195
Chapter nine: Thermoregulation
temperature, be it land, sea ice, water, or air. Convection is the loss of heat through the combined effect of conduction and the movement of air or water molecules. We commonly know
this in air as wind chill. Wind chill cannot be measured simply on a thermometer. When
the air temperature is −25°F, the absolute temperature will be the same whether the wind is
blowing or not, despite how much colder it may feel. This function is important not only for
marine mammals that haul out on land or ice, but also when animals encounter strong water
currents or are swimming rapidly through the water, which can drive the cooling power
up to almost 100 times that of air (Schmidt-Nielsen 1997). Radiation is the release of thermal
energy from all matter, which does not require a medium for transfer. The sun heats the
earth through radiation, as there is no air in space to facilitate conductive transfer of energy.
A seal warming on the ice surface by orienting to the sun is an excellent example of radiative
heat gain. Evaporation is the application of body heat to disperse surface water. Convection
can speed up the process by pulling away the moisture on the skin surface. Humans recognize this phenomenon as sweating. Evaporation can play a significant role in the thermoregulation of those marine mammal species that spend large amounts of time in air.
Body size and the ratio of surface area to volume (SA:V) are also important to the Fick
equation. Consider how the relative surface area of a simple sphere increases as the volume goes down (volume = 4/3Πr 3 ; surface area 4Πr 2 ). A sphere with a diameter of 10 cm has
surface area of 314 cm 2 , a volume of 524 cm 3 , and a SA:V 0.60. If we reduce the diameter to
5 cm, the surface area is 78.5 cm 2 , the volume is 65.5 cm 3 , and the SA:V is now increased to
1.2. A quick shortcut to this calculation is that the SA:V of a sphere is given by the value of
3/r. This SA:V ratio means that an otter will have a proportionately larger surface area than
a small dolphin. However, that same dolphin has a much higher SA:V than a whale. All
else being equal, the Fick equation dictates that a smaller animal will experience greater
heat loss than a larger animal because its surface area is proportionately larger.
Finally, when comparing heat loss in air or water, an essential point to emphasize is
that the heat capacity of water is about 25× that of air. Applied to the Fick principle, this
means that all other factors being held constant (temperature differential, surface area,
etc.), heat will flow out of a warm body 20–25 times faster in water than in air. For a marine
mammal, being in the water implies a heat challenge significantly greater than to a comparable terrestrial mammal, or even to itself when on land. Cetaceans are always confronted
with the greater heat loss capacity of water.
9.2.1 Heat flow applied to marine mammals
Marine mammals are endothermic homeotherms, subject to all of the components of
the Fick equation (conduction, convection, radiation, and evaporation), albeit to varying
degrees. They are primarily susceptible to heat loss through conduction (in all circumstances) and convection when in the water. Even though there is limited radiative heat
exchange in water, this component can come into play to a much larger degree for the
pinnipeds when hauled out on beaches or ice (Mellish et al. 2015). There is some evidence
that seals will haul out on beaches for pupping or during the annual molt when radiative
heat gain is most likely to offset the losses of conduction and convection (Boily 1995; Hind
and Gurney 1998). While there is no direct observation of sweating in marine mammals
as a method of heat loss, there is histological evidence of sweat glands in at least some
species (Ling 1965; Bryden and Button 1977; Rotherham et al. 2005; Khamas et al. 2012).
This makes evolutionary sense in that these species spend most of their time in the water,
where evaporation as a means to offset excess body heat is impossible. Instead, many
species will employ behavioral tactics to compensate when cooling is necessary, such as
Chapter nine: Thermoregulation
temperature, be it land, sea ice, water, or air. Convection is the loss of heat through the combined effect of conduction and the movement of air or water molecules. We commonly know
this in air as wind chill. Wind chill cannot be measured simply on a thermometer. When
the air temperature is −25°F, the absolute temperature will be the same whether the wind is
blowing or not, despite how much colder it may feel. This function is important not only for
marine mammals that haul out on land or ice, but also when animals encounter strong water
currents or are swimming rapidly through the water, which can drive the cooling power
up to almost 100 times that of air (Schmidt-Nielsen 1997). Radiation is the release of thermal
energy from all matter, which does not require a medium for transfer. The sun heats the
earth through radiation, as there is no air in space to facilitate conductive transfer of energy.
A seal warming on the ice surface by orienting to the sun is an excellent example of radiative
heat gain. Evaporation is the application of body heat to disperse surface water. Convection
can speed up the process by pulling away the moisture on the skin surface. Humans recognize this phenomenon as sweating. Evaporation can play a significant role in the thermoregulation of those marine mammal species that spend large amounts of time in air.
Body size and the ratio of surface area to volume (SA:V) are also important to the Fick
equation. Consider how the relative surface area of a simple sphere increases as the volume goes down (volume = 4/3Πr 3 ; surface area 4Πr 2 ). A sphere with a diameter of 10 cm has
surface area of 314 cm 2 , a volume of 524 cm 3 , and a SA:V 0.60. If we reduce the diameter to
5 cm, the surface area is 78.5 cm 2 , the volume is 65.5 cm 3 , and the SA:V is now increased to
1.2. A quick shortcut to this calculation is that the SA:V of a sphere is given by the value of
3/r. This SA:V ratio means that an otter will have a proportionately larger surface area than
a small dolphin. However, that same dolphin has a much higher SA:V than a whale. All
else being equal, the Fick equation dictates that a smaller animal will experience greater
heat loss than a larger animal because its surface area is proportionately larger.
Finally, when comparing heat loss in air or water, an essential point to emphasize is
that the heat capacity of water is about 25× that of air. Applied to the Fick principle, this
means that all other factors being held constant (temperature differential, surface area,
etc.), heat will flow out of a warm body 20–25 times faster in water than in air. For a marine
mammal, being in the water implies a heat challenge significantly greater than to a comparable terrestrial mammal, or even to itself when on land. Cetaceans are always confronted
with the greater heat loss capacity of water.
9.2.1 Heat flow applied to marine mammals
Marine mammals are endothermic homeotherms, subject to all of the components of
the Fick equation (conduction, convection, radiation, and evaporation), albeit to varying
degrees. They are primarily susceptible to heat loss through conduction (in all circumstances) and convection when in the water. Even though there is limited radiative heat
exchange in water, this component can come into play to a much larger degree for the
pinnipeds when hauled out on beaches or ice (Mellish et al. 2015). There is some evidence
that seals will haul out on beaches for pupping or during the annual molt when radiative
heat gain is most likely to offset the losses of conduction and convection (Boily 1995; Hind
and Gurney 1998). While there is no direct observation of sweating in marine mammals
as a method of heat loss, there is histological evidence of sweat glands in at least some
species (Ling 1965; Bryden and Button 1977; Rotherham et al. 2005; Khamas et al. 2012).
This makes evolutionary sense in that these species spend most of their time in the water,
where evaporation as a means to offset excess body heat is impossible. Instead, many
species will employ behavioral tactics to compensate when cooling is necessary, such as