Nature Notes: Busy Beavers-Pt. 2
In Part One of Busy Beavers, we explored the amazing physical features of beavers that allow them to do their important work. These incredible ecosystem engineers reshape the landscape by cutting down trees, building dams, and creating wetlands that support a wide variety of plants and animals. Have you ever wondered why beavers build dams, how they move all of that "stuff" or what life inside the lodge is really like? From clever construction techniques to cozy winter survival strategies, these hardworking critters have plenty of secrets to share. Here's a closer look at the wonders of beavers.
Beavers as Timber Harvesters & Ecological Engineers
Beavers are well-known but less-often-seen Minnesota critter. More often, we see their dams and/or lodges, which, let's be clear, are NOT the same thing. 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 to eat. But beavers are actually herbivores, meaning they only eat plant materials! They eat the bark and cambium (layer under the bark) on trees and a variety of aquatic vegetation. Beavers need to cut down trees for food and also for their building materials.
Canoeing up to an old beaver dam, no longer in use. Look how tall it was at one point on the right! Beaver dam on a mountain stream in Chile. An unexpected obstacle in our paddle!
A variety of beaver dams that I've come across in my travels! Photos by Nora W.
Beavers are quick in the water, but extremely awkward on land. The less time on land, the less likely you are to be eaten if you're a beaver. [Recent research from Voyageurs Wolf Project revealed that some Minnesota wolf packs have specialized in hunting beavers, with up to 60% of spring and fall diets consisting of beavers!] By damming a river and creating a beaver pond, they raise the level of the water and therefore make more trees around the edge accessible by only a small trip from the water.
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. 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!
A beaver run (left) that led to a felled tree from a beaver (center). Notice the grooves left behind from the beaver's teeth! They go along the downed tree & remove the branches they need (right). Photos by Nora W.
Beaver dams can be quite large – in fact, the largest dam is located in Northern Alberta, Canada and it is so big, you can see it from space (in images dating back to 1990)! It's calculated to be about 2,790 feet long. Beaver colonies need to constantly maintain their dams in order to keep their ponds flooded. The sound of trickling water triggers beavers to enter "dam repair mode." In fact, during an infamous study from the 1960s, beavers ignored the silent running water and instead chose to "dam" the speaker that was playing sounds of running water, covering it with sticks and muds.
Livin' Life in the Lodge
Beaver lodges are no less impressive than their dams. The largest lodge on record was approximately 40 feet wide. These domes of mud and sticks often have multiple rooms within - a lower chamber that serves as a mudroom and feeding area and a higher chamber that serves as a bedroom and nursery. They swim up into the lodge through underwater entrances (!) and drip dry in the mudroom before entering the higher level, lined with wood shavings and moss to help keep it dry, warm, and cozy. Building dams not only gives them food and building materials, but it also maintains water levels to ensure the entrance to their lodge remains underwater and deep enough that it doesn't freeze during winter. This helps beavers escape from large terrestrial predators such as wolves, bears, lynx, fishers, and even otters. Their lodges have thick walls of mud which freeze solid during the winter months, creating an effective barrier. Humans continue to be the largest threat to this massive rodent through trapping, habitat alteration, and pollution.
Exploring in winter let's you get up close to beaver lodges!
Beaver Lodge Photos by Nora W.
These animals have developed unique behaviors for winter survival. Contrary to popular belief, beavers do not hibernate. They are active all winter long; we just can’t usually see them. Beavers can exit their underwater entrances in winter to swim out in the water, but once the surface freezes with a couple inches of ice, they're essentially trapped below the ice all winter. Therefore, 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 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 lodge 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. That's a lot of beaver bodies in a very small space, generating a lot of heat! The one foot thick mud walls of the lodge are fantastic insulation, trapping the heat inside 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. On a cold winter day, you can see steam rising from the vent, or chimney, at the top of a lodge!
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!
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