The Carlisle Mosquito Online

Friday, November 28, 2003


Biodiversity Corner River Otter (Lutra canadensis)

Where and when: Fresh otter signs (scat and scent mounds) found in several shoreline locations throughout the Carlisle Cranberry Bog Reservation on November 8, 2003. I have been monitoring otter populations along River Meadow (Great Brook) since the early nineties; and this sign was the most abundant to date; perhaps indicating resident otternot just a transient animal passing through.

(Drawing by Tom Wilson)

Description: Massachusetts' largest weasel species; river otter have a long cylindrical body with lengths to 52". Males weigh up to 30 pounds. Fur is a rich, chocolate brown to tan, often with silver throat, chest markings, and a pale belly. Broad, flattened head with small ears; large, bulbous nose and prominent whiskers. Paws are webbed, with small claws. Tail (to 20") tapers from a thick base to a point and effectively acts as a rudder while swimming.

Diet: Primarily fish and crayfish; also amphibians, waterfowl, water beetles. Fish are selected primarily by convenience and abundance, not specific species. Studies show that turtles and freshwater mussels oddly do not factor much into otter diet. Aquatic prey is ambushed from below or chased down; sensitive whiskers (vibrissae) aid in locating food in murky water. Otter may feed in groups, herding schools of fish into shallow coves.

Behavior: River otter breed around March and April, just after giving birth to a litter. Embryonic implantation is delayed until the following season. Two to six pups are born in early spring, venturing out of the natal den within 50-70 days. Otter mothers are extremely devoted parents, with female pups staying on for another year, acting as "aunts" for the new litter. Male pups disperse first and may form loose groups called "clans," but usually remain solitary. Matriarchal groups are known as "families." Territorial needs of males have been determined at up to 30 miles of shoreline habitat, while females require as little as one mile. Relatively clean, secluded waters are vital for river otter.

Signs: Scat, irregular droppings containing fish scales, fish bones and crayfish parts, is commonly deposited on beaver mounds and lodges. Search along shoreline margins but signs may occur several yards upland. Their extensive grooming habits leave behind "rolling spots"— bank areas worn bare from otter rubbing their bodies against the ground to dry their coats upon emergence from the water. Scraping and scenting activities create "haul outs" and "spraint mounds;" which combine scat deposits and mounded leaf-litter. Temporary dens within beaver burrows, hollow, fallen logs or woody snags often lay nearby.

Snow tracking will inevitably produce otter "slides"— long channels (roughly foot-wide) cut into sloping banks and along riparian areas. Otter will lunge forward, tucking front paws against their sides, and with a push from the rear limbs plunge along on their bellies down into the water. While a useful aid in locomotion, slides reveal otters' playful, rollicking nature. Tracks appear as 3" wide, webbed prints in 3-4 or 2-2 trail patterns; bounding tracks commonly left by weasel species. Signs persist for years at established latrine areas and runways as long as human disturbance is kept low.

References: Wild Mammals of North America: Biology, Management, Economics by Joseph A. Chapman & George A. Feldhamer; National Audubon Society Field Guide to North American Mammals by Alfred A. Knopf; Tracking and the Art of Seeing by Paul Rezendes; Mammal Track and Sign by Mark Elbroch;,

Tom Wilson is a resident of Chelmsford. He is a nature and wildlife artist and has displayed his work in the Gleason Library. He is an experienced tracker, with a special interest in the Mustelidae (weasel family), is active in several watershed organizations, and is a property steward for the Cranberry Bog in Chelmsford. His artwork is currently on display at Mass. Audubon's Broadmoor Sanctuary in Natick.

(Photo by Tom Wilson)

2003 The Carlisle Mosquito