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Naturally Curious: Scented Signposts

  • After establishing a territory, beavers construct scent mounds around its perimeter. (Mary Holland photograph)

    After establishing a territory, beavers construct scent mounds around its perimeter. (Mary Holland photograph)

  • Beavers create scent mounds which act as sentinels and sign posts, alerting beaver passersby that the nearby pond is occupied. (Mary Holland photograph)

    Beavers create scent mounds which act as sentinels and sign posts, alerting beaver passersby that the nearby pond is occupied. (Mary Holland photograph)

  • These are dried beaver castor sacs. When fresh, they measure roughly five inches long and two inches wide. Trappers use dried castor sacs to lure beavers as well as bobcats and foxes. (Mary Holland photograph)

    These are dried beaver castor sacs. When fresh, they measure roughly five inches long and two inches wide. Trappers use dried castor sacs to lure beavers as well as bobcats and foxes. (Mary Holland photograph)

  • After establishing a territory, beavers construct scent mounds around its perimeter. (Mary Holland photograph)
  • Beavers create scent mounds which act as sentinels and sign posts, alerting beaver passersby that the nearby pond is occupied. (Mary Holland photograph)
  • These are dried beaver castor sacs. When fresh, they measure roughly five inches long and two inches wide. Trappers use dried castor sacs to lure beavers as well as bobcats and foxes. (Mary Holland photograph)

In the spring, just prior to when a mother beaver is about to give birth, her 22-month-old offspring, who have been living with her since birth, leave the lodge. An innate urge tells them that it’s time to seek a mate and start a colony of their own. The parents don’t discourage this behavior, as it prevents inbreeding and prevents premature depletion of the food supply. The major challenge facing these young beavers is the fierce territoriality of their species, which can make the search for a new pond or stream to dam more than a little daunting.

When they disperse, most young beavers go downstream to look for unoccupied territory. Ideally they come upon an old, abandoned beaver pond that has regrown a good supply of aspens, willows and birches — a beaver’s preferred diet. However, young beavers are rarely that lucky. It’s not beyond the realm of possibility for these young upstarts to attempt to move into an inhabited pond site, so resident beavers take measures to alert these youngsters that the pond is spoken for.

In an attempt to discourage young beavers from lingering, one of the first things adult beavers do in the spring is to mark the perimeter of their territory. They do so by gathering mud and leaves from the bottom of their pond and making piles, or “scent mounds” to advertise their presence as well as ownership to any beavers passing by. They deposit castoreum, a secretion that conveys information such as the beaver’s age and sex, on each mound by straddling it, everting their castoreum sacs and dragging them across the mound. Scent mounds vary in size, from a height of just a few inches, to three feet or so and they are usually located within two feet from the water’s edge. The pheromones in the castoreum are broadcast far and wide from these mounds. An encroaching 2-year-old beaver detects the odor, and, if it is smart, continues on its way. If a stray male beaver deposits some of his own scent on a resident’s scent mound, or stops to feed, the resident male drives him off by hissing loudly, and if that doesn’t work, he attacks the interloper.

Castoreum is a combination of a thick, yellowish, aromatic secretion of the castor sac and the beaver’s urine. Both male and female beavers possess a pair of castor sacs (technically they are not glands) as well as a pair of anal glands. Castoreum is used for communication, whereas oil from the anal glands is used by the beaver for waterproofing its fur.

It’s not only beavers that use castoreum to convey information to their own species; humans do, as well, but strictly for attraction purposes. Dried and aged beaver castoreum is an ingredient in some perfumes that supposedly gives them the scent of leather. As one maker of perfume describes it, “The scent of castoreum is wild and bodily, lustful and passionate, bestowing the one who wears it a delicate aura of sensuality.” Fortunately for beavers, perfume makers have developed a chemically synthesized castoreum.

Beaver-produced castoreum, however, is still used for some strawberry, raspberry and vanilla flavoring in alcoholic and non-alcoholic beverages, baked goods, candy, chewing gum, puddings and frozen dairy products. The next time you indulge in food or drink with these flavors, you might want to read the list of ingredients on the bottle or container, though it might not be quite as transparent as you would like — the FDA considers castoreum safe, to the point of allowing companies to refer to it as “natural flavoring.”

Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England” and “Milkweed Visitors.” She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.