- 40–54 inches long, 10–30 pounds.
- Sleek, cylindrical body; small head; tail nearly one third of the body and tapers to a point; feet webbed; claws short; fur is dark dense brown.
- Ears and nostrils close when underwater; whiskers aid in locating prey.
- Most aquatic member of weasel family; generally found near water.
- Eat crayfish and fish; also frogs, turtles, sometimes young muskrats or beavers.
- Active year-round. Mostly crepuscular but have been seen at all times of the day.
- Breed in late March through April; one litter of two young per year. Females and offspring remain together until next litter; may temporarily join other family groups.
- Can swim underwater up to 6 miles per hour and for 2–3 minutes at a time.
- Not agile or fast on land unless they find snow or ice, then can move rapidly by alternating hops and slides; can reach speeds of 15 miles per hour.
- May move long distances between waterbodies.
Crait, J.R. et al. 2006. Late seasonal breeding of river ot- ters in Yellowstone National Park. American Midland Naturalist 156: 189–192.
Crait, J.R. and M. Ben-David. 2006. River otters in Yellowstone Lake depend on a declining cutthroat trout population. Journal of Mammalogy. 87: 485–494.