203
Chapter nine: Thermoregulation
Sea otters have a very high metabolic rate and probably even use the heat from food
digestion for thermal balance (Morrison et al. 1974; Costa and Kooyman 1984; Yeates et al.
2007). Remember our point about the SA:V ratio and that dumping of heat may be more
important for larger species. This leads to an essential point about the thermal biology of
marine mammals: they are not warmer than terrestrial mammals, yet they appear to generate more heat. Therefore, the fundamental difference between marine and terrestrial
mammals lies almost entirely in their thermoregulatory control of heat loss, rather than
specialized systems to generate heat.
9.5 Heat balance versus other metabolic demands
Exercise and digestion (specific dynamic action, SDA) are two activities that increase
Resting Metabolic Rate and provide additional heat sources in all mammals, and this is
no different for marine mammals (Costa and Kooyman 1984; Rosen et al. 2007). Under
most circumstances, this extra heat can be conserved to offset heat in colder water, or they
can release heat in a warmer environment. However, this balance becomes more complex
when the animals are diving. Williams and Maresh (Chapter 3) discussed the need to
reduce oxygen consumption while still meeting the increased metabolic demands of diving. Previously, we noted that ion pumping through cell wall channels accounted for most
of the daily metabolic demands of mammalian cells. The Hochachka laboratory argued
that if marine mammals could channel arrest and reduce ion leakiness, and therefore the
need for ion pumping, they could lower their metabolic rate and dive for longer times
(Hochachka and Guppy 1987). This would also reduce their heat production during diving
with the potential to create a thermal deficit (Elsner 2015). In the 1970s–1980s, Kooyman
et al. (1980) and Hill et al. (1987) were able to place central arterial temperature devices
into freely long-diving Weddell seals and found that deep circulating body temperature
did decrease during diving, but only by 1°C–2°C. Since that time, a suite of studies have
examined body temperature in diving seals (and penguins) and found that temperature
does fluctuate throughout the body during diving, and that even a few degrees drop can
provide some reduction in metabolic demand as defined by the Q 10 relationship (Ponganis
et al. 2003; Meir and Ponganis 2010). However, for species such as elephant seals that
remain at sea for months and dive over 90% of their time at sea, they would not be able to
tolerate a continued drop in body temperature. This returns us to the discussions in this
book in Chapters 1 through 3, and others about swimming patterns, muscle use and the
energetics of diving. There is a clear relationship between diving physiology, metabolic
rate, and thermal biology for these species. Therefore, while measuring thermal characteristics in the laboratory or on the beach is important, you also have to consider how those
relationships may change during diving. For example, we know that blood flow to the liver
and kidney (important for digestion) is reduced during free diving in Weddell seals (Davis
et al. 1983). This will reduce not only the body temperature, but delay any resulting SDA
from their nutrient clearance and blood filtering reactions.
Another consideration about the differences between studying thermal biology on land or
in a small pool is that a swimming marine mammal will be moving quickly (usually ~2 m/s)
and the convective heat loss will be significant. Of particular interest is the bowhead whale
with its extremely thick blubber swimming through ice-laden waters. It is possible that
these whales have some unusual thermal properties. Recall that their blubber layers can
be up to 50 cm thick, which is at the high end of all marine mammals. Some believe this
thick blubber is necessary as a fuel source for their long migrations and is selected more
for energy than for thermal needs. Others propose that the thick blubber acts as a shield
Chapter nine: Thermoregulation
Sea otters have a very high metabolic rate and probably even use the heat from food
digestion for thermal balance (Morrison et al. 1974; Costa and Kooyman 1984; Yeates et al.
2007). Remember our point about the SA:V ratio and that dumping of heat may be more
important for larger species. This leads to an essential point about the thermal biology of
marine mammals: they are not warmer than terrestrial mammals, yet they appear to generate more heat. Therefore, the fundamental difference between marine and terrestrial
mammals lies almost entirely in their thermoregulatory control of heat loss, rather than
specialized systems to generate heat.
9.5 Heat balance versus other metabolic demands
Exercise and digestion (specific dynamic action, SDA) are two activities that increase
Resting Metabolic Rate and provide additional heat sources in all mammals, and this is
no different for marine mammals (Costa and Kooyman 1984; Rosen et al. 2007). Under
most circumstances, this extra heat can be conserved to offset heat in colder water, or they
can release heat in a warmer environment. However, this balance becomes more complex
when the animals are diving. Williams and Maresh (Chapter 3) discussed the need to
reduce oxygen consumption while still meeting the increased metabolic demands of diving. Previously, we noted that ion pumping through cell wall channels accounted for most
of the daily metabolic demands of mammalian cells. The Hochachka laboratory argued
that if marine mammals could channel arrest and reduce ion leakiness, and therefore the
need for ion pumping, they could lower their metabolic rate and dive for longer times
(Hochachka and Guppy 1987). This would also reduce their heat production during diving
with the potential to create a thermal deficit (Elsner 2015). In the 1970s–1980s, Kooyman
et al. (1980) and Hill et al. (1987) were able to place central arterial temperature devices
into freely long-diving Weddell seals and found that deep circulating body temperature
did decrease during diving, but only by 1°C–2°C. Since that time, a suite of studies have
examined body temperature in diving seals (and penguins) and found that temperature
does fluctuate throughout the body during diving, and that even a few degrees drop can
provide some reduction in metabolic demand as defined by the Q 10 relationship (Ponganis
et al. 2003; Meir and Ponganis 2010). However, for species such as elephant seals that
remain at sea for months and dive over 90% of their time at sea, they would not be able to
tolerate a continued drop in body temperature. This returns us to the discussions in this
book in Chapters 1 through 3, and others about swimming patterns, muscle use and the
energetics of diving. There is a clear relationship between diving physiology, metabolic
rate, and thermal biology for these species. Therefore, while measuring thermal characteristics in the laboratory or on the beach is important, you also have to consider how those
relationships may change during diving. For example, we know that blood flow to the liver
and kidney (important for digestion) is reduced during free diving in Weddell seals (Davis
et al. 1983). This will reduce not only the body temperature, but delay any resulting SDA
from their nutrient clearance and blood filtering reactions.
Another consideration about the differences between studying thermal biology on land or
in a small pool is that a swimming marine mammal will be moving quickly (usually ~2 m/s)
and the convective heat loss will be significant. Of particular interest is the bowhead whale
with its extremely thick blubber swimming through ice-laden waters. It is possible that
these whales have some unusual thermal properties. Recall that their blubber layers can
be up to 50 cm thick, which is at the high end of all marine mammals. Some believe this
thick blubber is necessary as a fuel source for their long migrations and is selected more
for energy than for thermal needs. Others propose that the thick blubber acts as a shield