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Marine Mammal Physiology: Requisites for Ocean Living
Alaska to Japan. It was hunted to extinction by the same fur traders in search of otter
and seal pelts, and sadly the last one was seen in 1768.
Just how did the sea cow, a low-metabolic rate vegetarian, survive in subarctic waters?
While we do not have living representatives to apply our tools and models to, we do know
from historical records and recovered bones that the sea cow was the largest of all the noncetacean marine mammals. It approached 8 m in length, reached 10 metric tons in mass,
had very thick skin and may have had such a significant fat layer that it was unable to dive
efficiently because of its high buoyancy (Scheffer 1972; Anderson and Domning 2002). The
only reasonable answer to the Fick requirements is that their overwhelming mass and
blubber combined with a relatively small surface area to volume ratio afforded them a
much smaller heat loss than their much smaller current relatives. How interesting a situation would it be to have the large, low metabolic rate sea cow still swimming alongside
the sea otter: the smallest of all marine mammals with the highest of all metabolic rates?
Anderson (1995) wrote about the co-evolutionary aspects of sea otters and sea cows in the
same nearshore niche, with a discussion of the curious potential impact of overlapping
diets of sea cows and sea urchin prey.
9.7.2 Climate change
Of interest is the question about how climate change and ocean warming would impact
marine mammals. From the basic and fundamental aspects of heat balance discussed
here, the answer is that it would probably have no impact (Huntington and Moore 2008;
Castellini 2012). As you have learned, marine mammals maintain a constant internal
body temperature in the face of massive temperature swings seasonally, on migrations,
off and onto beaches, and in and out of freezing seawater. A 1°C–2°C water temperature
change due to climate shift is insignificant and easily accommodated by their physiology.
However, these oceanic temperature changes will noticeably impact the distribution and
abundance of their prey. Oceanic temperatures might impact the migration routes of the
large whales as ice, prey, or other oceanic patterns shift. Warming temperatures can and
will continue to alter the ice pack upon which the polar seals, walrus, and bears depend for
rest and breeding. It is on these bases of predicted changes in ice platform that the recent
listings of Endangered under the U.S. Endangered Species Act for polar bears and some of
the Arctic seals have been enacted (Kuhn 2010).
9.8 Conclusions
There are several concepts that are essential in a summary on the thermal biology of
marine mammals. First, they not only have to stay warm when in cold water but must
also be able to remove excess heat when needed. Second, they do not have any specialized
heat organs and must be able to conserve or dump heat from normal metabolic activity,
exercise, and food digestion. They do this with very fine control of blood and heat flow
through blubber or through the use of selective blood flow to skin areas without fur or
blubber. Because they span such a large size range (from sea otters to blue whales), the relative heat flow issues differ based on mass, volume, and surface area. Water impermeable
fur is used for insulation by a few species, but most use blubber for insulation. However,
blubber is also used as a fuel, a water source, and for buoyancy control, and therefore cannot be selected solely for thermal needs. It is very likely that only the smallest individuals
of a given species are ever in a cold stress situation, but rather the most common thermoregulatory challenge for marine mammals is how to prevent overheating.
Marine Mammal Physiology: Requisites for Ocean Living
Alaska to Japan. It was hunted to extinction by the same fur traders in search of otter
and seal pelts, and sadly the last one was seen in 1768.
Just how did the sea cow, a low-metabolic rate vegetarian, survive in subarctic waters?
While we do not have living representatives to apply our tools and models to, we do know
from historical records and recovered bones that the sea cow was the largest of all the noncetacean marine mammals. It approached 8 m in length, reached 10 metric tons in mass,
had very thick skin and may have had such a significant fat layer that it was unable to dive
efficiently because of its high buoyancy (Scheffer 1972; Anderson and Domning 2002). The
only reasonable answer to the Fick requirements is that their overwhelming mass and
blubber combined with a relatively small surface area to volume ratio afforded them a
much smaller heat loss than their much smaller current relatives. How interesting a situation would it be to have the large, low metabolic rate sea cow still swimming alongside
the sea otter: the smallest of all marine mammals with the highest of all metabolic rates?
Anderson (1995) wrote about the co-evolutionary aspects of sea otters and sea cows in the
same nearshore niche, with a discussion of the curious potential impact of overlapping
diets of sea cows and sea urchin prey.
9.7.2 Climate change
Of interest is the question about how climate change and ocean warming would impact
marine mammals. From the basic and fundamental aspects of heat balance discussed
here, the answer is that it would probably have no impact (Huntington and Moore 2008;
Castellini 2012). As you have learned, marine mammals maintain a constant internal
body temperature in the face of massive temperature swings seasonally, on migrations,
off and onto beaches, and in and out of freezing seawater. A 1°C–2°C water temperature
change due to climate shift is insignificant and easily accommodated by their physiology.
However, these oceanic temperature changes will noticeably impact the distribution and
abundance of their prey. Oceanic temperatures might impact the migration routes of the
large whales as ice, prey, or other oceanic patterns shift. Warming temperatures can and
will continue to alter the ice pack upon which the polar seals, walrus, and bears depend for
rest and breeding. It is on these bases of predicted changes in ice platform that the recent
listings of Endangered under the U.S. Endangered Species Act for polar bears and some of
the Arctic seals have been enacted (Kuhn 2010).
9.8 Conclusions
There are several concepts that are essential in a summary on the thermal biology of
marine mammals. First, they not only have to stay warm when in cold water but must
also be able to remove excess heat when needed. Second, they do not have any specialized
heat organs and must be able to conserve or dump heat from normal metabolic activity,
exercise, and food digestion. They do this with very fine control of blood and heat flow
through blubber or through the use of selective blood flow to skin areas without fur or
blubber. Because they span such a large size range (from sea otters to blue whales), the relative heat flow issues differ based on mass, volume, and surface area. Water impermeable
fur is used for insulation by a few species, but most use blubber for insulation. However,
blubber is also used as a fuel, a water source, and for buoyancy control, and therefore cannot be selected solely for thermal needs. It is very likely that only the smallest individuals
of a given species are ever in a cold stress situation, but rather the most common thermoregulatory challenge for marine mammals is how to prevent overheating.