Are Bats Mammals? The Only Mammals That Truly Fly

What makes a bat a mammal?

Scientists group animals by shared features inherited from common ancestors, not by what they do. Bats have all the key traits of mammals:

• Hair: bats are covered in fur, often soft and dense, which helps keep their small bodies warm.
• Milk: mothers have mammary glands and nurse their pups, usually from a pair of nipples near the armpits.
• Live birth: bats do not lay eggs. A pregnant female carries her pup inside her body, attached by a placenta.
• Mammal skeleton and teeth: bats have three tiny middle-ear bones, a single lower jawbone on each side and different types of teeth (incisors, canines, premolars and molars), as other mammals do.
• Warm blood: bats keep a high body temperature when active, although many lower it to save energy during the day or over winter.

A bat pup is unusually large at birth. In many small species, a newborn weighs around 20–30% of its mother’s weight, roughly like a woman giving birth to a toddler. Most bats have only one pup a year, which is why bat populations grow slowly.

How are bat wings built?

A bat’s wing is really a modified hand. The order name, Chiroptera, means “hand wing” in Greek. Four of the fingers are hugely lengthened, and a thin, elastic double layer of skin called the patagium stretches between them and back to the body and hind legs. The thumb stays short and free, with a claw used for climbing and gripping.

This design makes bat wings very flexible. A bat can change the shape of its wing with dozens of joints and muscles inside the membrane, which lets many species twist, hover and turn tightly in cluttered places. The skin also contains touch-sensitive hairs that help bats sense airflow.

You can see the long finger bones clearly in a bat such as the common pipistrelle when it spreads its wings against the light.

Bats vs birds

Bats and birds both fly, but they reached flight on separate evolutionary paths, a classic example of convergent evolution. The differences go deep:

For a full side-by-side guide, see bat vs bird.

Bats vs gliding mammals

Several other mammals travel through the air, but only bats truly fly. Flying squirrels, sugar gliders and other gliding possums, and the colugos of Southeast Asia have flaps of skin between their legs that let them glide from tree to tree. They can steer, but they always lose height and cannot climb or stay airborne by flapping.

Bats, by contrast, power their flight with strong chest muscles, take off from the ground or a perch, gain height and stay in the air for hours. Some, such as the Mexican free-tailed bat, fly high and fast over long distances each night.

How many kinds of bats are there?

There are more than 1,400 known species of bats, roughly a fifth of all mammal species, making Chiroptera the second-largest mammal order after rodents. They range from Kitti’s hog-nosed bat, which weighs about 2 g (0.07 oz), to large flying foxes with wingspans of around 1.5 m (5 ft).

Bats live on every continent except Antarctica. Most eat insects, but different species also feed on fruit, nectar, pollen, fish, frogs, small mammals and, in the case of three American vampire bats such as the common vampire bat, blood. Many small insect-eating bats, like the little brown bat of North America, find food by echolocation, while most fruit bats of the Old World rely mainly on sight and smell.

Where do bats come from?

The oldest well-preserved bat fossils are about 52 million years old and come from the Green River Formation in Wyoming. One of them, Onychonycteris finneyi, described in 2008, already had fully formed wings, showing that bats were flying very early in their history. Older ancestors without full wings have not yet been found, so exactly how flight began is still debated.

DNA studies show that bats are not related to rodents or primates as once thought. They belong to a large mammal group, called Laurasiatheria, that also includes carnivores, hoofed mammals, whales, hedgehogs and pangolins.

Common myths about bats

“Bats are flying mice.” Many languages call bats something like “flying mice”, for example Fledermaus in German, but bats are not rodents and are not closely related to mice.

“Bats are blind.” No bat is blind. Many have good night vision, and fruit bats see well. Echolocation is an extra sense, not a replacement for eyes.

“Bats are birds because they fly.” Flight alone does not decide what group an animal belongs to. Insects and some extinct reptiles also flew, but they are not birds either.