The mammalian diving reflex is a fantastic biological adaptation intrinsic not only to marine mammals but also to land mammals, including humans, whose ancestors once lived in the ocean. I am currently editing my wildlife book and was once again awed by how deep some marine mammals dive and how long they can stay below the surface without breathing. I think the mammalian diving reflex represents one of nature’s most incredible adjustments for air-breathing mammals required to find food in an inhospitable environment void of air.
What is the mammalian diving reflex? Biologists mostly have studied the reflex in harbor seals, so I will use a seal to explain the elements of the physiological changes. A harbor seal can dive as deep as 1640 ft. (500 m) and stay submerged for over twenty minutes. When it dives, a harbor seal’s heart rate slows from its normal rate between 75 to 120 beats per minute down to just four to six beats per minute. Blood shunts from peripheral tissues tolerant to low oxygen levels and flows to the heart, brain, and tissues dependent on a constant supply of oxygen to survive. These adaptations allow the seal to conserve oxygen while it dives and searches for food.
Seals utilize additional adaptations to conserve oxygen and withstand the rigors of increased pressure when they dive. Before a deep dive, seals exhale several times to collapse their lungs, and they then store their oxygen in blood and muscle tissues instead of in the lungs. Harbor seals have a proportionately higher blood volume than land mammals of the same size, and seals also possess ten times more myoglobin than humans. This oxygen-binding protein helps prevent muscle oxygen deficiency.
Researchers originally believed the diving reflex was an automatic response triggered by breath-holding and submergence in cool water. In recent studies, though, scientists attached a device similar to a Fitbit to harbor seals. The device records blood flow and oxygen levels in the seal’s brain, and the study produced some interesting results. Seals can control their diving reflex. Seals contract their peripheral blood vessels beginning 15 to 45 seconds before they dive, and they restore normal blood flow to their blubber several seconds before they reach the surface. When seals are feeding, they return to the surface to breathe but often don’t stay there long enough to restore normal oxygen levels. Researchers also learned that seals slow their heart rates more if they plan to stay underwater longer.
Biologists hope to learn if other animals, including humans, can also consciously control their dive reflex. The world’s top freedivers can descend to a depth of 426 ft. (130 meters) and return to the surface, and the record for breath-holding without moving tops 11 minutes. Are these free divers able to control the physiology of the diving reflex to accomplish these incredible feats?
The next time you see a harbor seal, pause for a moment to consider the rigors this animal must endure just to eat dinner.
I hope you are well and navigating our changed world. Life remains quiet here in the wilderness of Kodiak Island, and we feel oddly removed from the biological havoc wreaked by this virus. Even here, though, we have been touched by the economic disaster the world faces. I look forward to better times for all of us soon! Take care.
Join the Battle of the Books contest, and you could win a $500 Amazon Gift Card! I am very excited to have my novel, Karluk Bones, included in this contest.
Robin Barefield is the author of four Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter, and Karluk Bones. You are invited to watch her webinar about how she became an author and why she writes Alaska wilderness mysteries. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska, and listen to her podcast, Murder
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