Acadia's North American Beaver: The Ultimate Keystone Species

Front pic of beaver in water
Front view of beaver (Castor canadensis)

NPS Photo

Keystone Species

The North American beaver is a keystone species, a unique organism that supports the entire biological community. The term “keystone” refers to a wedge-shaped block that forms the apex of a stone arch, the brick that holds the entire span in place. If you remove the keystone the arch collapses. The same can be true of a keystone species—their removal can contribute to an ecosystem collapse.

“To acknowledge that beaver create environments that store water and help sustain other creatures is insufficient. Beaver are nothing less than continent-scale forces of nature and in part responsible for sculpting the land upon which Americans built their communities.”
- Ben Goldfarb in Eager: The Surprising Secret Life of Beaver, 2018

Ecosystem Engineers

Sometimes referred to as “ecosystem engineers” of the forest, beaver continuously work to create new wetlands and ponds by building dams and lodges. They are incredible workers and prolific builders, carrying mud and stones with their fore-paws and timber between their teeth to build well-maintained dams. These dams turn small streams into ponds and lakes which provide excellent areas for other animals to use and thrive.

Beaver are indispensable creators of conditions that support entire ecological communities, and considering the effects of climate change, water stewardship may prove to be the beaver’s greatest gift to humankind. - Francis Backhouse in Once They Were Hats: In Search of the Mighty Beaver, 2018

itch hole pond-acadia
Beaver lodge at Witch-hole pond, Acadia National Park

NPS Photo

Beaver Habitats

Beaver habitats are recognizable in their characteristics:

  • Always found in riparian ecosystems near or in water, including streams, rivers and lakes.

  • Near areas with access to aspen, birch, poplar and willow used for dam and lodge-building materials and a vital food source.

  • Lodges may be surrounded by water or located partially on land and water and consist of several tree branches and mud that form a hard barrier that protects them from predators who try to penetrate the lodge.

  • Lodges have underwater entrances and are home to two adults and their offspring.

Beaver swimming
Beaver swimming in water

NPS Photo

Physical Characteristics

Beaver are the largest rodent in North America. To carry out highly functional tasks, the beaver possesses some very special and unique physical features and adaptations. They are well suited for environments found in the park and are nocturnal creatures that almost always work at night. To carry out highly functional tasks, the beaver possesses some very special and unique physical features and adaptations:

Body, Fur Coat, and Tail

  • Beaver have an average body size of two feet in length (without the tail) and weigh between 35 and 60 pounds.

  • They can hold their breath underwater for about 15 minutes, allowing for them to store food underwater, maintain their home in water, and escape from predators.

  • A two-layered fur coat consisting of an undercoat with fine, dense, grey soft hairs that cling together and a rough outer edge that helps mat the hairs of the undercoat together trapping air.

  • Brown color and shiny appearance from guard hairs spreading oil produced by oil glands.

  • Guard hairs that molt and produce a new layer as winter arrives, providing exceptional insulation and warmth during the winter.

  • A flat, broad, and scaly tail around a foot long used for balance, swimming, and creating noise to alarm others of approaching predators.

Front Paws and Hind Webbed Feet

  • Large webbed hind feet made for swimming and stability on land during tree cutting. Their feet contain special nerves that sense vibration from predators on land.

  • Front paws with five widely splayed toes that are hand-like, allowing them to grasp logs and branches and perform their work of manipulating objects with great dexterity.

beaver front view in water
Close-up, front view of beaver's head in water

NPS photo

Eyes, Ears, and Nose

Beaver have the following sensory adaptations to help with various underwater tasks:

  • Beady little eyes comprised of three eyelids that help protect their eyes from branches and twigs. They can see clear underwater but display poor eyesight on land.

  • Small ears with excellent ability to detect sounds from far away. While in the water, ears lay flat against the side of their head. A membrane of skin inside the ear closes forming a seal to prevent water from entering the ear canal.
  • Nose and ear valves shut to keep water out while underwater.
  • A highly developed sense of smell used for food selection and identification of family members within their colonies. Castoreum, a substance secreted from two castor glands at the base of their tail, is used to groom their fur coats and scent-mark territory. Family members share some of the same scent markers which is why beaver can recognize its own and strangers.
Beaver mouth and teeth while in water
Detail of a beaver's mouth and teeth

NPS Photo

Mouth, Lips, and Teeth

  • A mouth with a special flap that closes while submerged and lips that close behind over sized front teeth, allowing it to transport building materials and food in water without drowning.
  • 20 teeth comprised of 16 molars used for grinding and four large, orange-colored (iron-enriched) incisors in the front of their mouth to chew and cut down trees and bushes. The four front incisors continuously grow throughout its life, so they are constantly chewing to sharpen and prevent overgrowth.

Eating Habits and Food Sources

Beaver are herbivores and prefer wood from trees like aspens, willow, and cottonwood. Other food sources include aquatic plants like pondweed, water lilies, grasses, algae, moss, various fruits and berries. Since beaver do not hibernate, they cut and store large quantities of tree bark and roots in their homes to sustain them during the winter months when food is scarce.

beaver parent and kit close encounter in water
beaver parent and kit close encounter in water

NPS Photo/Kent Miller

Social and Mating Activities

At Acadia National Park, beaver are considered one of its busiest residents. The phrase “busy as a beaver” is well-known and an appropriate title given to beaver for its work ethic. Beaver work independently and tend to have little contact with each other while working.

Beaver are very social animals that live in a large family groups called colonies. Families are comprised of an adult male and adult female in a monogamous pair with offspring called kits. Beaver pairs mate for life; however, if a beaver’s mate dies, it will partner with another one.

Mating generally occurs in the water from January to March with gestation averaging 120 days. Females usually give birth between May and June, producing two to six kits weighing an average of 8 to 24 ounces. Kits are early swimmers from day one and usually remain with their parents for two years. Offspring begin mating at two to three years old. The lifespan of beavers in the wild is between 16 and 24 years.


Backhouse, F. (2018) Once They Were Hats: In search of the Mighty Beaver, ECW Press, Toronto, Canada

Beaver Institute, Inc.

Goldfarb, B. (2018) Eager: The Surprising Secret Life of Beavers and Why They Matter, Chelsea Green Publishing, White River Junction, VT

Morgan, L. H. (1868), The American Beaver and His Works, J.B. Lippincott & Company, Philadelphia

Strong, P. (1997) Where Waters Run Beavers, NorthWord Press, Inc. Minocqua, WI

Acadia National Park

Last updated: October 6, 2021