Wily Weasels: The Math Behind Mustelid Identification

The following piece was written by OAS Communications Coordinator AnnaKathryn Kruger for the Snapshot Wisconsin newsletter. To subscribe to the newsletter, visit this link

Weasels are the most commonly misidentified animal in the entire Snapshot Wisconsin dataset. What might make them so tricky to identify, and just how do they differ from the other members of their family for whom they are often mistaken, like marten or mink?

As far as their phylogenic standing, weasels belong to the superfamily Musteloidea. Contained within Musteloidea are the families Mephitidae, which includes skunks; Mustelidae, including weasels, otters, ferrets and kin; and Procyonidae, with raccoons and their neotropical brethren. In examining the dataset of Snapshot Wisconsin photos that have received an expert classification, researchers have determined that weasels and mink are the two most difficult species for volunteers to classify.

Phylogenetic_Tree

The two avenues for classification available to volunteers through Snapshot Wisconsin are MySnapshot and Zooniverse. MySnapshot is the outlet available to those who monitor trail cameras, where they can classify the animals in the photos captured on their own camera. Zooniverse is a public forum where volunteers classify photos that are served up at random from Snapshot Wisconsin cameras across the state. Photos are captured in sets of three, called “triggers”, and volunteers classify the entire set at once.

recall_precision4When evaluating accuracy in classification, researchers focus on two variables: recall and precision. Both variables provide measures of accuracy for a group of volunteer classifiers, either from MySnapshot or Zooniverse, compared to expert classification. Recall addresses the question: out of all the weasel triggers in our dataset, how many did volunteers classify as weasels? Whereas precision addresses the question: how many triggers classified as weasels by volunteers were actually weasels?

Between both MySnapshot and Zooniverse, volunteers generally demonstrate high recall and precision when classifying animals that belong to the whole superfamily Musteloidea. When it comes to classifying individual species, we can see that animals like skunks, otters and raccoons are easier to classify correctly on account of their distinctive traits, but weasels are quite similar in physical appearance to the species with whom they share a family, namely mink. This makes weasels particularly easy to misidentify.

Triggers containing weasels and mink are most often missed completely on Zooniverse, with a recall value for these species of 41%. Out of an expertly classified sample size of 15 weasels, only 10 of the 15 were identified correctly by volunteers and 4 additional triggers classified as weasels on Zooniverse were not weasels. This puts recall and precision for weasel classification at a measly 67% and 71%, respectively.

For mink, Zooniverse and MySnapshot share low recall, with approximately 65% of mink photos missed completely on Zooniverse, and 39% missed on MySnapshot. However, the triggers that were classified were largely classified correctly, with perfect precision on Zooniverse and 87% precision on MySnapshot.

So how can we successfully identify a weasel versus some other mustelid, and vice versa? There are three types of weasels in Wisconsin. The long-tailed weasel is the largest of the three. They are typically 13-18 inches in length with a 4-6 inch black-tipped tail. Their coats are brown and their bellies and throats are cream-colored, though they transition completely to white in the winter. The short-tailed weasel is Wisconsin’s most common weasel. Smaller than the long-tailed weasel, the two share their coloring, which makes them more difficult to differentiate. The only discernible difference is the tail length. The third type of weasel is the least weasel, aptly named as it is the smallest of the three at roughly 6 inches. Though this weasel has coloring similar to the others, the least weasel has a short tail without the black tip.

Weasels look rather similar to mink, though mink are dark-colored and larger than weasels with long tails and glossy coats. They weigh between 1.5-2 lbs. Another mustelid closely resembling the weasel is the American pine marten, an endangered furbearer with a penchant for climbing. They have large rounded ears and a bushy tail, and their fur varies in shades of brown from almost yellow to almost black. Snapshot Wisconsin has only one confirmed photo of a marten, as the species is incredibly rare in Wisconsin.

Badgers seem like a no-brainer, with their characteristic striped heads and wide bodies. They are significantly larger than weasels and have long claws well-suited to digging. Despite their distinctive appearance, badgers are subject to misidentification as well. 22% of the triggers classified as badgers on MySnapshot were not badgers. The same goes for the fisher, another sizeable mustelid weighing in at an average of 15 pounds, with dark brown fur and a bushy tail.

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Classification is a tricky business, especially when it comes down to mustelids. Snapshot Wisconsin relies on thousands of volunteers to classify the nearly 34 million photos in our dataset, which they generally do with tremendous success. Though the weasel is a trickster, their phylogenic camouflage can be discerned with a trained eye – the same can be said for their Mustelidae cousins. Each accurately classified photo, mustelid or no, brings Snapshot Wisconsin closer to a complete representation of Wisconsin wildlife, and better informs our management of these species.

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