cease lactation during seasonal periods of fasting. Unlike many pinnipeds, the blubber of
female polar bears has a fairly consistent fatty acid composition across lactation.
10.3 Toolbox
The study of lactation and post-partum reproduction in marine mammals has focused
largely on pinnipeds because they are accessible on land while nursing their young. These
studies have investigated how females invest energy in their offspring, how the offspring
allocate the acquired energy toward growth and development, and the metabolic features
that enable production of high energy density milk while fasting. These studies use a combination of techniques that allow estimation of energy expenditure and energy intake,
ranging from determining mass and body condition, to using data loggers to measure
movement and behavior. As technology advances, we will continue to increase our knowledge about post-partum physiology, particularly in the groups such as cetaceans, where
data are lacking.
10.3.1 Mass and body condition
In the 1970s and 1980s, it was recognized that phocids provide an ideal study system in
which to investigate reproductive effort and the potential cost of reproduction because the
physiology of lactation in phocids is significantly impacted by constraints resulting from