Intermediate 8 minutes
Marine mammals: returning to the sea without giving up breathing air
Cetaceans, sirenians, and pinnipeds, and the adaptations that let them dive like no human ever could
About fifty million years ago, the ancestors of whales walked on four legs along the shore. They went back into the water and stayed there, but they never got their gills back: every breath of every whale on the planet still happens at the surface, just like yours.
In this article you'll learn about the major groups of marine mammals, the physiological adaptations that let them dive to depths impossible for us, and the guidelines for observing them responsibly when you cross paths with one.
Marine mammals are the encounter that stays with every diver the longest. They're also protected, sensitive animals: knowing how to behave around one determines whether the encounter is a shared privilege or a disturbance for the animal.
Who they are: the major groups
All marine mammals share the same traits that define the class: they breathe air with lungs, they're warm-blooded, and the females feed their young with milk.
Cetaceans are the group most fully adapted to the sea: they spend their whole lives in the water and can't even come out of it. They split into two major branches. Mysticetes, or baleen whales, filter enormous amounts of water through baleen plates to catch krill and small fish; the humpback whale and the blue whale belong to this branch. Odontocetes, or toothed cetaceans, hunt individual prey: dolphins, orcas, sperm whales, and porpoises.
Pinnipeds (seals, sea lions, and walruses) keep one foot in each world: they hunt in the water but need to come out onto land or ice to rest and breed. The simplest way to tell a seal from a sea lion is to look at its ears and how it moves out of the water: sea lions have visible external ears and can rotate their hind flippers forward to walk, while true seals lack external ears and drag themselves along.
Sirenians, manatees and dugongs, are the only group of herbivorous marine mammals: they graze on seagrass meadows, the same meadows you saw in the article on ecosystems. They're slow, peaceful, and live in shallow, warm waters, which makes them especially vulnerable to boat propellers.
The group is rounded out by a few special cases, such as sea otters and the polar bear, which depends entirely on sea ice to hunt.
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