Diving Physiology and Anatomy of Marine Mammals

Since we first discovered how marine mammals survive, scientists have always been amazed at their bodies’ abilities that allow them to live in some of the ocean’s extreme conditions. Many have questioned: how do they hold their breath for so long? How do they withstand such extreme pressures? And, how on earth do some of them live in such icy water? People have dedicated their lives to study what is now known as the physiology of aquatic mammals — particularly while diving— and have discovered many built-in anatomical adaptations in their bodies that allow them to go to such extremes.

There aren’t many species that can thrive in temperatures that are near or below freezing. In the order pinnipedia, seals, otters, and walruses can often be seen living semi-aquatically on the snowy banks of Antarctica. They have adapted to its harsh conditions with waters as cold as -2°C (or 36°F) using their body’s innervated temperature-sensing nerve cells, which send signals to their brain to start conserving its body heat. Maintaining a core temperature of 37°C (or 99°F), they utilize blubber, or a built-in body tissue containing fat, collagen, and elastin that stores heat. “Newborn harbor porpoises pack the most; some 43 percent of their total body mass is blubber” (scientificamerican.com). Many young semi-aquatic pinnipeds, such as Harp seals, are born with fur. Certain species hold 13,000 hairs per square centimeter. That is as many as on a whole human head alone! The fur is so insulating that, in between the root and the tip of the fur, an extra layer of hot air is enveloped and heated by the animal’s body heat. In addition, Scientific American gives another factor, “One way of minimizing heat loss is to have a relatively low surface area–to-volume ratio: a small amount of skin—across which heat is exchanged with the environment—compared to a large volume of body tissue—which generates heat. Large animals tend to have lower surface area–to-volume ratios, and most marine mammals are pretty big.”

You may be asking, why do these animals even need to be diving in the first place? Well, for one reason and one reason alone: to get food. And a difficult way to eat, too: these animals dive in as deep as nearly 10,000 feet down. According to ScubaDiverLife.com, a 2014 experiment allowed scientists to name Cuvier's beaked whales the deepest diving animals (with the longest dive time) in history when they tracked one on a 2 ½ hour dive to 9,874 feet (2,992 meters). But how did this whale go so deeply into the earth and come back up alive? These whales exhale the majority of the air in their lungs before taking the plunge. This helps them become negatively buoyant (resistant to floating back up to the surface) and allows them to descend more quickly to deeper depths. This elimination of air pockets makes the whale’s body more resistant to crushing pressure levels — otherwise known as what is known called Boil’s law. When diving, a pinniped’s blood oxygen capacity is reduced due to the shrunken lungs. The nerves sends a signal to the brain, causing a release of hormones by the pituitary gland, that is chemically released into the blood and received by tissue receptors. Eventually, this causes a shunt of blood flow to all systems except for the necessary ones (turn off digestion, kidney processes, and liver function). Essentially, the trick for marine mammals when diving is to bring as much oxygen with you as you can to create what is called oxygen stores in the lungs, blood, and muscle. They allow their lungs to basically collapse so that they can dive, acquire food, then come back up using surfactants in their lungs, allowing them to reopen at the breaching atmosphere (fancy words for surfacing). Physiologically, for some species, aerobic metabolism is able to be conducted by their enhanced oxygen stores at considerably low levels for the duration of the dives. “For other species, there is a general consensus that they must either be reducing their aerobic metabolic rate when diving, possibly by way of regional hypothermia, and/or producing ATP, at least partly, by anaerobiosis and metabolizing the lactic acid when at the surface” (via ncbi.nlm.nih.gov). Some issues that come with deep diving includes extreme tissue nitrogen tension and oxygen poisoning. Additionally, decompression sickness (aka DCS), which was once thought to be only experienced by humans, has been recently found to result in gas bubbles and tissue damage under certain circumstances in marine mammals.

In addition to the factors stated, another challenge present is the issue of pressure. When diving deep, the animals are exposed to hydrostatic pressures in excess of 100 atmospheric differences than when at the surface. Many aquatic mammals can only go down so far — and when they pass their maximum level, their chest and lungs collapse, and they die. HPNS, or High Pressure Nervous Syndrome, can cause mammals to experience headache, tremors, and an increase in heart rate. In accordance with DCS, another name it goes by is The Bends, which occurs “when nitrogen gas, compressed in the bloodstream at depth, expands during ascent, causing pain, [fatigue, itching, choking], and sometimes death” (Phys.org). In the ocean, a change of pressure indicates a change of volume (but not a change of mass!). When a marine mammal is diving, the increasing pressure causes its air to compress to allow its total volume to shrink. These animals are blessed with some incredible abilities. The ocean is a HUGE place, and we are only just beginning to scratch the surface (haha!).

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