Scientists Say: Mammal

Mammal (noun, “MAM-mal”)

Mammals are a diverse group of animals. They include about 6,600 known, living species. Many of them, including humans, cats and dogs, live on land. Others, such as dolphins and whales, live underwater. And some — bats — even fly.

Mammals tend to share a few distinguishing traits. All are vertebrates. That means they have a backbone. Almost all have fur or hair, even if it is just a tiny bit. Most mammals give birth to live young. And mother mammals produce milk to feed their babies.

Some exceptions to these rules do exist. Whales, for instance, are mostly hairless. This helps them swim faster. Even so, many whales have whiskerlike hairs on their chins or heads to detect their surroundings.

Biologists sort mammals into three groups based on how they reproduce. Those groups are placentals, marsupials and monotremes.

Most mammals are placentals. These mammals give birth to live, somewhat developed, young. Humans are placental mammals.

Marsupials come with built-in pouches and give birth to live, less-developed young. Kangaroos, for example, give birth to babies about the size of jellybeans. Infant kangaroos then clamber into their mother’s pouch as a safe harbor to grow. Most marsupials live in Australia. North America’s only native marsupial is the Virginia opossum.

Monotremes are a weird bunch that break some rules of being a mammal. They do not give birth to live young. Instead, they lay eggs. Only five species of these mammals exist today: the duck-billed platypus and four kinds of echidnas.

In a sentence

The urine from marine mammals can help disperse nitrogen and other nutrients throughout the ocean.

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