172
Marine Mammal Physiology: Requisites for Ocean Living
This fasting is likely due to limited food availability in these low-productivity waters
(Horwood 1990). This fasting is a trade-off for the warm waters that may be necessary to
rear young in a more hospitable thermal environment, or even to avoid predators (Corkeron
and Connor 1999; Mehta et al. 2007). Similar patterns are not generally observed in smaller
bodied odontocetes, but there is evidence of fasting during periodic “maintenance” migrations undertaken by Orcas (Orca orcinus) (Durban and Pitman 2011).
8.1.2 Energy substrates during a fast
Energy is constantly required to maintain cellular and biochemical homeostasis. As we
shall discuss later, although fasting can be associated with strategies to limit and streamline energy expenditure requirements, energy use will always exceed external energy
intake, which is zero. Energy requirements during fasting episodes are, therefore, met
solely by substantial contributions from endogenous (internal) body reserves. Catabolism is
the process by which energy-containing molecules, or substrates, are broken down to
release energy. Fuel stores are unequally accumulated within body tissues in three forms:
fat, protein, and carbohydrate.
The most common energy source for a feeding organism is glucose, a carbohydrate that
is polymerized and stored as glycogen in the liver and muscles. Despite the importance of
glucose in meeting the energy requirements of body tissues, including the brain and red
blood cells, glycogen stores are relatively small in most marine mammals. By far, lipids
and proteins provide the largest energy reserves. Lipids, stored as fat, are the preferred
fuel to sustain fasting for several reasons. They have the highest caloric density, meaning
that more energy is liberated per gram of fat catabolized (approx. 40 kJ g −1 depending on
the composition of lipid stores) compared to other substrates (proteins and carbohydrate
provide approx. 18 kJ g −1 ) (Schmidt-Nielsen 1997). Lipids also provide more metabolic water
on breakdown than do proteins (107 g versus 40 g 100 g −1 tissue), an important resource that
is also limited or lacking during fasting (Figure 8.1).
Unlike many terrestrial mammals that store fat in visceral deposits, the majority of
lipids used as an energy reserve for marine mammals are situated in the hypodermal (subcutaneous) blubber layer. This blubber layer can account for 50% of the total body mass,
Water
released
32 ml
27 ml
23 ml
Mass
required
Carbohydrates
Lipids
Proteins
60 g
25 g
57 g
Figure 8.1 Lipids are the most energy dense substrate; less lipid mass is required to generate 1000 kJ
of energy compared to carbohydrates or proteins. These estimates are respectively based on glucose as the carbohydrate source, the complete oxidation of an “average” fat, and the catabolism of
a protein to urea. Different sources or configurations of carbohydrate, lipid, and protein energy
stores will yield slightly different values. Although catabolism of carbohydrates to generate 1000 kJ
of energy produces the most metabolic water, lipids yield the most water per gram. (Data from
Schmidt-Nielsen, K., Animal Physiology: Adaptation and Environment, 5th edn., Cambridge University
Press, Cambridge, UK, 1997; Edney, E.B., Metabolic water, in: Water Balance in Land Arthropods,
Springer, 1977.)
Marine Mammal Physiology: Requisites for Ocean Living
This fasting is likely due to limited food availability in these low-productivity waters
(Horwood 1990). This fasting is a trade-off for the warm waters that may be necessary to
rear young in a more hospitable thermal environment, or even to avoid predators (Corkeron
and Connor 1999; Mehta et al. 2007). Similar patterns are not generally observed in smaller
bodied odontocetes, but there is evidence of fasting during periodic “maintenance” migrations undertaken by Orcas (Orca orcinus) (Durban and Pitman 2011).
8.1.2 Energy substrates during a fast
Energy is constantly required to maintain cellular and biochemical homeostasis. As we
shall discuss later, although fasting can be associated with strategies to limit and streamline energy expenditure requirements, energy use will always exceed external energy
intake, which is zero. Energy requirements during fasting episodes are, therefore, met
solely by substantial contributions from endogenous (internal) body reserves. Catabolism is
the process by which energy-containing molecules, or substrates, are broken down to
release energy. Fuel stores are unequally accumulated within body tissues in three forms:
fat, protein, and carbohydrate.
The most common energy source for a feeding organism is glucose, a carbohydrate that
is polymerized and stored as glycogen in the liver and muscles. Despite the importance of
glucose in meeting the energy requirements of body tissues, including the brain and red
blood cells, glycogen stores are relatively small in most marine mammals. By far, lipids
and proteins provide the largest energy reserves. Lipids, stored as fat, are the preferred
fuel to sustain fasting for several reasons. They have the highest caloric density, meaning
that more energy is liberated per gram of fat catabolized (approx. 40 kJ g −1 depending on
the composition of lipid stores) compared to other substrates (proteins and carbohydrate
provide approx. 18 kJ g −1 ) (Schmidt-Nielsen 1997). Lipids also provide more metabolic water
on breakdown than do proteins (107 g versus 40 g 100 g −1 tissue), an important resource that
is also limited or lacking during fasting (Figure 8.1).
Unlike many terrestrial mammals that store fat in visceral deposits, the majority of
lipids used as an energy reserve for marine mammals are situated in the hypodermal (subcutaneous) blubber layer. This blubber layer can account for 50% of the total body mass,
Water
released
32 ml
27 ml
23 ml
Mass
required
Carbohydrates
Lipids
Proteins
60 g
25 g
57 g
Figure 8.1 Lipids are the most energy dense substrate; less lipid mass is required to generate 1000 kJ
of energy compared to carbohydrates or proteins. These estimates are respectively based on glucose as the carbohydrate source, the complete oxidation of an “average” fat, and the catabolism of
a protein to urea. Different sources or configurations of carbohydrate, lipid, and protein energy
stores will yield slightly different values. Although catabolism of carbohydrates to generate 1000 kJ
of energy produces the most metabolic water, lipids yield the most water per gram. (Data from
Schmidt-Nielsen, K., Animal Physiology: Adaptation and Environment, 5th edn., Cambridge University
Press, Cambridge, UK, 1997; Edney, E.B., Metabolic water, in: Water Balance in Land Arthropods,
Springer, 1977.)