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June 2020 Natural Mystery Answered

posted Jul 8, 2020, 12:50 PM by Jonathan Poppele   [ updated Jul 22, 2020, 9:32 AM ]
We received a number of guesses about last month's Natural Mystery. Everyone who wrote in correctly identified this as the carcass of a semi-aquatic mammal. But there were a range of guesses about exactly who this unlucky critter was. Congratulations to Jeremy Britzius, Emily Goldberg & Kirsten Welge who correctly identified the species, and thanks to everyone for their thoughtful analysis of this mystery.

This is the carcass of an American Beaver, Castor canadensis.

Emily gets us started with with a basic analysis:

"It's clearly a mammal, and its fur has the dense, slick look of a water-dweller; in Minnesota, that leaves us with otter, beaver, or muskrat. Its front feet look small and don't seem strongly webbed (though it's hard to tell given their pose), which makes otter less likely, especially since otters are just less common in general."

Cheri Stockinger also notes that “Otters have long narrow slinky like bodies and short claws on wide stubby paws.”

That leaves us with muskrat and beaver—the two most common responses. Both of these aquatic rodents have relatively small front feet with long, stout claws that are well adapted for digging. Muskrat and beaver are quite different in size, but as Emily notes, “It's hard to judge size in this photo.” Distinguishing between the two from this photo is unquestionably tricky and many people who wrote in admitted they were essentially guessing.

Kirsten applied careful observation and some clever reasoning to determining the species:

"I realized I could use the duckweed for scale. I found a picture of duckweed on a muskrat. I'm noticing in this picture, the leaf size compared to the body of the muskrat is greater than what I'm seeing in the natural mystery photo. And if we want to quantify it, Texas A&M AgriLife Extension notes: 'Duckweed has 1 to 3 leaves, or fronds, of 1/16 to 1/8 inch in length.' If one leaf of duckweed is 1/16” to 1/8" in length, those feet are much larger than we'd expect for a muskrat.

Also, muskrat females show 8-10 nipples; beaver females only have four. The Natural Mystery carcass shows four. Beaver it is!"

Indeed, beaver it is. Or was, at least. I have found other sources that say duckweed fronds (apparently they are botanically distinct from leaves) can measure nearly 1/4", but even if we assume they are nearly that large, they offer enough of a scale to identify this as a beaver rather than muskrat. I'll admit, however, I did not notice the animals teats until I read Kirsten's response.

Everyone who wrote in also recognized this as the belly and front legs of the animal. The head, tail, and hind legs were all submerged. One of our contributors, who asked to remain anonymous, sent this exacting explanation of why the animal is in this posture in the water:

“Intestinal contents continue to ferment after death due to microbial activity, usually causing extensive abdominal distention (swelling due to internal pressure) and sometimes even over-extension of the back (which may be the case here since the pelvis is below the water surface). Since the thorax (upper torso) and the post-mortem ventral abdomen is more buoyant than the rest of the body, this animal is floating belly up. Head and back will be more dense relative to the distended abdomen.”

Congratulations again to Jeremy, Emily & Kirsten for identifying this carcass to species.

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