Marine mammals are distinguished by adaptations of their body to a saline environment in which the water is relatively cool and usually uniform in temperature (Scheffer 1952). Further, the diving physiology of marine vertebrate are determined by the “dual constraints of maintaining body function in the absence of access to oxygen and avoiding physiological trauma caused by rapid changes in pressure” (Hooker 2005). The main adaptations developed by pinnipeds include:
- Adaptations to a fluid medium (body physiology, oxygen consumption)
- Adaptations for temperature control (blubber)
- Adaptations to a saline medium (maintain body salt concentrations at a level scarcely higher than terrestrial mammals)
One of the first basic adaptations required for pinniped life in an aquatic environment was flippers! Flippers are believed to have come from limbs with true seals having more developed hind flippers and eared seals having more developed flippers. The walrus is the intermediate between the two. (Wikipedia)
The next adaptation required for pinnipeds survival in water was eyesight. Seals eyes are well adapted for seeing both above and below the water. More specifically, when diving the animal has a clear membrane that covers and protects its eyes. Also, its nostrils close automatically and its testicles and mammary glands are located in slits under the skin to help keep the seals streamlined shape helping it to swim with less friction. Seals have also developed whiskers that help them navigate and find food. Their whiskers work by the attached sensors in their skull that absorb sounds underwater and transmit them to the cochlea. (Wikipedia)
Swimming is energetically expensive for mammals and results in transport costs that are 2 to 23 times the levels predicted for fish (Williams 2000). To reduce these costs, marine mammals have developed a wide variety of energy-conserving behaviors. These behaviors include: narrow range of routine transit speeds, wave-riding, and porpoising in order to decrease the amount of energy expended when pinnipeds move near the water surface. Unfortunately, only one of these energy-conserving strategies is beneficial while underwater, cost-effective routine speeds while diving (Williams 2000).
Diving mammals such as seals and whales have adaptations that work off of a primarily fat-based metabolism when not diving. It is fat-based because many aquatic mammals use blubber as thermal insulation and therefore may have been pre-adapted for fat utilization. Since many marine mammals need to sustain long migrations, fat is the primary choice for fuel due to its high energetic content as an efficient fuel source. (Hochachka 1975) Blubber also provides buoyancy that helps in seal swimming and diving techniques.
Blubber as an adaptation for thermal insulation is just one of many characteristics that help pinnipeds survive in their unique aqueous environment. In fact, diving response which includes: apnea, bradycardia, peripheral vasoconstriction, and redistribution of cardiac output is another component of pinniped biological adaptations that are essential to pinniped success during deep-sea diving (Mottishaw 1999). Further, pinnipeds have an elevated mitochondrial volume density and elevated “citrate synthase and beta-hydroxyacylc-CoA dehydrogenase activities in their swimming muscles to maintain an aerobic, fat-based metabolism during diving” (Fuson 2003).
More specifically, pinnipeds have increased myoglobin levels in muscles and an increased blood volume, with a higher concentration of hemoglobin that is found in terrestrial mammals (Bron 1966). However, even these relatively great oxygen stores would be insufficient for extended dives if these mammals were not capable of making circulatory adjustments to conserve the available oxygen for the central nervous system and heart. These adaptations allow marine mammals to maintain greater oxygen concentrations in a given mass than a terrestrial mammal (Bron 1966). Further, pinnipeds are able to direct the oxygen to particular organs in the body during a prolonged dive so that those organs with the highest metabolic requirement are satisfied first (Bron 1966).
It is within the organs such as the liver, kidneys, and stomach that a heightened ability for aerobic, fat-based metabolism during hypoxia can be associated with routine diving. However, a “heightened lactate dehydrogenase activity in the heart and liver indicates an adaptation for the anaerobic production of ATP on dives that exceed the animal’s aerobic dive limit” (Fuson 2003). In result, the heart, liver, kidneys and gastrointestinal organs of seals do have adaptations that promote a aerobic, fat-based metabolism under hypoxic conditions. However, these organs can also provide ATP anaerobically if needed. Nevertheless, most pinnipeds maintain aerobic metabolism during most free-ranging dives (Fuson 2003).
When a seal begins its deep-sea dive they:
- Take a breath above the ocean's surface
- Hold their breath (apnea) right as they dive into the water
- Then as the seal gets deeper, they undergo bradycardia which involved a fall in cardiac output (Rowell 1986). In fact, when an animal starts diving its heart rate slows to about one-tenth of the usual speed (Microsoft 2007).
- Soon after, their arteries squeeze and development of extensive peripheral vasoconstriction occurs leading to a limit in blood supply to the peripheral tissues (Hochachka 1975).
Figure 1 provided by http://scicom.ucsc.edu/scinotes/9701/full/features/seal/dive.html with permission
As shown in Figure 1, the process of seal diving and all of their deep-sea diving adaptations allow for available oxygen to be conserved for use by tissues having an obligatory oxygen requirement. Although the degree of bradycardia and peripheral vasoconstriction may vary with the dive duration or level of exertion, “all organs and tissues experience a reduction in convection oxygen delivery resulting from both hypoxic (decrease in O2 supply without a decrease in blood flow) and ischemic hypoxia (blood flow is reduced or stopped)” (Fuson 2003) this includes the heart, kidneys, and splanchnic organs.
Some research even suggests that marine mammals have increased spleen mass, blood volume, and red blood cell mass that helps contribute to their ability to successful dive for extended periods of time (Mottishaw 1999). Additionally, diving mammals that descend to depths of 50-70 meters or greater fully collapse the gas exchanging portions of their lungs and then re-expand these areas when they ascend (Spragg 2004). It is said then that one might expect that adapted surfactant function, which allows for rapid expansion of collapsed alveoli, might be found in those seals diving below 50 meters or more.
Now, some of you might be wondering…what about decompression sickness? Decompression sickness is caused when elevated levels of dissolved nitrogen in the blood and tissues expand during ascent, causing bubbles and the formation of emboli. Shallow-water blackout is caused by depletion of blood oxygen often associated with declining partial pressure of lung oxygen during ascent (Hooker 2005). In order to avoid decompression sickness, seals have developed a way in which they can prevent gas exchange. Through their adaptation of a strengthened airway they can exhale before diving which helps to promote alveolar collapse at relatively shallow depths (30-50 meters) (Hooker 2005). Because alveolar collapse happens early on it helps seals avoid decompression sickness and shallow-water blackouts. Further, alveolar collapse at these depths allows seals to exploit their lung oxygen storage while diving, as “increase partial pressure increase gas absorption and partly counteract the pulmonary shunt caused by reduction in alveolar surface area” (Hooker 2005).
Figure 2 provided for free use for student audiences and their educators by http://www.afsc.noaa.gov/nmml/education/science/studymmsealexample.php
The above figure shows the diving behavior of a male elephant seal. As you can notice, the diving pattern is repetitive showing that without these adaptations that prevent decompression sickness and allow for long dives, seals would not be able to survive. Take this elephant seal depicted in Figure 2 for example. The dives seem to range from 20 to 40 minutes and all include a very rapid descent that most mammals would not survive.
If all these adaptations already mentioned were not enough, seals have even more! Most seals are known to also consistently exhale during the last 50-85% of their ascent from all dives whether shallow or deep (Sato 2002). In fact, even the inhaled diving lung volume is independent of diving depth and in turn a constant amount (Sato et al. 2002). Even at shallow dive depths bubbles may be released before ascent, whereas during deep dives there is always a period of ascent before bubble release. This suggests that there “is some form of pressure threshold before which venting begins, possibly related to the need for higher buccal air pressure than ambient water pressure prior to opening the mouth” (Hooker 2005).
Further, it has been found that the thrusting intensity in the final phase of ascent was greater for dives in which ascent exhalation began at a greater depth, “suggesting an energetic cost to this behavior, probably as a result of loss of buoyancy from reduced lung volume” (Hooker 2005). Prolonged gliding (greater than 78% descent duration) occurs during dives exceeding 80 meters n depth (Williams 2000). Along with the thrusting, gliding was attributed to buoyancy changes with lung compression at depth. By modifiying their locomotor activity, seals have developed a way in which they can reduce the energetic cost of diving. This energy-conserving strategy along with the many others previously mentioned allows pinnipeds to increase aerobic dive duration and achieve remarkable depths despite limited oxygen availability while submerged (Williams 2000).