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Marine Mammal Physiology: Requisites for Ocean Living
2.4 Unanswered questions
The primary challenge in understanding the physiology underlying extended dive durations remains making measurements in freely diving mammals. There are significant
differences in the physiological responses to mammals undergoing forced submersion
experiments and animals freely diving in their natural environment. Measurements in
free-diving cetaceans are especially difficult due to their completely aquatic lifestyle and
to the difficulties of recorder attachment.
A number of the physiological parameters discussed above may also vary depending
on the type of dive or even within dives; these include diving lung volume, O 2 extraction
from lung or blood, hematocrit, and initial/final blood O 2 saturations. How these parameters change during or between dives remains an unanswered question.
As more physiological data are collected in freely diving marine mammals, we are
learning how the dive response and O 2 store management vary during different types of
dives and between species. We now know that heart rates and O 2 depletion rates can vary
considerably depending on the nature and circumstances of a given dive. Yet, the how and
why these parameters vary remains a challenge for future studies and new technological
advances. Finally, the ADL has been defined by the accumulation of blood lactate during
dives, yet we still have not conquered the challenge of making continuous blood lactate
measurements. Such measurements are essential to interpretations of diving/foraging
behavior and to evaluation of the ADL hypothesis that most dives are aerobic.
Glossary
Aerobic dive limit (ADL): The shortest dive duration associated with an elevation in
post-dive blood lactate concentration. This is determined by lactate measurements in post-dive blood samples. The ADL is important because it indicates an
increased reliance on anaerobic metabolism, the result of which is lactate accumulation. In the Weddell seal, the accumulation of lactate has been associated
with increased time at the surface, which results in less time spent underwater for
important activities, such as foraging.
Arterialization: The arterialization of venous blood means that more O 2 than normal is
found in the venous circulation. For a diving mammal, such an increase in predive venous O 2 saturation (arterialization) would result in an increased blood O 2
store, allowing these mammals to dive aerobically for longer periods.
Behavioral aerobic dive limit (ADL b ): The behavioral ADL is often considered the dive
duration below which are 90%–97% of all dives of a given animal. The behavioral
ADL has also been estimated as the dive duration after which the time spent at the
surface (post-dive surface interval) is increased significantly.
Bradycardia: A bradycardia is a heart rate that is below resting levels. In diving mammals, a bradycardia during diving is part of the dive response and is considered
to reduce O 2 consumption during the dive.
Calculated aerobic dive limit (ADL c ): The total sum of all three oxygen stores in a species
divided by the diving metabolic rate. The ADL c is subject to many assumptions
and is often used when the aerobic dive limit cannot be determined.
Dive response: The cardiovascular response during a breath-hold or dive. This response
involves both a decrease in heart rate and an increase in peripheral vasoconstriction
via activation of the parasympathetic and sympathetic nervous systems. Dive reflex
usually refers to a most extreme dive response as elicited by forced submersion.
Marine Mammal Physiology: Requisites for Ocean Living
2.4 Unanswered questions
The primary challenge in understanding the physiology underlying extended dive durations remains making measurements in freely diving mammals. There are significant
differences in the physiological responses to mammals undergoing forced submersion
experiments and animals freely diving in their natural environment. Measurements in
free-diving cetaceans are especially difficult due to their completely aquatic lifestyle and
to the difficulties of recorder attachment.
A number of the physiological parameters discussed above may also vary depending
on the type of dive or even within dives; these include diving lung volume, O 2 extraction
from lung or blood, hematocrit, and initial/final blood O 2 saturations. How these parameters change during or between dives remains an unanswered question.
As more physiological data are collected in freely diving marine mammals, we are
learning how the dive response and O 2 store management vary during different types of
dives and between species. We now know that heart rates and O 2 depletion rates can vary
considerably depending on the nature and circumstances of a given dive. Yet, the how and
why these parameters vary remains a challenge for future studies and new technological
advances. Finally, the ADL has been defined by the accumulation of blood lactate during
dives, yet we still have not conquered the challenge of making continuous blood lactate
measurements. Such measurements are essential to interpretations of diving/foraging
behavior and to evaluation of the ADL hypothesis that most dives are aerobic.
Glossary
Aerobic dive limit (ADL): The shortest dive duration associated with an elevation in
post-dive blood lactate concentration. This is determined by lactate measurements in post-dive blood samples. The ADL is important because it indicates an
increased reliance on anaerobic metabolism, the result of which is lactate accumulation. In the Weddell seal, the accumulation of lactate has been associated
with increased time at the surface, which results in less time spent underwater for
important activities, such as foraging.
Arterialization: The arterialization of venous blood means that more O 2 than normal is
found in the venous circulation. For a diving mammal, such an increase in predive venous O 2 saturation (arterialization) would result in an increased blood O 2
store, allowing these mammals to dive aerobically for longer periods.
Behavioral aerobic dive limit (ADL b ): The behavioral ADL is often considered the dive
duration below which are 90%–97% of all dives of a given animal. The behavioral
ADL has also been estimated as the dive duration after which the time spent at the
surface (post-dive surface interval) is increased significantly.
Bradycardia: A bradycardia is a heart rate that is below resting levels. In diving mammals, a bradycardia during diving is part of the dive response and is considered
to reduce O 2 consumption during the dive.
Calculated aerobic dive limit (ADL c ): The total sum of all three oxygen stores in a species
divided by the diving metabolic rate. The ADL c is subject to many assumptions
and is often used when the aerobic dive limit cannot be determined.
Dive response: The cardiovascular response during a breath-hold or dive. This response
involves both a decrease in heart rate and an increase in peripheral vasoconstriction
via activation of the parasympathetic and sympathetic nervous systems. Dive reflex
usually refers to a most extreme dive response as elicited by forced submersion.