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Evolution of Marine Mammalian Respiratory Systems

Updated: Dec 4, 2023

INTRODUCTION

It’s a wonder how mammals such as dolphins, whales, seals, manatees, walruses, and sea lions are able to hold their breath for so long underwater. Unlike their fish and crustacean counterparts, marine mammals are not able to breathe underwater. But, mammals such as the Cuvier’s beaked whale and elephant seal are both able to stay underwater for over an hour at a time. Other mammals such as dolphins and killer whales can only hold their breaths for a little under thirty minutes, though their lung capacity is still impressive. (King, 2021) Their incredible abilities beg the question - were these oceanic mammals always in the water or did they evolve from something entirely different?


EVOLUTION

At first glance, it may seem that these marine mammals have always been accustomed to underwater life in the Earth’s oceans, though scientific research proves that they have rather evolved over the centuries from land ancestors. In addition, they do not all come from the same land mammals, as each group of marine mammals, such as cetaceans (dolphins and whales), pinnipeds (seals and walruses), and sirenians (dugongs and manatees), have descended from different mammals. (Jane, 2018)


  • The cetaceans originated 50 million years ago from hoofed ancestors in the order called Artiodactyla, similar to modern-day hippos, deer, cows, and elk.



  • The sirenians, such as the docile manatees and dugongs, evolved in the same time period as the cetaceans but are not related to them. They are more related to elephants and hyraxes, descending from wading herbivores.


  • The pinnipeds originated 25 million years ago, and are believed to have evolved from primitive bears. Therefore they are more related to modern bears, weasels, and otters.


CONVERGENT EVOLUTION

According to Prof. Masanori Kasahara, “Convergent evolution is defined as the process whereby distantly related organisms independently evolve similar traits to adapt to similar necessities.” (Kasahara, n.d., 7-36) This is a perfect explanation for the physical similarities between these three groups of marine mammals, as flippers, dorsal tails, and blubber are all signs of evolving under the same environments, not of evolving from the same ancestor.




RESPIRATORY SYSTEM/BREATHING

Although marine mammals can hold their breath for so long, at one point or another they’ll have to come to the surface or they’ll drown. Breathing is a step-by-step process, but even more so if you’re a whale about to dive 2,000 meters. You need to be sure that you’ll have enough in the tank to make it down and up with that hour-old air.

Upon inhalation, dolphins and whales suck air through their blowhole into their nasal duct, larynx, trachea, and then the lungs. Blowholes are the equivalent of nostrils on whales and dolphins. Baleen whales have two while toothed whales, such as dolphins, narwhals, belugas, and porpoises have one. The second blowhole on toothed whales has, like the marine mammals themselves, evolved.

It is now used for echolocation, a complex process used for communication and locating objects with sound. Blowholes sit on the top of these mammals’ heads, so it is only possible to realize that they have “moved” there through evolution. (Ashcroft, n.d.) With these marine mammals spending so much time underwater or at the surface, it’s simply more efficient to have their “nose” at the top of their head where it's closest to the surface.


Marine mammals such as dolphins and whales cannot breathe through their mouth as their breathing and digestion are kept entirely separate. This is to allow them to take in fish or krill and not choke on water in the process. (About Whales & Dolphins - How Do Whales and Dolphins Breathe?, n.d.) In fact, it’s said that their respiratory systems are 63% more efficient than humans due to their ability to dive deeper depths. (Learn About Dolphins & Whales, n.d.)

Next in the path of the air is the larynx, followed by the trachea, a muscle-lined tube that enters the lungs. Ironically, whales’ lungs are way smaller than those of land mammals. This is because their small lungs can minimize the tension of diving so deep into highly pressurized water.


HOLD YOUR BREATH!

The record for the most time spent holding their breath underwater was held by an elephant seal until a Cuvier’s beaked whale broke the record for a whopping 3 hours! The reason these whales and even dolphins can only stay underwater for max 15 minutes is due to the adaptation of storing oxygen in their blood and muscles instead of their lungs like land mammals. Proteins such as hemoglobin and myoglobin are incredibly prominent in these marine mammals and are the reason they can store so much oxygen in their blood and muscles. The darker the blood color, the more oxygen is prominent. This is why whales have super dark red, nearly black blood. But this adaptation isn’t the sole reason marine mammals, especially those who dive, can store much more oxygen than humans and land mammals. Of course, the lungs aid in this, and the difference in oxygen retention is a big cause for their breath-holding ability. But whales and other diving animals are also able to slow their heart rates and even stop blood from traveling to other organs, rerouting blood cells (essentially oxygen) to the muscles, heart, and brain.



References

About whales & dolphins - How do whales and dolphins breathe? (n.d.). Whale and Dolphin Conservation. Retrieved August 5, 2023, from https://us.whales.org/whales-dolphins/how-do-whales-and-dolphins-breathe/

Ashcroft, K. (n.d.). Respiratory System. Baleines en direct. Retrieved August 5, 2023, from https://baleinesendirect.org/en/discover/life-of-whales/physiology/respiratory-system/

Conry, D. (2018, January 17). THE ORIGINS OF MARINE MAMMALS. Ocean Blue Adventures. Retrieved August 5, 2023, from https://oceanadventures.co.za/origins-marine-mammals/

Kasahara, M. (n.d.). Genome Duplication and T Cell Immunity. Progress in Molecular Biology and Translational Science, 92(2010), 7-36. https://www.sciencedirect.com/science/article/abs/pii/S1877117310920024

King, L. (2021, February 22). What Animal Can Hold its Breath the Longest? The Big Zoo. Retrieved August 5, 2023, from https://thebigzoo.com/animal-hold-breath-longest/

Learn About Dolphins & Whales. (n.d.). Sea Life Park Hawaii. Retrieved August 5, 2023, from https://www.sealifeparkhawaii.com/conservation-n-education/animal-profiles/dolphins---whales



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