What's in a Swim? Part 1

Marine tetrapods swim in incredibly diverse ways. This is because the land ancestors of these marine animals were also diverse in terrestrial form and function, and also diverse in the marine ecologies and environments they evolved to occupy. A large gentle filter feeding whale swims differently than a sea lion in pursuit of fast moving fish, which swims differently than a leopard seal chasing penguins. A manatee grazing along the bottom for shoal grass swims differently than a plesiosaur with its unique highly elongate neck as part of is aquatic form, which swims differently than a sea turtle which has a shell and lays its eggs on the shore. To me, the most amazing thing about all of these examples and more is that while all of these marine tetrapods are different in form and swimming function, there is one thing they all did the same and that's transition to the water from a terrestrial environment.

So all of these once terrestrial groups of animals evolved to swim once more, returning to the water, the environment where all tetrapods initially emerged from. I'm here to talk about how these animals do that from a functional perspective. This post is Part 1 in a series of how marine tetrapods swim and will start with marine mammals.

Modern marine mammals' swimming behavior is based on movements of either their flippers/paddles or their caudal flukes. Lateral pelvic oscillation is the type of swimming seen in true seals (phocids) and is done by rapidly alternating their hind flipper position side to side like a biological propeller. Pectoral oscillation is utilized by eared seals (otariids) where they flap their forelimbs and "fly" through the water. Walruses fall slightly in between both true and eared seals with regards to swimming function mainly using pelvic oscillation, but they do also rely on their forelimbs a little bit more heavily compared to true seals. These different types of limb-based swimming methods are reflected by the terrestrial postures of these animals. Eared seals have an active terrestrial posture with their weight mainly supported by their flexible limbs. Their hind limbs can face forward and they can even run on land.

True seals on the other hand, in all their blubbery glory often look like this on land:

Why? True seals actually cannot rotate their hindlimbs forward and so their body is entirely supported by their bellies and forelimbs. A consequence/trade-off of swimming via lateral pelvic oscillation. Walruses can face their hind limbs forward too, but not to the same degree as eared seals and are still primarily supported by their bellies on land.

Cetaceans (whales and dolphins) and sirenians (manatees and sea cows) swim via vertical caudal movement of their laterally expanded tails, or flukes, driven by the muscles which support the vertebral column. Modern cetaceans specifically swim via a locomotor function called caudal oscillation and they have an important soft tissue structure which enables this type of aquatic locomotion called the Subdermal Connective Tissue Sheath (SCTS) which is a strong network of collagen that maintains rigidity in the lumbar (lower back) and caudal (tail) vertebral column. It also provides more surface area for the important muscle attachments that alternate in contraction to power a rigid tail and fluke through the dense medium of water. The tail flukes are the most important part of whale locomotion and while most are convex at the trailing edge (the backmost edge of the fluke where it meets the water), they are differently shaped depending on the ecology of the animal and their hydrodynamic needs. All flukes are made up of a cutaneous layer, thin blubber layer, ligamentous layer, and a very dense fibrous material which keep the fluke rigid.

Whales also swim with the assistance of a dorsal fin, a unique convergent evolutionary feature some marine tetrapods share with highly specialized marine vertebrates like fish and sharks. The dorsal fin provides hydrodynamic stability and balance, as well as recognition within and outside of a species, and thermoregulation. The last component of a whale's swimming functions are their forelimbs. Whale forelimbs are reduced and the external paddle that you see is just the forearm and hand, no elbow, which restricts the range of motion of the forelimb. This is also seen in marine reptiles, but more on that later! The paddles in whales help provide maneuverability, support, and balance, but not nearly as much of the aquatic thrusting strength that comes from the caudal oscillation.

The evolution of how whales arrived at full caudal oscillation is awesome because we actually have great fossils which show the path from their terrestrial ancestors to now. The semi-aquatic ancestors of whales likely utilized some type of fore and hindlimb padding, then transitioned to pelvic paddling similar to sea otters and seals. As limbs reduced and the tail elongated, they began to swim via caudal undulation which is a more fluid type of alternating movement with multiple waves present in the motion. Finally, as the tails further elongated with fluke expansion, hindlimbs becoming completely reduced, and their bodies because more rigid and hydrodynamic, whales evolved the caudal oscillation that we see today.

Onto manatees! When directly compared to whales, even though manatees use their tail flukes in a superficially similar up and down fashion, they are actually quite "poor" swimmers in the sense that they are incapable of reaching fast speeds. However, the hydrodynamic function of a manatee is reflected by its ecology and based on a 2007 paper in the Journal of Experimental Biology, manatees, despite using caudal undulation and a smaller rounded fluke as compared to cetaceans, actually swim highly efficiently, but with lower power outputs compared to caudal oscillation of the larger aspect ratio flukes in cetaceans (Kojeszewski & Fish, 2007). This makes sense because there are few natural predators of manatees and they aren't exactly chasing after mobile prey. They do however spend a lot of time migrating to new places to graze. So they don't have to swim fast, just easily. A plump, slow moving, and sea grass filled aquatic existence is all a manatee needs.

Pinnipeds, cetaceans, and sirenians are the primary marine mammals people think about because of their derived (more different from their terrestrial ancestors) form, but sea otters, in the mustelid family, and even polar bears are considered marine mammals. Sea otters have a unique marine morphology where their hindlimbs are far larger than their forelimbs. This is what the use to push through the water. Sea otters use pelvic paddling on the surface and pelvic undulation underwater. They are slow on land, moving with the back half of their body higher than the front. Polar bears, the most terrestrial in form out of all marine mammals, swim in a crawling motion where they use their forelimbs to pull water towards them and thrust through forward with minimal action from the hindlimbs which are mostly used for maneuvering.

So there you have it! This read is a broad summary of the varied aquatic locomotor functions of the marine mammal groups so many of us love and adore. Stayed tuned for "What's in a Swim, Part 2" where I'll be delving into the different swimming mechanics of marine reptiles!

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Sources

Fish FE. 1996. American Zoology. Science.

Kojeszewski and Fish, F.E. 2007. Journal of Experimental Biology.

Pysenson ND et al.. 2014. Palaeogeography, Palaeoclimatology, Palaeoecology.

Thewissen JGM, Fish FE. 1997. Paleobiology.

Turner JP. 2009. Marine Mammal Locomotion (Lecture Slides).

Uhen M. 2007. The Anatomical Record.

Williams TM. 1999.