Nature Notes: Busy Beavers
Last week, we took a look at how squirrels are tirelessly preparing for winter by collecting and stashing food away for the harsh season. This week, we’re focusing on another MN native that caches food for winter – the beavers! They are the largest rodent in North America. While most adults weigh-in at about 45 pounds, beavers can weigh over 70 pounds! Historically, beaver fur has been important economically (trading), which led to beavers being introduced in other regions of the world. A few years back I traveled to Teirra del Fuego, Chile at the southern tip of the Americas. Beavers were introduced to this region of Patagonia in the mid-20th century in hopes of increasing economic prosperity. However, beavers have no natural predators in Tierra del Fuego and have run rampant ever since, causing millions of dollars in damage to the ecosystem. There is now a widespread campaign to eradicate this nonnative species in this region of Chile. In Minnesota, beavers are not only a native species, but also a keystone species, meaning they play a very important role in their ecosystem as timber harvester, architect, and engineer! Here habitat modification by beavers creates aquatic habitats for many other animals, helps prevent flooding and erosion, and aids in filtering and cleaning water. (Note: Click on a photo to enlarge.)
Beavers are well adapted to the roles they play in their aquatic, cold climate habitat. They have two layers of fur – a dense undercoat to provide insulation and warmth, and a topcoat of waterproof “guard hairs”.
Beavers are well-known but less-often-seen Minnesota critters that are second only to humans in their capabilities for modifying our landscape. More often, we see their dams and/or lodges, which you can see in the videos below! A dam is what they build to block a stream or river and flood the area upstream. A lodge is the dome of sticks and mud that a beaver actually live inside of. But why do beavers build dams? Many people think it is to create a pond where the beaver can catch fish. Beavers are actually herbivores, meaning they only eat plant materials! They eat the bark and cambium (layer under the bark) on trees as well as water vegetation in the pond.
Beavers are very graceful and quick in the water, but extremely awkward and slow on land. By damming a river and creating a beaver pond, they raise the level of the water and therefore more trees around the edge accessible by only a small trip from the water. You can often find beaver “runs” through wetlands and even notice their paths on land. While beavers can lift their weight in wood, it is much easier to float the wood, so again, having trees close to the water is very useful! Beaver dams can be quite large – in fact, the largest one is located in Canada and it is so big, you can see it from space (above)! Beaver colonies need to constantly maintain their dams in order to keep their ponds flooded. The sound of trickling water kicks them into maintenance gear.
Beavers need to cut down trees for food and also for their building materials. They cannot climb trees, so in order to access the smaller branches that they can drag to the water, a beaver needs to fell the whole tree! Once it is on the ground, they can move up and down the trunk, chewing off the branches that are of an appropriate size. The Minnesota DNR claims a beaver can chew through a tree with a 6 inch diameter in just 15 minutes!
The other large benefit of beaver ponds is it creates water that is deep enough so the underwater entrances to a beaver lodge do not freeze during the winter. That’s right – underwater entrances! This helps the beavers escape from large terrestrial predators. Their lodges have thick walls of mud which freeze solid during the winter months. Predators often leave large adult beavers alone, but young beavers may fall prey to wolves, bears, lynx, fishers, and even otters. Humans continue to be the largest threat to this massive rodent, as we impact the population through trapping, habitat alteration, and pollution.
These animals have developed unique behaviors for winter survival. First, beavers do not hibernate! They are active all winter long; we just can’t usually see them! Due to the underwater entrances of their lodges, beavers can go out into their pond/lake during the winter. However, once the surface freezes with a couple inches of ice, they’re essentially trapped below the ice all winter. Therefore, there needs to be enough food in the water to last until spring. So beavers, like squirrels, are busy throughout the fall collecting enough food to last them through winter! They fell many trees, collecting small branches and twigs and pile them up right in front of their lodges. This cache serves as a beaver refrigerator! You may be able to see these piles accumulating above the surface of the water, but most of their food is down below, where they will be able to access it even when the water freezes!
A beaver is quite cozy inside the lodge all winter. A mating pair often resides in the lodge with their offspring, resulting in a beaver “colony” of up to 2-10 beavers. Kits, or baby beavers, arrive between April and June, and will stay with their parents in the den for two years. That means there are usually older siblings in the lodge helping collect food, care for younger siblings, and work on dam repairs. In fact, throughout winter, the beavers take turns creating heat through motion. No matter what time of day it is, there is typically one beaver up and moving around in the lodge to generate body heat. The construction of the lodge heavily relies on mud, which is a fantastic insulator, and heat is easily trapped within the lodge. Even when temperatures drop to -40°F outside, the inside of the lodge is always above freezing, providing a warm den for beavers throughout winter.
So get outside this month and see if you can spot any evidence of beavers getting ready for winter! Are there any “beaver chews” in your woods? Can you find any beaver runs in our ponds and wetlands? Do you notice a cache of sticks in front of a beaver lodge near you? Get out and investigate! Fall is an extremely busy time for many of our animals and there is a lot to observe. As always, enjoy!