How do you sex a beaver? Squeeze and sniff

Unlocking the secrets of rodent scent glands could help restore Western watersheds.


Pop quiz for all you amateur wildlife biologists: How do you determine the sex of a beaver?

That question might sound like the set-up to a raunchy punchline, but for the Methow Valley Beaver Project, it’s a pressing concern. The Methow Project, as I reported this month for High Country News, captures tree-felling, ditch-clogging nuisance beavers in eastern Washington and relocates them to public lands in the Cascades. There, the buck-toothed engineers construct salmon-sheltering wetlands, recharge groundwater and create habitat for wildlife from salamanders to moose.

In between live-trapping and release, the Project houses its wards at the Winthrop National Fish Hatchery, where males and females form pair-bonds that help them better survive in the wild. Of course, to set up Harry with Sally, you have to know who’s Harry and who’s Sally. That’s where the critters make matters difficult.

Beavers, you see, lack familiar mammalian plug-and-socket genitalia. Instead, the creatures possess cloacas — fleshy vents, analogous to the anatomy of birds and reptiles, that do triple duty in the departments of waste disposal, scent secretion, and, yes, reproduction. A male beaver’s cloaca looks almost exactly like a female’s. Not even the sharpest-eyed matchmaker can reliably tell the sexes apart — at least not visually.

Hendrix, a 44-pound male, surveys his human captors from his enclosure at the Winthrop National Fish Hatchery
Ben Goldfarb

This, in a roundabout way, is how I ended up kneeling on the concrete floor of the Winthrop hatchery — one hand pressed against the damp fur of a beaver’s belly, another swabbing its hindquarters with a tissue, nostrils puckered by the potent odors rising from below its leathery tail. Harry or Sally? My nose, in theory, would know. 

Katie Weber, Methow Project biologist and beaver-sexing coach, peered over my shoulder at the rodent writhing in my uncertain grasp. “Once you get the anal gland expressed, put some pressure on it, and you’ll see oil,” she offered, like a cornerman urging a boxer to lead with a left jab. Biologist Catherine Means had wrestled the beaver, a weathered male named Half-Tail Dale, into a blue cloth sack, leaving only his nether regions exposed. The bag’s darkness had calmed Dale, though he still occasionally kicked out with his clawed feet. I didn’t blame him — the experience must have been like visiting a very clumsy proctologist. 

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Methow Valley Beaver Project

Beneath my fumbling, Latex-wrapped fingers, the scent glands — angry twin volcanos of pinkish flesh — popped from the cloaca. A drop of amber liquid glistened on one tip, and I dabbed at it gingerly. Weber encouraged me to squeeze a bit harder. “Be careful where you position yourself,” called Torre Stockard, another scientist, from behind the safety of a fence. “You’re in the splash zone!” 

The scientists seemed to think they were offering me a once-in-a-lifetime opportunity. To me the experience seemed more akin to a hazing ritual. With a whispered apology to Dale, I applied additional pressure on his glands, which poured forth a viscous stream of caramel beaver juice. (From the Beaver Restoration Guidebook: “Obviously this procedure should be done with your face a reasonable distance from the cloaca, with your mouth shut.”) Weber swooped in to wipe up the mess. Dale’s ordeal at the inexpert hands of a journalist had, mercifully, concluded.

Weber held up the tissue, blotched with hard-won scent secretion. Males, she explained, excreted darker, thicker fluid than females. The odor provided another diagnostic key. A hint of motor oil indicated a Harry. If you smelled old cheese, you had a Sally on your hands. “Once you’ve done five, you can pretty well tell,” Weber assured me. According to a recent genetic analysis, the Methow Project has misidentified the gender of just one beaver since it began using the glandular technique.

I took the tissue and, against my better judgment, inhaled deeply. Motor oil? Maybe. But the bouquet also contained notes of overripe fruit, pet store interior, dead muskrat, paint, and countless other olfactory sensations. Indeed, a single anal secretion may contain upwards of 100 different chemical compounds. It wasn’t unpleasant, but it was powerful. I felt a touch lightheaded, and wondered if the Food and Drug Administration had ever seen reason to ban the use of beaver glands as a narcotic.

Methow Project biologist Katie Weber ponders a tissue full of diagnostic beaver secretions.
Ben Goldfarb

While glandular discharges aid the efforts of beaver relocators like Weber, they’re also fundamental to the lives of the rodents themselves. Beavers have poor eyesight, which means they experience the world primarily through their noses (their hearing is excellent, too). Each beaver has a distinct, fingerprint-like scent, which the animals use to identify relatives — three generations typically share a single lodge — and mark “scent mounds,” piles of leaves, mud and sticks that delineate the domain of individual colonies. (Incidentally, don’t confuse beavers’ anal glands with their castor sacs, oil-producing organs that beavers use to map their territory and waterproof their fur. Castoreum has some applications in the world of humans, though contrary to the claims of the Food Babe, there is virtually no chance that “beaver butt” flavors your vanilla ice cream. Your perfume? That’s another story.) 

Understanding the creatures’ sensory abilities isn’t just an academic question: In one 1990 study, scientists in upstate New York used manmade scent mounds to manipulate the establishment of wild beaver families. For their part, the Methow Project’s scientists use scent lures — gloppy brown paste concocted from beaver secretions and other substances — to lure the creatures into live traps. Further unraveling the animals’ aromatic mysteries, therefore, could help advance C. canadensis’ restoration. And that, Weber told me once we were safely removed from the line of fire, would be a very good thing. 

“It’s amazing how many people start out saying, ‘Hmm, beavers, I don’t like those guys,’” Weber said as we watched Dale, now free and reconfirmed as male, cruise the waters of his concrete-walled enclosure. “And then you show them the benefits, and they say, ‘Oh — I didn’t think of it that way.’” The best smell of all, perhaps, is that of success.  

Ben Goldfarb is a correspondent for High Country News. 

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