85
Chapter four: Pressure regulation
renal bubbles (Dennison et al. 2012). However, controlled studies in the bottlenose dolphin
(Houser et al. 2010) or Steller sea lion (Moore, unpub. obs.) have failed to replicate these findings. One possible reason could be that the dive history of wild-stranded animals is much
more extensive than those starting off with near sea surface blood and tissue PN 2 levels. The
development of technology to continuously monitor free ranging animals will be needed to
test if free-ranging animals do experience gas bubbles while foraging.
4.4 Future of this topic
It is difficult to formulate a list of the lingering questions for this topic, since there remain
so many gaps in our knowledge and understanding of how marine mammals cope with
repeated exposure to high pressure. This is largely because experimental research, at least
on obligate aquatic mammals diving to pressure has been a relatively intractable problem
(Kooyman 2006). This continues to be the case, although advances in electronics are beginning to allow the pursuit of new avenues. However, in trying to make more sense of this
field, it will be crucial to understand more about the phylogenetic variation in species
behavior and physiology. We need to be open-minded in terms of questioning some of the
assumptions, speculation, and hypotheses that have been suggested previously.
4.4.1 Phylogenetic variation
The vast majority of studies of the diving physiology of marine mammals have been done
on seals, with a handful of studies of trained bottlenose dolphins (Noren et al. 2004, 2012;
Ridgway and Howard 1979; Ridgway et al. 1969; Williams et al. 1993, 2015). An illustration
of the type of problem that can arise is oft-quoted wisdom that “seals dive on exhalation.” In fact, this was initially shown for some phocid seals, but studies of several otariid
seals suggest that these seals are inhalation divers (Hooker et al. 2005; Kooyman 1973;
McDonald and Ponganis 2012). Thus care is needed in assuming that particular species or
species groups are representative of an overall pattern.
Our knowledge of marine mammal adaptations to pressure remains extremely limited. We need a better understanding of both physiology and physiological plasticity particularly in terms of phylogenetic variability. Thus far, we have only a handful of studies
suggesting the depth of alveolar collapse, even fewer documenting changes in blood flow
with diving and yet these are crucial to our ability to build a framework to understand
changes in gas uptake that animals are exposed to during dives.
4.4.2 Form and function
Even in terms of more tractable studies such as the investigation of beach-cast animals
to better describe anatomical adaptations, there are often difficulties inferring function
from anatomical form. Although we see certain features of form that we speculate are
functional in preventing problems due to pressure, we rarely have the opportunity for
establishing definitive proof. There are many features that are suggested to be adaptations
for diving, such as the reinforced conducting airways, the rete mirabilia and bronchial
sphincters, but for which we can only speculate as to function.
While work with marine mammals under human care is controversial, access to animals that voluntarily participate in studies can allow us to perform experiments under
physiologically normal conditions that would be logistically and ethically difficult with
wild animals, and that may provide vital information for conservation efforts.
Chapter four: Pressure regulation
renal bubbles (Dennison et al. 2012). However, controlled studies in the bottlenose dolphin
(Houser et al. 2010) or Steller sea lion (Moore, unpub. obs.) have failed to replicate these findings. One possible reason could be that the dive history of wild-stranded animals is much
more extensive than those starting off with near sea surface blood and tissue PN 2 levels. The
development of technology to continuously monitor free ranging animals will be needed to
test if free-ranging animals do experience gas bubbles while foraging.
4.4 Future of this topic
It is difficult to formulate a list of the lingering questions for this topic, since there remain
so many gaps in our knowledge and understanding of how marine mammals cope with
repeated exposure to high pressure. This is largely because experimental research, at least
on obligate aquatic mammals diving to pressure has been a relatively intractable problem
(Kooyman 2006). This continues to be the case, although advances in electronics are beginning to allow the pursuit of new avenues. However, in trying to make more sense of this
field, it will be crucial to understand more about the phylogenetic variation in species
behavior and physiology. We need to be open-minded in terms of questioning some of the
assumptions, speculation, and hypotheses that have been suggested previously.
4.4.1 Phylogenetic variation
The vast majority of studies of the diving physiology of marine mammals have been done
on seals, with a handful of studies of trained bottlenose dolphins (Noren et al. 2004, 2012;
Ridgway and Howard 1979; Ridgway et al. 1969; Williams et al. 1993, 2015). An illustration
of the type of problem that can arise is oft-quoted wisdom that “seals dive on exhalation.” In fact, this was initially shown for some phocid seals, but studies of several otariid
seals suggest that these seals are inhalation divers (Hooker et al. 2005; Kooyman 1973;
McDonald and Ponganis 2012). Thus care is needed in assuming that particular species or
species groups are representative of an overall pattern.
Our knowledge of marine mammal adaptations to pressure remains extremely limited. We need a better understanding of both physiology and physiological plasticity particularly in terms of phylogenetic variability. Thus far, we have only a handful of studies
suggesting the depth of alveolar collapse, even fewer documenting changes in blood flow
with diving and yet these are crucial to our ability to build a framework to understand
changes in gas uptake that animals are exposed to during dives.
4.4.2 Form and function
Even in terms of more tractable studies such as the investigation of beach-cast animals
to better describe anatomical adaptations, there are often difficulties inferring function
from anatomical form. Although we see certain features of form that we speculate are
functional in preventing problems due to pressure, we rarely have the opportunity for
establishing definitive proof. There are many features that are suggested to be adaptations
for diving, such as the reinforced conducting airways, the rete mirabilia and bronchial
sphincters, but for which we can only speculate as to function.
While work with marine mammals under human care is controversial, access to animals that voluntarily participate in studies can allow us to perform experiments under
physiologically normal conditions that would be logistically and ethically difficult with
wild animals, and that may provide vital information for conservation efforts.