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Chapter nine: Thermoregulation
a suite of blood vessels that run through it to the skin surface. These veins and arterioles
can be opened or shut and blood flow to the skin surface can be controlled. When the
animal is too hot, it opens up the vessels, essentially blushes, and hot blood flows to the
skin surface to effectively dump the heat. Both authors have had the unusual fortune to
see seals steaming on the ice surface, even in subzero temperatures. This effect is often
described as a window in thermal imaging literature (Mauck et al. 2003; Nienaber et al.
2010). By contrast, if the animal is cold and needs to conserve heat, it will shut down the
blood flow through the blubber and reduce heat loss. Therefore, the very same animal that
can be on the surface giving off enough heat to steam and melt into the ice surface, can
under other conditions have skin and fur cool enough to not even melt snow that has fallen
on it during a storm. In both pinnipeds and cetaceans, this is taken even further by controlling blood flow to the relatively free of blubber, flippers, and flukes. These appendages
have almost no insulation through blubber or fur, allowing for large amounts of heat to be
released when blood is shunted through them (MacArthur 1989). Conversely, when these
appendages are shut down, there is almost no blood flow to those sites. This phenomenon
is routinely experienced by field biologists attempting blood sample collection by venipuncture in a flipper or fluke. If the animal is warm, blood flow will allow for relatively
easy sample collection if the needle is placed properly, however, if the animal is cold, it is
essentially impossible to collect a sample regardless of the skill of the collector. The very
large surface areas of the flippers of sea lions are thought to play a major role in dumping
of heat (Beenijes 2006), especially the hind limbs that are not used for propulsion.
Cooling mechanisms are not limited to the extremities. For example, the internal testes of male cetaceans are completely enclosed inside the blubber layer of the animal. There
is anatomical evidence that male cetaceans route cool blood coming from the dorsal fins to
the testes, presumably to keep these reproductive organs from overheating (Rommel et al.
1992). Direct measurement of surface temperatures in sea lions has shown that the shoulders and hips are warmer than the trunk in active and inactive animals alike, and this
was not a function of blubber depth (Willis and Horning 2005). The fact that blubber alone
does not dictate heat loss but merely plays a role in the Fick regime has been reinforced in
several species (Mellish et al. 2013).
Before we leave the discussion on the biochemical nature of blubber, it is important to discuss brown adipose fat. Some terrestrial newborn mammals contain an amount of a specialized
fat termed brown fat at key locations in their bodies for the first days after birth. This fat is
extremely metabolically active through massive lipid oxidation and futile biochemical cycling
that generates a large amount of heat as a waste product. The brown fat does not appear to
have a role in caloric energy balance, instead it is used as a heat-generating organ. Many
decades ago, there were studies that investigated whether marine mammals had brown fat
to help newborn pups survive being born in extremely cold conditions. The only evidence of
brown fat was its discovery in harp seals in 1979 (Blix et al. 1979), along with a recent description of the uncoupling mechanisms that allow the futile cycling (Pearson et al. 2014). The use
of brown fat to generate heat is generally not thought to be of significance for most marine
species. Pabst et al. (1999) provide a summary of many of the functional morphology aspects
of heat flow and blubber characteristics in marine mammals.
9.4 Heat generation in marine mammals
We now turn to the other side of the equation and discuss the mechanisms of heat generation
by marine species. As noted above, they are endothermic homeotherms and create most of
their heat (barring external radiative or conductive heat gain) through metabolic processes.
Chapter nine: Thermoregulation
a suite of blood vessels that run through it to the skin surface. These veins and arterioles
can be opened or shut and blood flow to the skin surface can be controlled. When the
animal is too hot, it opens up the vessels, essentially blushes, and hot blood flows to the
skin surface to effectively dump the heat. Both authors have had the unusual fortune to
see seals steaming on the ice surface, even in subzero temperatures. This effect is often
described as a window in thermal imaging literature (Mauck et al. 2003; Nienaber et al.
2010). By contrast, if the animal is cold and needs to conserve heat, it will shut down the
blood flow through the blubber and reduce heat loss. Therefore, the very same animal that
can be on the surface giving off enough heat to steam and melt into the ice surface, can
under other conditions have skin and fur cool enough to not even melt snow that has fallen
on it during a storm. In both pinnipeds and cetaceans, this is taken even further by controlling blood flow to the relatively free of blubber, flippers, and flukes. These appendages
have almost no insulation through blubber or fur, allowing for large amounts of heat to be
released when blood is shunted through them (MacArthur 1989). Conversely, when these
appendages are shut down, there is almost no blood flow to those sites. This phenomenon
is routinely experienced by field biologists attempting blood sample collection by venipuncture in a flipper or fluke. If the animal is warm, blood flow will allow for relatively
easy sample collection if the needle is placed properly, however, if the animal is cold, it is
essentially impossible to collect a sample regardless of the skill of the collector. The very
large surface areas of the flippers of sea lions are thought to play a major role in dumping
of heat (Beenijes 2006), especially the hind limbs that are not used for propulsion.
Cooling mechanisms are not limited to the extremities. For example, the internal testes of male cetaceans are completely enclosed inside the blubber layer of the animal. There
is anatomical evidence that male cetaceans route cool blood coming from the dorsal fins to
the testes, presumably to keep these reproductive organs from overheating (Rommel et al.
1992). Direct measurement of surface temperatures in sea lions has shown that the shoulders and hips are warmer than the trunk in active and inactive animals alike, and this
was not a function of blubber depth (Willis and Horning 2005). The fact that blubber alone
does not dictate heat loss but merely plays a role in the Fick regime has been reinforced in
several species (Mellish et al. 2013).
Before we leave the discussion on the biochemical nature of blubber, it is important to discuss brown adipose fat. Some terrestrial newborn mammals contain an amount of a specialized
fat termed brown fat at key locations in their bodies for the first days after birth. This fat is
extremely metabolically active through massive lipid oxidation and futile biochemical cycling
that generates a large amount of heat as a waste product. The brown fat does not appear to
have a role in caloric energy balance, instead it is used as a heat-generating organ. Many
decades ago, there were studies that investigated whether marine mammals had brown fat
to help newborn pups survive being born in extremely cold conditions. The only evidence of
brown fat was its discovery in harp seals in 1979 (Blix et al. 1979), along with a recent description of the uncoupling mechanisms that allow the futile cycling (Pearson et al. 2014). The use
of brown fat to generate heat is generally not thought to be of significance for most marine
species. Pabst et al. (1999) provide a summary of many of the functional morphology aspects
of heat flow and blubber characteristics in marine mammals.
9.4 Heat generation in marine mammals
We now turn to the other side of the equation and discuss the mechanisms of heat generation
by marine species. As noted above, they are endothermic homeotherms and create most of
their heat (barring external radiative or conductive heat gain) through metabolic processes.