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Solved: T’was a Northern River Otter

April 5th, 2008 · 3 Comments · mink, otter, photography, Radnor Lake

I have finally come to a rock-solid conclusion as to the species of the elongate mammal I filmed as it ran up a steep hill along the South Cove trail at Radnor Lake.

otter running behind tree at Radnor Lake, near South Cove trail

It was an otter — specifically, a northern river otter — for certain. The giveaway was the tail!

I came very close to deleting the second photograph I snapped of this animal as it ran; after all, the shot was lousy because the mammal just happened to be directly behind a large tree as the light pattern was digitally recorded. But at the last minute I decided to open the image to see if any part of the animal was visible, especially the tail.

At this point, I had been told by Radnor Lake park ranger Leslie Ann of the differences in the tails of the mink and the otter. It turns out the tail is one of the main ways to tell the two mammals apart.

The mink’s tail is somewhat bushy. The otter’s tail is thick and not so bushy, and flattened horizontally to propel itself through the water by waving the tail up and down.

northern river otter, swimming

It is rather interesting to me that the otter’s tail apparently does not go back and forth. It seems back and forth would be the most natural way to use the tail for acceleration or movement through the water. Alligators, snakes, and other animals use a back-and-forth motion.

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3 Comments so far ↓

  • Patti

    Love the otters! It is unusual to see them away from the water, so you had a treat! I have two ferrets and they run just like that.

    I’ve only seen the otters twice at Radnor. Both times there were three of them, swimming in the lake. I’ll have to keep an eye on the land for them!

  • Rob

    Very neat that otters are present in radnor lake! I wonder if they found their way in on their own, or were introduced by people? That is interesting about how they move when they swim; if you think about it, fish, amphibians, and reptiles all swim with a side-to-side motion, and mammals (whales are a good example) swim with an up-and-down motion. At any rate, you are quite lucky to have seen the animal on land!

  • Leslie

    In the north bay area of San Francisco there is a small pond that I visit and river otters go there in the summer. They have to cross overland to get there as this pond doesn’t connect to the waterways system. In fact, the only crossing is through an 18 hole golf course! I suppose they go at night. I’ve watched them tease my golden retriever pup when he was swimming by popping their head up, then disappearing and reappearing in an other location. They eat crayfish in the pond and have very identifiable scat.