Up at the Tree House… For the Love of Weasels

News // April 06, 2023

I love weasels. Technically, I love members of the mammal family Mustelidae, named for the genus of Old World weasels, Mustela. Members of this family are called mustelids. Besides weasels, in North America this group includes otters, badgers, fishers, martens, and wolverines. Seven species of weasels are found in Pennsylvania. It’s unknown how many sightings occur in Philadelphia. Without good photos or specimens, it can be difficult to confirm identification from sight records. As a native Philadelphian and avid naturalist, I enjoy seeing wildlife within city limits. Friends and family have seen mink, river otters, and long-tailed weasels in Philly. I saw giant otters in Brazil and had an ermine scamper over my boot in the Arctic before I saw a Philly mustelid. It’s silly, but I was getting a little frustrated!

I was 43 when I finally saw a mustelid in Philadelphia. I was working from home during the start of the pandemic and took a mountain bike ride on the Yellow Trail during lunch. As I approached the ponds on Wise’s Mill Road, an American mink darted across the trail. I finally saw a Philly weasel, and it was in my beloved Wissahickon!

Minks are semiaquatic weasels, and the Wise’s Mill ponds, the Wissahickon Creek, and its tributaries provide excellent habitat. Minks eat frogs, crayfish, fish, birds, and mammals. Despite only weighing between one and three pounds, mink have been known to kill prey as large as gulls and cormorants! They are also a major predator of muskrats. Minks have brown fur that can look black when wet; they usually have a white chin that helps distinguish them from river otters and other weasels. Minks are more stout than other weasels but less robust than otters. Their pointy nose and shaggier fur also help to distinguish them from otters.

The American mink (Neogale vison), was named for its resemblance to the European mink (Mustela lutreola) and its similar semiaquatic habits. These species are not closely related. Mustela includes many species called weasels, as well as ferrets, stoats, ermines, and polecats. American minks share their genus with only three extant species: Amazon weasel, Colombian weasel, and long tailed weasel. The sea mink, which lived in coastal New England and the Canadian Maritime Provinces, was made extinct by the fur trade. Unfortunately, the American mink’s popularity as an economically farmed fur species has led to its spread to Europe, Asia, and South America, where it is an invasive species.

Besides mink, I have heard reports of river otters and weasels being seen in the Wissahickon. I know two people who swear they have seen fishers, one in the Wissahickon and one on private land near the Schuylkill Center. I have even heard of one spotted on a street in the Bronx!

In Philadelphia, the most reliable place to spot a mink is at John Heinz National Wildlife Refuge at Tinicum. This is likely due to the sheer number of visitors and extensive network of walkways and vantage points through aquatic environments. In the Wissahickon, it seems that these creatures are usually seen accidentally while doing something else, especially fishing or birding. I wish I had better advice on how to see a mink in the Wissahickon, but I guess the best way is to spend more time in the park!

Tony Croasdale, MS, is an environmental education program specialist at the Wissahickon Environmental Center.

Photo by Liam Hart