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Marine Mammal Physiology: Requisites for Ocean Living
Like all mammals, the normal body temperature of most marine species is around 37°C.
There is nothing particularly special about 37°C, but rather it is the constancy of that temperature that is more important (Hochachka and Somero 1984). Homeotherms aim to hold
body temperature constant because it is biochemically more efficient than having a body
temperature that fluctuates widely. There is a chemical principle called the Q 10 concept
that has described the change in metabolic rate driven by changes in temperature. For
mammals, a 10°C tissue temperature change induces a biochemical change in reaction
of about 2x, but this varies through the many thousands of biochemical reactions in the
body. Because biochemical homeostatis (constancy of biochemical reactions) is important
for effective metabolic control, it is advantageous to not allow body temperature to fluctuate too widely. Therefore, it is not the 37°C that is so important, but rather that it is mostly
constant. Hyperthermia, or when the body temperature rises to dangerous levels, is a problem because it causes significant disruption of the delicate biochemical reaction balances
in the body. For example, some Antarctic fish that live in water at −1.8°C will die of hyperthermia at only 6°C–7°C (Hochachka and Somero 1984). They do not die because tissues
are breaking down from the temperature, but because of Q 10 imbalances in the many biochemical reactions in their bodies. Similarly, hyperthermia in a marine mammal would be
dangerous because of loss of biochemical balance. Therefore, the need to thermoregulate
in a marine mammal is to provide a relatively constant body temperature and get neither
too cold nor too hot.
Surprisingly, the generation of heat by all mammals, including the marine species, is
actually a highly inefficient waste product of basic cellular metabolism. Almost all energy
in cellular metabolism is needed to maintain ion balance across cell walls. Mammals have
relatively leaky cell walls compared to ectotherms, and the cellular ion pumps must constantly work to keep the ion balance between the inside of outside of the cells (Stevens 1973;
Else and Hulbert 1981; Hochachka and Guppy 1987). This means that they consume vast
amounts of ATP to maintain those ion pumps, and the consumption of ATP in general, is
only about 25% efficient. That is, only 25% of the ATP is used to drive the ion pumps, and
the rest is lost as waste heat. The ion leakiness of mammalian cells and the vast amount
of waste heat is thought to have been important in the evolution of the group, because it
allowed the expansion of mammals to colder regions on the planet (Hammel 1976).
The Kleiber principle, covered in-depth in Chapter 3, is also involved in endothermic
heat production and energy consumption. It describes how the relationship between
metabolic rate and body mass decreases in a logarithmic function, so that small animals
have a much higher metabolic rate per body mass than larger animals. The sea otter has a
very high metabolic rate/body mass compared to a larger pinniped or cetacean (Morrison
et al. 1974). Recall that the smaller sea otter also has a higher SA:V ratio. It is tempting to
relate the loss of heat due to the higher SA:V ratio, but that ratio has a log exponent of
−0.66, while the Kleiber line has a log exponent of −0.75. Further, the Kleiber line also has
been seen in ectothermic species, meaning the loss of heat by a mammal as the surface
area goes up relative to body mass is not the driver for the Kleiber-derived increase in
metabolic rate.
Knowing these relationships, we can return to an aspect of body mass and metabolic
rate that was discussed by Williams and Maresh (Figure 3.2, Chapter 3). They showed
that the metabolic rate of a marine mammal is roughly 2× that of a terrestrial mammal of
the same mass. For many years it was thought that this was to offset the heat loss of being
in water, which is 25× more a thermal challenge than being on land. However, as pointed
out by Williams and Maresh, “thermoregulation may not be a problem for all but the
smallest of marine mammals and that other traits in marine mammals set metabolism.”
Marine Mammal Physiology: Requisites for Ocean Living
Like all mammals, the normal body temperature of most marine species is around 37°C.
There is nothing particularly special about 37°C, but rather it is the constancy of that temperature that is more important (Hochachka and Somero 1984). Homeotherms aim to hold
body temperature constant because it is biochemically more efficient than having a body
temperature that fluctuates widely. There is a chemical principle called the Q 10 concept
that has described the change in metabolic rate driven by changes in temperature. For
mammals, a 10°C tissue temperature change induces a biochemical change in reaction
of about 2x, but this varies through the many thousands of biochemical reactions in the
body. Because biochemical homeostatis (constancy of biochemical reactions) is important
for effective metabolic control, it is advantageous to not allow body temperature to fluctuate too widely. Therefore, it is not the 37°C that is so important, but rather that it is mostly
constant. Hyperthermia, or when the body temperature rises to dangerous levels, is a problem because it causes significant disruption of the delicate biochemical reaction balances
in the body. For example, some Antarctic fish that live in water at −1.8°C will die of hyperthermia at only 6°C–7°C (Hochachka and Somero 1984). They do not die because tissues
are breaking down from the temperature, but because of Q 10 imbalances in the many biochemical reactions in their bodies. Similarly, hyperthermia in a marine mammal would be
dangerous because of loss of biochemical balance. Therefore, the need to thermoregulate
in a marine mammal is to provide a relatively constant body temperature and get neither
too cold nor too hot.
Surprisingly, the generation of heat by all mammals, including the marine species, is
actually a highly inefficient waste product of basic cellular metabolism. Almost all energy
in cellular metabolism is needed to maintain ion balance across cell walls. Mammals have
relatively leaky cell walls compared to ectotherms, and the cellular ion pumps must constantly work to keep the ion balance between the inside of outside of the cells (Stevens 1973;
Else and Hulbert 1981; Hochachka and Guppy 1987). This means that they consume vast
amounts of ATP to maintain those ion pumps, and the consumption of ATP in general, is
only about 25% efficient. That is, only 25% of the ATP is used to drive the ion pumps, and
the rest is lost as waste heat. The ion leakiness of mammalian cells and the vast amount
of waste heat is thought to have been important in the evolution of the group, because it
allowed the expansion of mammals to colder regions on the planet (Hammel 1976).
The Kleiber principle, covered in-depth in Chapter 3, is also involved in endothermic
heat production and energy consumption. It describes how the relationship between
metabolic rate and body mass decreases in a logarithmic function, so that small animals
have a much higher metabolic rate per body mass than larger animals. The sea otter has a
very high metabolic rate/body mass compared to a larger pinniped or cetacean (Morrison
et al. 1974). Recall that the smaller sea otter also has a higher SA:V ratio. It is tempting to
relate the loss of heat due to the higher SA:V ratio, but that ratio has a log exponent of
−0.66, while the Kleiber line has a log exponent of −0.75. Further, the Kleiber line also has
been seen in ectothermic species, meaning the loss of heat by a mammal as the surface
area goes up relative to body mass is not the driver for the Kleiber-derived increase in
metabolic rate.
Knowing these relationships, we can return to an aspect of body mass and metabolic
rate that was discussed by Williams and Maresh (Figure 3.2, Chapter 3). They showed
that the metabolic rate of a marine mammal is roughly 2× that of a terrestrial mammal of
the same mass. For many years it was thought that this was to offset the heat loss of being
in water, which is 25× more a thermal challenge than being on land. However, as pointed
out by Williams and Maresh, “thermoregulation may not be a problem for all but the
smallest of marine mammals and that other traits in marine mammals set metabolism.”