What defines a marine mammal?

Marine mammals are found in marine ecosystems around the globe. They are a diverse group of mammals with unique physical adaptations that allow them to thrive in the marine environment with extreme temperatures, depths, pressure, and darkness.

What defines a marine mammal?

  • They all breathe air through lungs
  • They are all warm blooded
  • They all have hair (at some point in their life)
  • They all produce milk to nurse their young


Marine mammals represent a variety of ecological roles, including herbivores (manatees), filter feeders (baleen whales), and top predators (killer whales). Mammals evolved on land around 160 million years ago. Each taxonomic marine mammal group evolved from a different group of land mammals, whose ancestors separately ventured back into the ocean environment. Despite these different origins, many marine mammals evolved similar features — streamlined bodies,  paddle-like limbs and tails. 

Marine mammals are classified into four different taxonomic groups (a group of organisms within a species that differ in trivial ways from similar groups).

  1. Cetaceans (whales, dolphins, and porpoises), 
  2. Pinnipeds (seals, sea lions, and walruses), 
  3. Sirenians (manatees and dugongs), and 
  4. Marine Fissipeds (polar bears and sea otters).


Cetaceans are categorized into two main groups: 

  • Baleen whales (mysticetes) 
  • Toothed whales (odontocetes). 

They spend their entire lives in water, and have many adaptations to their aquatic lifestyle. There are over 70 different species of cetacean. 



Pinnipeds are in the suborder pinnipedia, which means “fin-footed.” These carnivores use flippers to move both on land and in the water. Pinnipeds spend the majority of their lives swimming and eating in water and come onto land or ice floes to bear their young, rest, and molt. This group includes seals, sea lions and walruses. Millions of years ago, the ancestors of pinnipeds lived on land.



Like cetaceans, sirenians also spend their whole lives in water. They are the only entirely herbivorous group of marine mammals. Sirenians are named for sirens, the legendary Greek sea beauties who lured sailors into the sea. Some think historic mermaid sightings were actually sirenians, not the mythical half women, half fish.


Marine Fissipeds 

The marine fissipeds are considered marine mammals, but spend most of their time on land and only part of the time in the water, mainly to hunt for their food. They include the Polar Bear and Sea Otter. 

There is also a class of land based fissipeds, which include animals like racoons, dogs and cats.

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