Why Are Beaver Teeth Orange? Iron-Strengthened Enamel

What makes beaver teeth orange?

All mammal teeth have a hard outer layer called enamel and a softer inner layer called dentin. In beavers and many other rodents, the enamel on the front face of the incisors contains iron. While the tooth forms, special cells deposit iron-rich material into the enamel, and this is what gives it the rusty orange color.

Scientists call this pigmented enamel. The iron is laid down in the last stage of enamel formation, so it sits in the outermost layer of the tooth, the part that does the cutting.

The color is not stain from food or a sign of disease. A healthy adult beaver’s incisors are bright orange. The color is strongest on the outer front surface of the tooth, while the back of the tooth is pale. The same feature shows up in other rodents, including rats, squirrels and the nutria, though the shade varies from pale yellow to deep orange.

How does iron make teeth stronger?

Human enamel is made mostly of tiny crystals of a calcium phosphate mineral, and it can be dissolved by acid. In rodents, studies of the enamel’s fine structure found that iron-rich material fills the spaces between these crystals. This makes the pigmented enamel harder and noticeably more resistant to acid attack than unpigmented enamel.

For a beaver, this is a big advantage. Its incisors take a beating every day as it cuts through wood, peels bark and chews tough plants. Tougher enamel means the cutting edge wears more slowly and is less likely to chip.

Self-sharpening teeth that never stop growing

Like all rodents, beavers have incisors that grow throughout their life. They have open roots, so new tooth material is added at the base all the time. If the beaver did not wear the teeth down by gnawing, they would grow too long.

The way the tooth is built keeps it sharp. The hard orange enamel covers only the front surface, while the back is made of softer dentin. As the beaver gnaws, the dentin wears away faster than the enamel, leaving a sharp, angled edge of enamel at the tip. The result is a self-sharpening chisel.

Behind the incisors is a wide gap, then the cheek teeth. A beaver has 20 teeth in all. The broad molars have folded ridges for grinding bark and plants. You can read more about the animal itself in our North American beaver profile.

How do beavers cut down trees with their teeth?

A beaver usually stands upright beside the trunk, propped on its broad tail. It grips the bark with its upper incisors as an anchor and chisels out chips of wood with the lower incisors, switching sides as it goes. Strong jaw muscles and a large skull give it a powerful bite.

Working around the trunk, the beaver carves the familiar hourglass shape, until the tree falls. A tree up to about 15 cm (6 in) across can come down in less than an hour, while thicker trees may take several nights. The beaver then cuts the branches into lengths it can carry or float. Some of this wood goes into the dam, as explained in why do beavers build dams?, and the inner bark becomes food, as covered in what do beavers eat?

Gnawing underwater

Beavers often gnaw and carry branches in the water. They can close their lips behind their incisors, so the teeth stick out while the mouth stays sealed. This lets them bite and carry sticks without water flooding into their mouths or lungs. Valves in the nose and ears close when they dive, and a clear third eyelid protects their eyes.

Myths and facts about beaver teeth

Myth: orange teeth mean a beaver is unhealthy. The opposite is true. Healthy adult beavers have strongly colored incisors.

Myth: beavers eat the wood they chew. They chew through wood to reach bark and to collect building material. The bark is the food.

Fact: the teeth are a lifelong tool kit. Because the incisors keep growing and keep sharpening themselves, a beaver can cut trees and peel bark every night of its life without the teeth wearing out.

Fact: both beaver species have orange teeth. The Eurasian beaver of Europe and Asia has the same iron-colored incisors as the North American beaver.