30
Marine Mammal Physiology: Requisites for Ocean Living
of O 2 is essential to diving ability. Such O 2 store management is achieved through the dive
response (decreased heart rate or bradycardia and redistribution of blood flow to tissues) as
well as through efficient locomotory patterns. Finally, while terrestrial mammals are not
able to withstand low levels of blood O 2 , some marine mammals have demonstrated the
ability to dive with very little O 2 remaining in the blood (hypoxemic tolerance). Thus, marine
mammals can extend dive durations by (1) bringing more O 2 with them, (2) using it more
efficiently, and (3) depleting it to very low levels during dives.
Many marine mammals dive repetitively with very short surface intervals. Due to the
cost efficiency of aerobic metabolism for ATP production, such diving patterns in many
species have led to the concept that most dive are aerobic and less than an aerobic dive limit
(ADL, the dive duration associated with the onset of post-dive blood lactate accumulation).
Our understanding of O 2 storage, rates of O 2 depletion, the dive response, and aerobic
dive limits in marine mammals is limited by the difficulty of measuring these parameters in freely diving animals. Early work on the diving physiology of seals involved
forced submersions, an approach that allowed for measurements under extreme conditions
(Scholander 1940). Advances in microprocessor technology have led to the development
of small physiological loggers that can record some of these parameters in freely diving
animals (Ponganis 2015). Much of this recent work has been performed on pinnipeds,
due to the ability to capture these animals on land. Since cetaceans are not easily accessible and dive in the open ocean, they present the greatest challenge in measuring these
variables.
Measurement of O 2 stores, recording of heart rate during diving, and determination of
ADLs each has its own challenges. Body O 2 stores, located in the lungs, blood, and muscle,
are dependent on many different variables: diving lung volume, blood volume, hemoglobin (Hb) concentration, muscle mass, and myoglobin (Mb) concentration. Accurate recording of heart rate during dives as deep as 500 m is not a simple matter. And documentation
of post-dive blood lactate concentrations has been achieved in only a few species under
special circumstances. This chapter will review (1) the magnitude and distribution of O 2
stores in marine mammals, (2) the dive response, (3) the ADL concept, and (4) limitations
in the measurement and documentation of these parameters and variables.
2.2 What is known
2.2.1 Magnitude and distribution of total body O 2 stores
How much O 2 is stored and where it is stored varies among species; the best divers tend
to store more O 2 in the muscle and blood than in the lungs. In comparison to humans
(“a non-diver”), mass-specific body O 2 stores are elevated 1.5- to almost 5-fold in all marine
mammals except the manatee (Table 2.1). The largest mass-specific O 2 stores are found
in the phocid seals; the northern elephant seal (Mirounga angustirostris), with some of the
longest routine dive durations of any pinniped, has the highest mass-specific O 2 store at
94 ml O 2 kg −1 (Simpson et al. 1970; Bryden 1972; Thorson and Le Boeuf 1994). The manatee (Trichechus manutus), as the shallowest diver, has the lowest O 2 store at 21 ml O 2 kg −1
(Lenfant et al. 1970; Gallivan et al. 1986). In general, diving mammals with greater diving
capacities have significantly larger O 2 stores.
As a percentage of the total body O 2 store in diving mammals, the blood and muscle O 2
stores range from 54% in the sea otter (Enhdyra lutris) to 97% in the northern elephant seal
(Table 2.1). In deeper divers, there appears to be decreased dependence on the respiratory
Marine Mammal Physiology: Requisites for Ocean Living
of O 2 is essential to diving ability. Such O 2 store management is achieved through the dive
response (decreased heart rate or bradycardia and redistribution of blood flow to tissues) as
well as through efficient locomotory patterns. Finally, while terrestrial mammals are not
able to withstand low levels of blood O 2 , some marine mammals have demonstrated the
ability to dive with very little O 2 remaining in the blood (hypoxemic tolerance). Thus, marine
mammals can extend dive durations by (1) bringing more O 2 with them, (2) using it more
efficiently, and (3) depleting it to very low levels during dives.
Many marine mammals dive repetitively with very short surface intervals. Due to the
cost efficiency of aerobic metabolism for ATP production, such diving patterns in many
species have led to the concept that most dive are aerobic and less than an aerobic dive limit
(ADL, the dive duration associated with the onset of post-dive blood lactate accumulation).
Our understanding of O 2 storage, rates of O 2 depletion, the dive response, and aerobic
dive limits in marine mammals is limited by the difficulty of measuring these parameters in freely diving animals. Early work on the diving physiology of seals involved
forced submersions, an approach that allowed for measurements under extreme conditions
(Scholander 1940). Advances in microprocessor technology have led to the development
of small physiological loggers that can record some of these parameters in freely diving
animals (Ponganis 2015). Much of this recent work has been performed on pinnipeds,
due to the ability to capture these animals on land. Since cetaceans are not easily accessible and dive in the open ocean, they present the greatest challenge in measuring these
variables.
Measurement of O 2 stores, recording of heart rate during diving, and determination of
ADLs each has its own challenges. Body O 2 stores, located in the lungs, blood, and muscle,
are dependent on many different variables: diving lung volume, blood volume, hemoglobin (Hb) concentration, muscle mass, and myoglobin (Mb) concentration. Accurate recording of heart rate during dives as deep as 500 m is not a simple matter. And documentation
of post-dive blood lactate concentrations has been achieved in only a few species under
special circumstances. This chapter will review (1) the magnitude and distribution of O 2
stores in marine mammals, (2) the dive response, (3) the ADL concept, and (4) limitations
in the measurement and documentation of these parameters and variables.
2.2 What is known
2.2.1 Magnitude and distribution of total body O 2 stores
How much O 2 is stored and where it is stored varies among species; the best divers tend
to store more O 2 in the muscle and blood than in the lungs. In comparison to humans
(“a non-diver”), mass-specific body O 2 stores are elevated 1.5- to almost 5-fold in all marine
mammals except the manatee (Table 2.1). The largest mass-specific O 2 stores are found
in the phocid seals; the northern elephant seal (Mirounga angustirostris), with some of the
longest routine dive durations of any pinniped, has the highest mass-specific O 2 store at
94 ml O 2 kg −1 (Simpson et al. 1970; Bryden 1972; Thorson and Le Boeuf 1994). The manatee (Trichechus manutus), as the shallowest diver, has the lowest O 2 store at 21 ml O 2 kg −1
(Lenfant et al. 1970; Gallivan et al. 1986). In general, diving mammals with greater diving
capacities have significantly larger O 2 stores.
As a percentage of the total body O 2 store in diving mammals, the blood and muscle O 2
stores range from 54% in the sea otter (Enhdyra lutris) to 97% in the northern elephant seal
(Table 2.1). In deeper divers, there appears to be decreased dependence on the respiratory