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Marine Mammal Physiology: Requisites for Ocean Living
to great depths, while others, such as dugongs, spend much of their lives in relatively shallow sea-grass environments (Chilvers et al. 2004). Even among the deep divers, some are
relatively sedate divers, while others descend and ascend at high speeds (Figure 4.1). With
the advent of sophisticated microelectronics, we are building up knowledge of the diving
behavior of many marine mammal species although many gaps remain, especially in our
understanding of how physiology may limit diving.
There is a general tendency for greater body size to favor greater dive capacity
because of the differential scaling between body O 2 stores and metabolic rate (Halsey
et al. 2006). However, other factors may also play a role, such as the ability to reduce body
temperature (Boyd and Croxall 1996) or the costs associated with engulfment capacity
(Goldbogen et al. 2012). Given constraints on swimming speed, there is also a general
relationship between dive time and dive depth, such that the deepest dives are necessarily the longest. Thus, the deepest divers also tend to be relatively large (Table 4.1). This
relationship has also led to predictions that as animals grow, their diving capacity will
increase. There is some support for this, and, at least among pinniped species, it appears
that there is some ontogeny of the ability to dive to depth, although this is possibly
driven more by O 2 stores and metabolism than by adaptation to pressure (Fowler et al.
2006; Horning and Trillmich 1997; McLellan et al. 2002). Similarly, babysitting behavior
in sperm whales is thought necessary because young animals are unable to dive as deep
or for as long as adults (Whitehead 1996). Some preliminary work on the mechanical
properties of the lung in pinnipeds showed higher and more variable lung compliance
in wild pinnipeds compared with those raised under human care, possibly indicating
that lung conditioning is important for diving animals and that repeated diving helps
protect against lung squeeze (Fahlman et al. 2014a).
4.2.2 Physics of pressure, gas volume, and solubility
Before looking at the effects of pressure on animals, it is worth briefly considering some
of the physics and chemistry that underlie problems with pressure. Pressure is a measure
of the force exerted over a given surface area. Hydrostatic pressure is the pressure exerted
by a fluid due to the force of gravity, increasing in proportion to depth from the surface
because of the increasing weight of fluid exerting downward force from above. Thus in
the ocean, from atmospheric pressure (1 ATA) at the surface, pressure increases by 1 ATA
for every 10 m descended. So, at 1000 m depth the pressure is 100 times greater than that
at the surface.
The volume of 1 mole of gas (6.02 × 10 23 molecules) without water vapor (dry) under
standard temperature (0°C), and pressure (1 ATA) is 22.7 L for all gases. In a mixture,
each gas exerts its own pressure (partial pressure) and the total gas pressure is the sum
of all partial pressures (Dalton’s law; Table 4.2). Boyle’s law describes the inverse relationship between gas volume and pressure of a gas phase (Table 4.2). A popular experiment
to show this is to attach a styrofoam cup to deep-ocean sampling equipment. Such cups
return from depth thimble-sized as the pressure has compressed the air in them, forcing this structural change. Airspaces in animals similarly become compressed at depth,
potentially causing damage to any surrounding rigid structures, causing, for example,
barotrauma or lung squeeze.
While liquids are essentially incompressible, increasing pressure does have an effect
on the solubility of dissolved gases (Henry’s law, Table 4.2). This is well illustrated by
the gas dissolved in carbonated drink, which is “released” from solution as the pressure drops (as the lid is opened). Gases dissolved in liquids are measured in terms of the
Marine Mammal Physiology: Requisites for Ocean Living
to great depths, while others, such as dugongs, spend much of their lives in relatively shallow sea-grass environments (Chilvers et al. 2004). Even among the deep divers, some are
relatively sedate divers, while others descend and ascend at high speeds (Figure 4.1). With
the advent of sophisticated microelectronics, we are building up knowledge of the diving
behavior of many marine mammal species although many gaps remain, especially in our
understanding of how physiology may limit diving.
There is a general tendency for greater body size to favor greater dive capacity
because of the differential scaling between body O 2 stores and metabolic rate (Halsey
et al. 2006). However, other factors may also play a role, such as the ability to reduce body
temperature (Boyd and Croxall 1996) or the costs associated with engulfment capacity
(Goldbogen et al. 2012). Given constraints on swimming speed, there is also a general
relationship between dive time and dive depth, such that the deepest dives are necessarily the longest. Thus, the deepest divers also tend to be relatively large (Table 4.1). This
relationship has also led to predictions that as animals grow, their diving capacity will
increase. There is some support for this, and, at least among pinniped species, it appears
that there is some ontogeny of the ability to dive to depth, although this is possibly
driven more by O 2 stores and metabolism than by adaptation to pressure (Fowler et al.
2006; Horning and Trillmich 1997; McLellan et al. 2002). Similarly, babysitting behavior
in sperm whales is thought necessary because young animals are unable to dive as deep
or for as long as adults (Whitehead 1996). Some preliminary work on the mechanical
properties of the lung in pinnipeds showed higher and more variable lung compliance
in wild pinnipeds compared with those raised under human care, possibly indicating
that lung conditioning is important for diving animals and that repeated diving helps
protect against lung squeeze (Fahlman et al. 2014a).
4.2.2 Physics of pressure, gas volume, and solubility
Before looking at the effects of pressure on animals, it is worth briefly considering some
of the physics and chemistry that underlie problems with pressure. Pressure is a measure
of the force exerted over a given surface area. Hydrostatic pressure is the pressure exerted
by a fluid due to the force of gravity, increasing in proportion to depth from the surface
because of the increasing weight of fluid exerting downward force from above. Thus in
the ocean, from atmospheric pressure (1 ATA) at the surface, pressure increases by 1 ATA
for every 10 m descended. So, at 1000 m depth the pressure is 100 times greater than that
at the surface.
The volume of 1 mole of gas (6.02 × 10 23 molecules) without water vapor (dry) under
standard temperature (0°C), and pressure (1 ATA) is 22.7 L for all gases. In a mixture,
each gas exerts its own pressure (partial pressure) and the total gas pressure is the sum
of all partial pressures (Dalton’s law; Table 4.2). Boyle’s law describes the inverse relationship between gas volume and pressure of a gas phase (Table 4.2). A popular experiment
to show this is to attach a styrofoam cup to deep-ocean sampling equipment. Such cups
return from depth thimble-sized as the pressure has compressed the air in them, forcing this structural change. Airspaces in animals similarly become compressed at depth,
potentially causing damage to any surrounding rigid structures, causing, for example,
barotrauma or lung squeeze.
While liquids are essentially incompressible, increasing pressure does have an effect
on the solubility of dissolved gases (Henry’s law, Table 4.2). This is well illustrated by
the gas dissolved in carbonated drink, which is “released” from solution as the pressure drops (as the lid is opened). Gases dissolved in liquids are measured in terms of the