published week of July 5th 2021
A friend who lives in Dover and I routinely meet at the Cocheco River to kayak on warm summer afternoons. I’m always amazed at the wildlife along this river that runs through downtown Dover. Just a half-mile outside the city, as soon as paved surfaces give way to trees, life explodes: great blue herons, green herons, kingfishers and osprey hunt for fish, huge snapping turtles laze near the sun-dappled surface, little painted turtles bask on logs and wild grapes dangle over the water. We’ve found a hidden marsh off of the main river that is a fun tangle of lily pads, bladderwort (one of my favorites- a carnivorous aquatic plant!) cattails and pickerelweed. We are able to kayak through this aquatic jungle thanks to the muskrats that carve narrow passages through the dense vegetation.
Muskrats tend to get overlooked — we’re all aware of beavers and their role in building wetlands with their dams and impacting surrounding woodlots by cutting down valuable timber, but you don’t hear much about muskrats. Muskrats are, in fact, invaluable wetland engineers, removing extra plants and making sure waterways are clear. They carve channels through dense cattail or pickerel stands that lead into and out of their lodges (trappers routinely set their traps along these canals); these provide space for other plants and animals, helping to keep a marsh from becoming a monoculture. They also slow the process of succession in a marsh-where the buildup of dead vegetation causes the marsh to fill in and become a field. Muskrats help keep the perfect mix of water and vegetation in marshes.
Muskrats are not beavers, however, like beavers, they are ecosystem engineers.
Muskrats are not closely related to beavers, nor are they true rats-if you took a mouse and made it bigger and aquatic you would have a muskrat. They are closely related to voles and lemmings, with the characteristic rodent incisors that will grow through the skull if they are not constantly in use. Like their cousin the beaver, muskrats live in the water and build lodges, but are much smaller and are composed of mud, cattails and bulrushes (vs the beavers who use sticks). They also construct floating rafts of vegetation on top of the water to use as feeding platforms. Like beavers they will also burrow into river banks if conditions don’t support building a lodge.
Muskrats have taken the basic field mouse body plan and tweaked it for life-aquatic (or rather the environment has selected traits that help muskrats survive in the water). Instead of the broad, flattened tail of a beaver, muskrats have a rounder, thinner tail that is flattened side-to-side. They use this tail, plus their slightly-webbed rear feet to propel themselves through the water. Their fur is dense, waterproof and buoyant. My favorite adaptation to aquatic life is their ability to chew with their mouth closed while feeding underwater. Their lips can close behind their incisors (front teeth) so that they can keep their mouth closed while nibbling on underwater plants.
Muskrats are basically large field mice adapted for a life aquatic.
Another great adaptation to aquatic life is their ability to stay underwater for up to 17 minutes. Most non-aquatic mammals can’t do this because of the need for oxygen and to get rid of carbon dioxide building up in the bloodstream. Muskrats reduce their heart rate and relax their muscles when submerged to slow down the rate at which oxygen is used and carbon dioxide is produced. They also can store extra oxygen in their muscles and can tolerate more carbon dioxide in their blood than non-aquatic mammals. This is important, they need to be able to stay underwater for long periods of time while foraging for submerged stems and roots, travelling under the ice in winter and escaping from enemies.
Without the muskrat our secret marsh would probably be a monoculture of cattails or perhaps would have become so clogged with dead cattails and sediment that it would be well on its way to becoming dry land. This hidden marsh is a reminder to me that nature is a wonderful balancing act; that unlikely characters, the muskrat in this case, can have subtle feedbacks on a system that are critical for the health of that system–in this case maintaining a healthy marsh.