Class: Mammalia
Order: Various
Suborder: Various
Family: Various

Minnesota's lakes, rivers, streams, ponds, and wetlands are home to semi-aquatic mammals such as the River Otter, Nothern Bog Lemming, and Beaver. All mammals are warm-blooded vertebrates (vertebrates have backbones). With the exception of a few notable species, mammals nurse their young on milk produced by the female’s mammary glands, give birth to live young, and have bodies insulated by hair. Semi-aquatic mammals are mammals that dwell partly or entirely in bodies of water. These semi-aquatic mammals that depend on the freshwater systems of Minnesota tend to spend a lot of time in and around cold water, so they have special traits like oils and fats that keep them warm, long tails, and webbed feet to help them swim.

There are only about 4,000 kinds of mammals. This sounds like a lot, but when you consider there are 21,000 kinds of fish and a whopping 800,000 kinds of insects you’ll realize mammals are a pretty small class! Around one in four of the world’s mammals are currently threatened with extinction. For semi-aquatic mammals, their major threat is the loss of their bodies of water they need to survive.

All mammals share four traits*:

  • Hair
  • Mammary glands
  • Hinged jaw
  • Three tiny middle ear bones.
    *Most have specialized teeth and moveable external ears.


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What are essential needs for your species?

Life Cycle

All mammals reproduce sexually—sperm from the male fertilizes the female's egg—and the female gives birth to live young. Both beaver and river otter females reach sexual maturity at about three years of age. Male otters have multiple female mates, whereas beavers stay with the same mate for life. Beavers tend to have larger litters of babies or kits; otters usually just have one offspring at a time. Both young are able to swim at birth. The female will nurse and care for the young for 1-2 years before they're old enough to go off on their own. The average lifespan of these mammals is 15-20 years.


Energy Web

Otters are carnivores, living on fish, amphibians, aquatic reptiles and invertebrates and often taking the role of top predator in freshwater ecosystems. Beavers, on the other hand, are herbivores, eating mostly leaves, roots, bark and twigs. Beavers are especially fond of cambium - the soft tissue that grows under the bark of a tree.

Beavers play a very important role in the energy web as a keystone species. This means their presence in nature greatly affects other wildlife. Beavers build dams and create wetlands upon which many species depend. Almost half of all endangered and threatened species in North America rely on wetlands to survive, and eighty-five percent of all North American wild animal species depend on wetlands. Beaver dams also help purify and control water by filtering silt from the water bodies. They can also slow flood waters and aid in containing forest fires. Beavers can serve as "ecological indicators." Their presence in an area lets us know the ecosystem is healthy.


Both otters and beavers are semiaquatic, spending much of their time in the water. Otters can thrive in any water habitat, such as ponds, marshes, lakes, rivers, and estuaries—and in cold, warm, or even high-elevation areas—as long as the habitat provides adequate food. Beavers, however, live exclusively in freshwater habitats. Both mammals build their shelters (called dens for otters, lodges for beavers) along waterways with the entrance underwater.




Are your species' needs being met?

You should have a pretty good idea of what your species needs in its environment to be healthy and vibrant. Do you know if your watershed is healthy for your species?