Tuleyome's Science Corner - River Otters

Geoffrey Benn • October 25, 2023

River Otters at Yolo Bypass by Jonathan Eisen

Meet one of California's cutest and most voracious predators – the North American river otter! River otters are found throughout most of Northern California, including in much of Berryessa Snow Mountain National Monument. They live in aquatic habitats including rivers, lakes, streams, and marshes and can be found at Yolo Bypass, UC Davis Riparian Reserve, and in some of the many sloughs bordering rice fields.

Range of the North American River Otter in California (CDFW)

River otters will utilize a variety of locations as nesting sites, including thickets, cavities in riverbanks or logs, or burrows abandoned by other animals. In California, female river otters will typically give birth in March or April and the pups will remain with their mother until the following winter. Adult male otters have also been known to form social groups of up to 17 individuals.


The diet of river otters varies by location and time of year, but typically consists of fish and crustaceans such as crayfish. River otters have also been known to eat amphibians and birds, particularly migratory water birds during the winter.

River otter enjoying a crayfish at the UC Davis Riparian Preserve by Jonathan Eisen

River otters are also found along the coast and will hunt in bays and inlets. They can be distinguished from sea otters by size (sea otters are much larger) and by their behavior in the water – river otters are generally always moving in the water and don’t float on their backs like sea otters. According to a recent phylogenetic study, North American river otters (Lontra canadensis) are actually more closely related to several other species of river otters from Central and South America, than they are to sea otters (Enhydra lutris). The ancestor of these American river otters is thought to have crossed into the Americas from Asia via the Bering land bridge.


This study also showed that North American river otters have significantly higher genetic diversity than sea otters, likely because the latter were hunted almost to extinction in the 19th century for the fur trade. River otters were also trapped for fur until the practice was banned in California in 1961. Since then, river otter populations and ranges have expanded, with otters returning to the Bay Area and other parts of their original range. Due to their genetic diversity, sizable population, and wide range, North American river otters are not considered to be threatened, though water pollution and habitat destruction are still a concern.


This means that river otters are relatively easy to find in our area. The best places to look are bodies of water that are surrounded by thickets of vegetation. Signs of otters in the area include mud slides leading into the water and latrine sites on shore with scat containing crayfish shells. If you are lucky enough to find otters, you should resist the urge to try to cuddle them, as they are wild animals and have been occasionally known to attack people. So, the next time you’re heading to an area with rivers or lakes, be sure to bring your binoculars and watch the water and shoreline for signs of our furry neighbors!

-Geoff Benn (gbenn@tuleyome.org)


Education Associate


About Geoffrey Benn

Geoff is Tuleyome's Education Associate. He has a Ph.D. in Plant Biology and previously taught introductory biology at UC Davis and American River College.

RECENT ARTICLES

By Ellen Jenkins July 2, 2026
From left to right, Horticulture Interns Ellen Jenkins, Rithika Warrier, and Diego Barraza Hernandez, with Education Associate Geoff Benn, who coordinates the internship program. From September to June, I worked as a Horticultural Intern at Tuleyome. Each week at the Woodland Regional Park Preserve, I would meet with my mentor Teri Barry. Over the course of the year, we worked together identifying, documenting, and analyzing the preserve's growth. Through this opportunity, I gained experience in hands-on restoration efforts, familiarity with plant taxonomy and classification, and community outreach.
By Kristie Ehrhardt July 2, 2026
I’m sure you already know that the Bald Eagle is used as a symbol of our Nation’s freedom, strength, perseverance and independence. It is said that our founding fathers chose the Bald Eagle to adorn our Great Seal because it is indigenous to North America and if you look closely at it, you’ll see that it is holding 13 arrows and an olive branch simultaneously symbolizing the power of war and the power of peace. But, what about our national mammal? Er, maybe you didn’t even realize that we had a national mammal… President Barack Obama designated the American Bison as our national mammal in May of 2016. It too is endemic to North America and in prehistoric times, millions of them roamed through roughly two-thirds of the United States. Their range was known as the “great bison belt” - a stretch of habitat that encompassed the forests of Alaska, the grasslands of the North American plains and stretched from the Great Basin east to the eastern Appalachian Mountains. Side note - I’m just gonna come clean right now and tell you that the historic range of the American bison, American Buffalo or just plain bison or buffalo does NOT include the Berryessa Snow Mountain National Monument (Monument). I’m supposed to be writing about the Monument region and I picked a topic that is clearly not there but it’s important and they are impressive so please don’t tattle. Another side note - the American Bison and the American Buffalo are one in the same and both names can be used interchangeably. Its scientific name (genus and species) is actually Bison bison so I guess calling them bison is probably more scientifically accurate but either works. Bison are North America’s largest land animal; an adult male averages about 2,000 pounds - that’s a ton (thanks Schoolhouse Rock, I will never forget this one!), are about 12 feet long from nose to rump and about six feet tall. Females max out at around 1,000 pounds and are a little over nine feet long. In the wild bison can live ten to twenty years. Both females and males have massive forequarters and sport what looks like a shaggy, long-sleeved sweater. Both sexes have short, curved horns that are an integral part of their status in the herd and defensive strategy. During the snowy winters, bison display a dark brown winter coat and in the summer they show off their lighter brown summer-bods. Calves typically arrive from March through May depending on weather conditions. When they’re first born calves are an orangey-red color which earned them the nickname “red dogs”. Within a few months they become darker brown like their parents and their little horns start to poke through. Around this time they also begin to develop their iconic shoulder hump. The hump is made of solid muscle buttressed by elongated vertebrae. This dense network of muscles act as a powerhouse that allows the animal to swing its massive head from side to side and plow through icy snow to access buried forage enabling them to survive even the harshest winters. Although they are not known for keen eyesight, bison have superb senses of hearing and smell. As monumental (get it?) as these animals are, they are shockingly quick and agile. Bison have been clocked at running 35 miles per hour (!), they can spin and turn on a dime, leap over high fences and are incredibly adept swimmers. All these mad skills on a strictly vegetarian diet - that’s ruminant efficiency. For thousands of years, before the European settlers arrived, Native Americans of all tribes and regions revered the bison. They were the cultural, spiritual and resource backbone of indigenous societies across the United States. Every part of the animal was used; the meat was consumed, the hide was used for clothing, shoes and shelters, and the bones and hooves were used for tools and weapons. Even the sinew was used for sewing and bow strings. Everything was copacetic then came the Europeans with their domestic cattle diseases and thoughtless greed. Bison were hunted to near extinction within roughly just 100 years. Their numbers went from an estimated 60 million (!) to less than 550 individual animals by 1889. They were hunted mercilessly and their habitat destroyed, some say as a way to control the Native Americans by removing their primary resource. One of the buffalo’s biggest fans was President Teddy Roosevelt. In the early 1880’s he traveled to what is now North Dakota (shout out to my people!) to hunt. After recognizing the bison’s diminishing population numbers he co-founded the American Bison Society in 1905 to help conserve and protect this American emblem. For many years the few buffalo that remained were only found in national parks and preserves but primarily on Native American reservations. Had it not been for the willingness of tribes across the country to work with interested individuals and state and federal governments, the American Bison would be gone today. Much like the Bald Eagle, the bison is an authentic symbol of American character and as it turns out, another one of the greatest conservation success stories in U.S. history. The bison within the Yellowstone National Park boundaries are direct descendants of the original animals that roamed our country’s grasslands and are the only herd that still occupy their original prehistoric location. This free-ranging herd numbers approximately 5,500 animals and is the largest herd on public land. Recovery efforts paid off and today bison can be found in all 50 states including private and tribal lands, national parks and wildlife refuges.