Beavers gnawed trees at the Clark Street Beach Bird Sanctuary, Photo by Libby Hill

Beavers live at the interface of land and water where there is sufficient woody vegetation for their homes, lifestyle and food. The North American beaver, Castor canadensis, is the second-largest rodent in the world (the largest is the capybara, native to South America).

Beavers can be at loggerheads with humans, despite their benefit of creating wetland habitats that support a diversity of wildlife. Damming a stream creates a pond that provides them access to trees to fell, while simultaneously providing them a protected water avenue.  They harvest trees near water when they can, but will travel as much as 100 feet from shore as long as they do not have to travel too far on open ground, where they are unprotected from predators. 

Although we humans are predisposed to having things our way and often do not appreciate the flooding ponds and loss of trees, we marvel at beavers’ engineering feats and industry.  How do they do it?

Teeth

Beavers must chew to keep their front teeth from growing too long. Their large incisors that we see, two on top, two on the bottom, have enamel fronts that include iron rather than calcium, explaining their orange color. The back side is dentin. Beaver teeth are sharp, because the softer dentin wears away faster, forming an angle on the teeth and creating very sharp instruments for chiseling wood.  Beavers are vegetarians; their diet is heavy on hard-to-digest cellulose.

Larry Heaney, a mammalogist at the Field Museum, further explains: “Beavers have three molars on each side, upper and lower.  There is a long gap between the incisors and the molars. These molars are very sturdy, with lots of ridges of hard enamel that allow beavers to chew up bark and leaves into finely ground pulp that they can then digest, with the help of lots and lots of bacteria in their gut (they have big pot-bellies).”

The Body of a Swimmer

Beavers are uniquely adapted to aquatic life. Human swimmers often use wet suits, goggles, and earplugs for protection and have to learn breathing techniques. 

Beavers have all those built in.  When they hit the water, folds of skin close over their nostrils and ears. Nictitating membranes that are nearly transparent close over their eyes so they can see underwater.  Fur lining on their lips behind those large incisors blocks water from entering their mouth and lungs.  Their short, soft underfur, covered by bristly guard hairs, insulates them and keeps them dry. They waterproof their fur by grooming it with castoreum oil, produced in glands near the base of their tail. They spread the oil using the claws on their front feet and the “grooming” claws on their hind feet.

They groom their fur at least once a day, and they often groom each other. These adaptations allow them to stay submerged for up to 15 minutes, as opposed to humans who can typically stay underwater for at most two minutes.  Their unique scaly, long, flat tail acts as a rudder, steering them as they swim.  The beaver’s tail is also used to slap the water as a warning signal, to prop them up as they are gnawing at a tree, and to store fat for the winter.

Home and Family

Beavers’ well-insulated conical lodges are constructed of mud and sticks and have at least one water-filled tunnel entrance. Lodges have a ventilation hole at the top for fresh air. They have a generous social chamber for eating, sleeping and grooming, and a nursery, all above water and kept clean by regularly changing the bedding of grasses, reeds and wood chips.  Their heating system is their own body warmth, which keeps the lodge above freezing in winter.  (The beavers that live along the North Shore Channel construct “bank” lodges built into the sides of the channel, usually underneath the roots of a large tree.)

A typical beaver colony averages four to eight beavers, including the monogamous parents, the kits from the current year, and some from the previous brood.  They breed in January or February and the kits are born in spring. There are typically two or three kits; some sources report as many as five.

Young beavers usually leave the colony at age 2 and can reproduce at age 3. Beavers can live in the wild for 15 years or more; sources vary.

Basic facts about beaver life can only be generalized. Geography, location and habitat quality differ. The same can be said for humans: There are ballpark or typical figures.

Some examples:

  • ·        Beavers are considered monagamous, but research on some Illinois beaver DNA suggests they may take an occasional fling.
  • ·        Gestation period is likewise variable; it is around 106 days, more or less.
  • ·        Beaver young play and learn, but how soon after birth can they swim? It may be around 24 hours, but they may not head to water for two to three months.  
  • ·        Beavers are usually nocturnal but may be active during the day when it suits them.
  • ·        The maximum distance they will waddle on land or swim from the lodge for food varies by need and conditions.

·        Beaver lodges will be deserted when the food supply diminishes beyond sustainable levels, only to be reoccupied at some later date, perhaps when the habitat recovers.

What Defines “Sustainable”?

The year 2020 brought, relentlessly, the pandemic, and then the presidential election. But first, it brought a record-high Lake Michigan lake level and fierce waves.  In a wild January storm, Lake Michigan invaded the Clark Street Beach Bird Sanctuary (CSBBS) and knocked down the lakeside fence. 

Almost immediately, the beavers, which have lived in the Northwestern lagoon off and on for 25 years, took advantage of the high water.  They swam under the bridge, around the Sailing Center, and took down several cottonwoods in what had been CSBBS. 

City staff rebuilt the fence farther from the lake. 

Twice. 

No more beaver activity.

Temporarily. 

Then came fall.

Beavers and the Bird Sanctuary

Beavers do not hibernate, so they need to gather in their food for the long winter. They harvest full-grown trees and saplings and bring them to their lodge to refrigerate them in an underwater stash until needed. 

Favorite foods are cottonwood and willows, particularly the fairly nutritious cambium layer just under the bark, as well as buds, leaves and twigs. After taking more cottonwoods that shaded a small section of beach, they gnawed through the snow fence into the bird sanctuary. Altogether, they took more than 30 cottonwoods.

Up to then, volunteers at Clark Street Beach Bird Sanctuary (of which this writer is one) knew little about beaver life.  It was time to learn how they live, and how we could learn to live with them.  Volunteers fenced off the cottonwood habitat and put four-foot high hardware cloth around 100 individual trees to protect them, leaving most of the area south of the beach house for beaver foraging.

Interest in beaver reintroduction has accelerated, particularly in the Northwest U.S. Often thought to be freshwater mammals, researchers have found they do well in brackish water coastal areas and their ponds can be safe havens for salmon fry.  

Can introducing beaver to arid land where fires are frequent create ponds that will serve as firebreaks? Scientists think this idea holds promise. But predicting good habitats for relocating beavers is an art; the beavers have the last word.

The best beaver watching at Northwestern is in the very early morning hours near the bridge at the south end of the lagoon.