340
Marine Mammal Physiology: Requisites for Ocean Living
varies with numerous variables including prey species and time of year. The field of
digestive physiology focuses on assimilation and digestive efficiency by using a suite
of metabolic tracers in food items, combined with analyzing calories ingested versus
the calories remaining in fecal material. These tracers include protein and amino acid
markers to estimate protein metabolism among a host of other methods for specific tissues or types of metabolism. This returns us to the question of physiological ecology
and policy management in the wild, particularly as it relates to the overlap of fisheries
and marine mammal prey. While it may be simple enough to determine the species and
general caloric value of a prey species, it only provides the roughest approximation of
what is available to the marine mammal in question, which may or may not be even
remotely close to the true energetic value to the consumer. This was an essential component of the “Is it food” policy debate during the listing of the Steller sea lion under the
Endangered Species Act. One major theory during this process was that despite their
energetic content, some fish species should be considered junk food (Rosen and Trites
2000), precluding their need to be included in fishing regulations. As techniques for
monitoring animals in the wild grow ever more detailed and comprehensive, we will
hopefully be in a better position to answer these sometimes politically charged questions with solid scientific data. For an in-depth review of marine mammal digestive
physiology (see Worthy 1990).
15.4.3 Sleep physiology
Dive recorders have shown that elephant seals dive repetitively and without stopping for
months at sea. When do they sleep if they are diving hundreds of times per day with no
long surface periods? It turns out that many species of phocids are able to sleep while
underwater. Some cetaceans remarkably sleep with one hemisphere of their brain while
the other half maintains vigilance. Sleeping and diving have become so physiologically
intertwined in phocids that seals will hold their breath while sleeping on dry land. This
phenomenon enables us to study many of the same attributes of breath-holding seen in diving (e.g., bradycardia, hematocrit regulation) but without the conflict of exercise, as noted
in Chapter 3. Electroencephalograms of brain activity show that seals will stay sleeping on
the beach, as they come in and out of breath-hold periods. In the case of elephant seals, this
effect of sleep apnea can be as long as 20 min. Comparisons to estimates of diving metabolic rate in Weddell seals suggested that it was only slightly lower than during sleeping.
For a review of sleep apnea physiology (see Castellini 1996).
15.4.4 Tissue level biochemistry and molecular biology
The field of molecular medicine and biochemistry is touched on many times in this book,
but overall, this research area is far behind that of laboratory-based medical research on
terrestrial mammals. The difficulty lies in obtaining samples in sufficiently large numbers
for marine mammal species. Genetic analysis has been essential for some Endangered
Species Act determinations, and cell culture has been essential to the fields of contaminant chemistry and responses to external stressors. Many of the basic questions on cellular metabolic pathway regulation, impacts of temperature, pressure, and pH on cell
function and enzymatic reactions remain at the very edge of marine mammal biochemistry. New work on oxygen free radical damage and oxidative stress is important and is
an area that touches diving physiology and other medical aspects of marine mammal
biology (e.g., Hindle et al. 2009).
Marine Mammal Physiology: Requisites for Ocean Living
varies with numerous variables including prey species and time of year. The field of
digestive physiology focuses on assimilation and digestive efficiency by using a suite
of metabolic tracers in food items, combined with analyzing calories ingested versus
the calories remaining in fecal material. These tracers include protein and amino acid
markers to estimate protein metabolism among a host of other methods for specific tissues or types of metabolism. This returns us to the question of physiological ecology
and policy management in the wild, particularly as it relates to the overlap of fisheries
and marine mammal prey. While it may be simple enough to determine the species and
general caloric value of a prey species, it only provides the roughest approximation of
what is available to the marine mammal in question, which may or may not be even
remotely close to the true energetic value to the consumer. This was an essential component of the “Is it food” policy debate during the listing of the Steller sea lion under the
Endangered Species Act. One major theory during this process was that despite their
energetic content, some fish species should be considered junk food (Rosen and Trites
2000), precluding their need to be included in fishing regulations. As techniques for
monitoring animals in the wild grow ever more detailed and comprehensive, we will
hopefully be in a better position to answer these sometimes politically charged questions with solid scientific data. For an in-depth review of marine mammal digestive
physiology (see Worthy 1990).
15.4.3 Sleep physiology
Dive recorders have shown that elephant seals dive repetitively and without stopping for
months at sea. When do they sleep if they are diving hundreds of times per day with no
long surface periods? It turns out that many species of phocids are able to sleep while
underwater. Some cetaceans remarkably sleep with one hemisphere of their brain while
the other half maintains vigilance. Sleeping and diving have become so physiologically
intertwined in phocids that seals will hold their breath while sleeping on dry land. This
phenomenon enables us to study many of the same attributes of breath-holding seen in diving (e.g., bradycardia, hematocrit regulation) but without the conflict of exercise, as noted
in Chapter 3. Electroencephalograms of brain activity show that seals will stay sleeping on
the beach, as they come in and out of breath-hold periods. In the case of elephant seals, this
effect of sleep apnea can be as long as 20 min. Comparisons to estimates of diving metabolic rate in Weddell seals suggested that it was only slightly lower than during sleeping.
For a review of sleep apnea physiology (see Castellini 1996).
15.4.4 Tissue level biochemistry and molecular biology
The field of molecular medicine and biochemistry is touched on many times in this book,
but overall, this research area is far behind that of laboratory-based medical research on
terrestrial mammals. The difficulty lies in obtaining samples in sufficiently large numbers
for marine mammal species. Genetic analysis has been essential for some Endangered
Species Act determinations, and cell culture has been essential to the fields of contaminant chemistry and responses to external stressors. Many of the basic questions on cellular metabolic pathway regulation, impacts of temperature, pressure, and pH on cell
function and enzymatic reactions remain at the very edge of marine mammal biochemistry. New work on oxygen free radical damage and oxidative stress is important and is
an area that touches diving physiology and other medical aspects of marine mammal
biology (e.g., Hindle et al. 2009).