North American River Otters Mating and Giving Birth
North American River Otters are induced ovulators – copulation releases the female’s egg from the ovary. Once the egg is released and fertilized, however, there is a nine to eleven month delay before the embryo begins actively developing (delayed implantation). Actual gestation takes about two months. Thus, otters sometimes give birth up to a year after mating, just before their next breeding cycle. April and May are busy months for this semiaquatic mammal.
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North American River Otters Use Abandoned Beaver Lodges As Dens
North American River Otters use dens (called holts) for giving birth and for shelter from weather extremes. Den sites are usually close to the water line of rivers and lakes, and have multiple entrances underwater as well as on dry land. They are often excavated under trees or rocks or in river banks, but otters also use abandoned muskrat burrows and beaver lodges as shelters (cohabitation with beavers has also been documented – see https://naturallycuriouswithmaryholland.wordpress.com/2013/02/22/beaver-otter-cohabitation/).
One of the most obvious and distinctive signs of otter occupancy of a den is the presence of their scat in the vicinity. It usually has little form; rather, it consists of loose piles primarily composed of fish scales. Pictured is an abandoned beaver lodge that is currently occupied by several otters whose scat in the foreground and tracks and slides in the vicinity confirm their presence. A lack of any beaver sign indicates the lodge has been abandoned by its original inhabitants.
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North American River Otters Sliding & Gliding
North American River Otters spend much of their time foraging. They often have a circuit they travel along rivers and lakes which takes them up to a week or more to complete. In between bodies of water, they travel overland on well-used paths, often during the day in winter.
These circuits are miles long, and for much of the time otters lope along in typical weasel fashion. However, in winter the snow permits them to occasionally flop down on their bellies, tuck their front feet next to their chest and push off with their hind feet as they slide effortlessly on top of the snow, both down slopes as well as along flat surfaces. Once they obtain a certain speed, they give their hind legs a rest and lift them off the ground so as not to slow them down as they slide (see photo). Otters have been clocked up to 17 miles per hour running and sliding in this manner.
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River Otter Scat
Because fish make up a large part of their diet, North American River Otters live along streams, lakes and wetlands. Although crayfish, hibernating frogs and turtles, insects and other aquatic invertebrates are also consumed in the winter, the telltale identifying feature of otter scat (spraint) this time of year is the presence of fish scales.
Look for otter scat on raised areas near water, especially the shortest distance between two water bodies or on peninsulas. It is usually found on the ground, but occasionally on logs and at the intersection of two streams. Otters frequently form large latrines of multiple scats.
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Otter Holes
A pond can have one to two feet of ice on top of it, yet North American River Otters somehow manage to find a way to come up through the ice. How do they do this? Careful observation reveals that the ice where otter holes are found is thinner than most of the pond’s ice. Otters rely on spring-fed open water and areas of thin ice for surfacing holes. Here they break through the ice and often drag larger fish out onto the ice to eat them.
If otters are feeding in a beaver pond and they can’t find openings in the ice for fishing, they have been known to tunnel into beaver dams for access to open water. In late winter, water levels sometime drop below the ice, leaving an air space that lets otters swim and hunt beneath the ice without the need for holes. (Look closely at opening to see the actual hole beneath the surface water that they emerge from.)
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North American River Otters Foraging For Fish
Whether or not North American River Otters made the original holes evident in Wednesday’s Mystery Photo, they were responsible for keeping them open by frequently poking their heads up through them for some air. Congratulations to Noel K. for being the first to correctly identify their surface holes. This was a tricky Mystery Photo, as there were none of the usual signs of otter activity (tracks, fish remains, etc.) on the ice surrounding the holes. This is probably because the ice was too thin to support the weight of an otter. To find the most humorous response, scroll down on Wednesday’s Mystery Photo comments until you get to Peg Emerson’s.
These semiaquatic members of the weasel family are active year-round and while they are mainly nocturnal and crepuscular during the summer, they are frequently spotted during the day in winter. If otters encounter open water, they rarely resist the urge to enter it and pursue resident fish.
Thanks to their webbed feet and streamlined body, otters are accomplished swimmers and divers. They are able to reach a depth of around five feet and remain submerged for up to four minutes as they hunt underwater. Top swimming speed is seven miles per hour. (They can achieve a speed of up to 18 miles per hour when running and sliding on snow or ice.) While fish are their mainstay, these carnivores also consume frogs, snakes, turtles, insects, birds and bird eggs and the occasional mammal (mainly muskrat). Though called “river” otters, they forage in fresh, salt and brackish waters. (Thanks to Rita and Dave Boynton for photo op.)
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Otter Brown-Out
North American River Otters are not territorial in the classic sense of marking territorial boundaries. Instead they mark prime resource areas within their territory. Often, the site is near their den or a productive food area. They visit these sites repeatedly to urinate, defecate and roll around on the ground – so much so that the surrounding vegetation is often dead or dying and is referred to as a “brown-out.”
If an otter has been eating fish, its scat is often just a pile of fish scales. However, if it has been dining on crayfish and it is fresh, the scat can be tubular. No matter what form otter scat takes, a tell-tale sign (in addition to fish scales and/or crayfish exoskeletons) is the presence of clear, white or yellow mucus (scat-jellies). It is not always deposited, but occasionally you do find it. The origin of this mucus is not known – most likely it’s from the otter’s intestinal tract or its anal glands. Research shows that the presence of mucous deposits in some otter species indicates reduced prey availability or reproductive state. (Photo: Tubular otter scat is circled in red. Mucus is on right side of photo. Thanks to David Putnam and Natalie Starr for yesterday’s and today’s photo op.)
Reasons why Mystery Photo was not
Black Bear: Scat consists primarily of crayfish remains.
Beaver: Beavers defecate only in water, and individual pellets consist of tiny woody fragments resembling sawdust.
Raccoon: Raccoons have latrines where multiple scat is deposited, similar to otters. However, only otters deposit mucus.
River Otter Tails
From their nose to the tip of their tail, North American River Otters measure three to four feet long. Their tail makes up anywhere from a third to nearly a half of their length. A River Otter’s tail is very thick at its base, packed with muscles, flexible, and tapers to a point. It is used to steer when an otter is swimming slowly, propel the otter when it is swimming at high speed and to help the otter balance when it stands upright on its hind legs. River Otters, known for their powerful swimming, can reach speeds of six to seven miles per hour with the help of this appendage.
When loping through the snow, River Otters often hold their tails up off the surface of the snow, but not always. Occasionally drag marks can be seen. In the accompanying photograph, an otter had leapt up an incline, and in so doing left an imprint of its impressive tail in the snow.
(Thanks to Alfred Balch for photo op, and Joan Waltermire for her sharp eyes.)
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North American River Otters Sliding
There is no denying that North American River Otters know how to travel and have fun in winter. Whether on a flat surface such as a frozen pond or river, field, or down slopes, otters take advantage of the snow, bounding then dropping to their belly and sliding, saving precious energy. Most slides are relatively short, around 10 feet long and 6-10 inches wide, though they can be as long as 25 feet long on slippery ice. At the beginning and end of a slide there are tracks (from where they push off with their hind feet, and cease sliding and begin bounding again), creating a dot-dash pattern. Sometimes a downhill slide is used repeatedly and when it is, bobsledders have nothing over otters, as water from the otters’ coats creates an icy and very slippery slide.
For those of you who would like to view an excellent video of an otter sliding, go to https://www.youtube.com/watch?v=iBB0OLOkvIU .
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Low Water Levels Provide Palette for Tracks
The low water levels in our ponds this fall do have one benefit – visitors leave obvious signs in the exposed muddy banks. It is fairly astounding how much nocturnal and crepuscular wildlife regularly visits these spots and remains undetected by humans under normal conditions.
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Young Otters Just Beginning to Explore Outside Their Den
Between February and April, female North American River Otters give birth to an average of two or three young inside their dens, which tend to be near water, usually in an excavated cavity under tree roots, logs or thickets. Occasionally they have been known to give birth in abandoned beaver and muskrat lodges as well as woodchuck burrows. The mother doesn’t allow her young to exit their den until they are three or four months old, which, for the oldest litters, is in May. At this point, after their waterproof adult coats have grown in, she begins to teach them how to swim (surprisingly, this skill is not innate). The mother continues to be very protective and secretive, hiding and shielding the young (even from their father) until they are about six month old. Thus, glimpses of young otters are not likely to be had until late summer, and only then if you’re lucky. (Photo: adult river otter. Thanks to Heidi Marcotte and Tom Wetmore for photo op.)
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North American River Otters Running & Gliding
One hundred years ago, naturalists were puzzled by the disappearance of otters in the winter. Some people theorized that they must be hibernating, but in fact, otters are active throughout the year — they just aren’t observed as often in the winter as in the summer. Because they inhabit water, where the fish that they eat are found, and often reside in bank dens (above water level), otters can spend most of their winter under cover of ice. However, some do travel over land from open water to open water, upon occasion. In fact, it is not unusual for them to cover several miles in their search for an open stream or a spring. Run and glide, run and glide, often along a frozen river or marsh, but also through the woods, where otter tracks seem so incongruous. Paired foot prints stop and start each slide; this combination is a dead giveaway as to who is making grooves in the snow. Ultimately, if you persevere when following them, you will find that these slides disappear into open water.
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Otter Tracks
North American River Otters have four webbed feet and strong claws that assist them in water as well as on land. There is relatively little hair on the soles of otter feet, and therefore the individual pads are often well defined in good tracking snow. Each foot leaves a five-toed track, with the inside toe on the front feet being somewhat smaller than the others. Otters have four plantar pad glands in the center of each hind foot with which they mark mounds of vegetation they create.
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Otter Ingenuity
North American River Otter tracks are usually found where otters have travelled from one body of water to another. In the winter, this can be on frozen streams as well as over land. If they come to an incline, even a small one, they often take advantage of it by letting gravity do the work on the way down. In this photograph, two otters travelling together had the same idea at the same time, and reached the marsh at the bottom of the hill by effortlessly sliding down the hill on their bellies, leaving two 12-inch-wide grooves in the snow. Although John James Audubon, in the mid-1800s, observed a pair of otters sliding down an embankment over and over 22 times, stopping only when they discovered that they were being observed, the otters that made the tracks in this photograph were intent on getting where they were going, and only slid down once.
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Otter Holes
River otters are one of the most aquatic members of the weasel family. They can swim up to six or seven miles per hour on the surface of the water as well as underneath it, and can remain submerged for up to two minutes. Otters spend a great deal of time fishing under the ice, and obtain oxygen from open holes such as the one pictured (as well as from air bubbles under the ice). As their tracks indicate, otters come up onto the ice to eat their prey, be it fish or crayfish, their two favorite winter meals.
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More Otter Sign
It’s often a lot easier to find signs of otters than otters themselves. Recently I discovered two dead crayfish on the shore of a mostly iced-over pond that I knew was inhabited by otters. Nearby otter scat confirmed that these crayfish were probably left by satiated otters. The most important prey item in a majority of otter scat analysis studies is fish, followed closely by crayfish. Otters will take advantage of other prey, such as frogs, salamanders, ducks, muskrats, an occasional young beaver, mice, snakes, insects and even turtles when readily obtainable, but fish and crayfish are first and second choices.
Otter Slides
Otters will travel long distances from one pond to the next, and when they do, they frequently alternate between bounding and sliding. They often slide down hills, but they also slide on level ground, as in this photograph, and sometimes even uphill. While sliding, the otter holds its front feet back along its sides with its hind feet out behind it, leaving a trough roughly 6” to 12” wide and up to 25 feet long. Two footprints (actually four, but the hind feet land on top of where the front feet landed so it looks like two) can be found at the end and at the beginning of each slide, where the otter stopped sliding, bounded and began sliding again. You can see at least five separate slides in this photograph. Occasionally, in deep snow on level ground, an otter will use its foot to help push it along, either inside or outside of the trough. Otters slide at all times of the year, on mud as well as snow and ice, and appear to do so in order to get from one place to another, as well as purely for fun, as when they repeatedly slide down the same slope over and over. (Thanks to Mark and Susan Boutwell for sharing their discovery.)
Beaver & Otter Cohabitation
It is not coincidental that you often find otters residing in beaver ponds. There appears to be a commensal (one animal benefits while the other is unaffected) relationship between these two animals. The beaver is unaffected – it is a herbivore, so its food supply is not threatened by the presence of otters. (While an occasional beaver is eaten by an otter, it is a rare occurrence.) The otter, on the other hand, benefits from abandoned as well as active den sites (both beaver bank dens and lodges) as well as an ample supply of fish due to the impoundment of streams by beavers. While I was aware that otters often take over abandoned beaver lodges, I only recently learned that the lodge does not have to be uninhabited for otters to move in. This was confirmed when I discovered a large amount of otter scat (mostly fish scales and crayfish shells) on top of a beaver lodge, right next to the hind foot print of a beaver. Freshly placed sticks on the lodge (it is in open water) indicated that it was occupied by beavers, while an otter’s stream of air bubbles could be seen as it exited the lodge and popped its head up above the surface of the water.
North American River Otters
River otters are the most aquatic members of the weasel family. They eat the whole gamut of aquatic prey, including fish, frogs, crayfish, salamanders and turtles – not to mention snakes, small birds, mammals, earthworms and insects. It would not be an exaggeration to say that this threesome consumed a minimum of 15 fish and crayfish within half an hour. Look for signs of their sliding in the snow, both down slopes as well as on level ground.
A Great Christmas Present!
If you’re looking for a present for someone that will be used year round, year after year, Naturally Curious may just fit the bill. A relative, a friend, your child’s school teacher – it’s the gift that keeps on giving to both young and old!
One reader wrote, “This is a unique book as far as I know. I have several naturalists’ books covering Vermont and the Northeast, and have seen nothing of this breadth, covered to this depth. So much interesting information about birds, amphibians, mammals, insects, plants. This would be useful to those in the mid-Atlantic, New York, and even wider geographic regions. The author gives a month-by-month look at what’s going on in the natural world, and so much of the information would simply be moved forward or back a month in other regions, but would still be relevant because of the wide overlap of species. Very readable. Couldn’t put it down. I consider myself pretty knowledgeable about the natural world, but there was much that was new to me in this book. I would have loved to have this to use as a text when I was teaching. Suitable for a wide range of ages.”
In a recent email to me a parent wrote, “Naturally Curious is our five year old’s unqualified f-a-v-o-r-I-t-e book. He spends hours regularly returning to it to study it’s vivid pictures and have us read to him about all the different creatures. It is a ‘must have’ for any family with children living in New England…or for anyone that simply shares a love of the outdoors.”
I am a firm believer in fostering a love of nature in young children – the younger the better — but I admit that when I wrote Naturally Curious, I was writing it with adults in mind. It delights me no end to know that children don’t even need a grown-up middleman to enjoy it!
Naturally Curious with Mary Holland 
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