American beaver
North America's largest rodent, a keystone species whose dams reshape whole streams at Yellowstone, Glacier, and ten other parks along the route.
American beaver at a glance
Castor canadensis BEE-ver; Castor canadensis is KAS-tor kan-uh-DEN-sis
- Type
- Mammal · rodents
- Diet
- Herbivore
- Size
- 35 to 40 inches long including the tail, 30 to 60 pounds — about as long as a baseball bat and roughly as heavy as a full-grown Labrador retriever.
- Lifespan
- Averages around 5 years in the wild.
- Conservation
- Least concern
Eats willow and aspen bark and twigs, pond lilies and other water plants, conifer bark when little else is available
A beaver gnaws down willow and aspen for both food and building material, felling trees and hauling the branches to its dam and lodge.
Should you worry?
Beavers are shy and would rather swim off than confront anyone. Watch dams and lodges from the bank, and keep dogs leashed near the water's edge.
At dusk, the flat water below a beaver dam starts to move against the current — a slow V-shaped ripple that means dinner is about to start gnawing. Beavers are crepuscular, busiest at dawn and dusk, and North America’s largest rodent, up to 60 pounds and 40 inches long counting the flat, paddle-shaped tail.
What the dam actually does is the real trick. Felling willow and aspen floods the streambank behind it into a pond, and that flooded margin is exactly where more willow and aspen grow back in thick, which is exactly what the beaver eats and exactly what it builds its lodge and dam from next. The pond you’re looking at was made by the same animal now feeding off what the pond grew — a loop with no end, run by an animal living in a family colony of about six related beavers, one of the few mammals anywhere that organizes its life that way.
The loop can break down and start over. Yellowstone’s northern-range beaver colonies bottomed out at just 8 in a 1953 survey, after decades of low water and heavy grazing on the willow they needed, then climbed back to 127 by 2007 as the streambanks recovered enough to hold beaver-built water again. The current estimate still hovers around that number, one gnawed cottonwood and one rebuilt dam at a time.
Did you know?
- Yellowstone's northern-range beaver colonies bottomed out at just 8 in a 1953 survey, then climbed back to 127 by 2007 as habitat recovered.
- A beaver colony usually has about six beavers, all related, living and denning together — fewer than 5% of mammal species organize their families this way.
- Beavers are crepuscular, most active at dawn and dusk, which is also when their dams and lodges get built or repaired.
Common questions
Why are beavers called a "keystone species"?
Because their dams flood streambanks into ponds, which grows the willow and aspen the beaver eats and builds with, and that same pond habitat supports fish, birds, and other wildlife too.
What do beavers eat?
Willow and aspen bark and twigs mostly, plus water plants like pond lilies, and conifer bark when nothing better is around.
How big do beavers get?
35 to 40 inches long including the tail, and 30 to 60 pounds — North America's largest rodent.
How long do beavers live?
Around 5 years on average in the wild, in a family colony of about six related beavers.
Where can you see a beaver dam in a national park?
Yellowstone's southeast, southwest, and northwest corners all have active colonies. Look for a dam or lodge along a slow stream at dawn or dusk.