Are Dolphins Mammals?

Playful, agile, and intelligent, dolphins have both fascinated and confused people for thousands of years. As far back as 77 A.D., Roman author Pliny the Elder referred to dolphins and whales as fish in his “Natural History” book. While there is still some confusion about where dolphins fit in the taxonomy, dolphins are not fish. They are part of a group of marine mammals called cetaceans, which also include whales and porpoises.

There are around 40 dolphin species and they can be found in every ocean in the world. Although they breathe air, dolphins spend their entire lives in the water. It may seem contradictory, but dolphins have several characteristics that they share with land mammals. Keep reading to find out exactly why dolphins are mammals.

Dolphins Breathe Through Their Lungs

If there is a single defining feature that almost all fish share it’s gills. These respiratory organs enable them to filter oxygen from the water in order to breathe. However, dolphins breathe using their lungs like mammals. Dolphins can hold their breaths for several minutes at a time, but they usually need to breathe 4-5 times a minute.

The blowhole on its head is like a human’s nasal passage. This adaption makes it easier for them to breathe while in the water. The dolphin uses its blowhole to expel stale air upward and draw in fresh oxygen. Since dolphins breathe exclusively through their blowholes, they never have to worry about sucking water into their lungs as they swallow prey.

They Give Birth to Live Young

With the exception of monotremes like the platypus and the echidna, all mammals have a more developed placenta that supports live birth. This allows them to give birth to live young. So, the baby dolphin develops in the mother’s uterus. All dolphins are born with belly buttons marking where they were connected to their mothers via umbilical cord.

Dolphin pregnancies last between 10-18 months depending on species. Dolphin mothers give birth to one calf every 1-6 years. Unlike most mammals, however, the babies are delivered tail first to ensure they do not drown.

Dolphins Can Produce Milk

Like all mammals, female dolphins have mammary glands that produce milk for their young. Since the calves have no lips, they form a water-tight seal by rolling their tongues into a straw-like shape. This allows them to nurse while keeping out the seawater.

Most dolphins nurse their calves for 2-3 years. Because of the high-fat content, the milk helps the baby dolphin build up the blubber it needs to survive in the water. It also helps the calves to grow more quickly.

Calves Are Born with Hair

Mammals all grow hair at some point in their lives. Dolphin calves are born with hair protruding from their snouts. However, most dolphins lose these hairs quickly after birth and never grow more.

The Amazon River dolphin is an exception. It retains the stiff, whisker-like hairs into adulthood. Researchers believe these hairs serve as a sensory instrument to help the dolphin locate food while foraging in the river mud.

Dolphins Are Warm-Blooded

Dolphins are warm-blooded, or endothermic, as all mammals are. They have the ability to maintain their body temperatures internally, regardless of the outside temperature. Since they lose any hair they had shortly after birth, dolphins rely on a thick layer of blubber to help them stay warm.

Dolphins generally have higher metabolisms than land mammals of similar sizes. This means their bodies can generate more heat. The higher metabolic rate also helps them retain body heat. Warm-blooded animals are faster than cold-blooded animals, so dolphins have an advantage when hunting in cold water.

So, even though dolphins spend their lives in the ocean, they are not fish, they are mammals. As discussed above, they share important characteristics common to all mammals, despite their aquatic habitat.