Monogamous minks? Not quite.

Post by Brett Peto. All mink images and footage by John D. Kavc.

Yes, it’s almost that time of year. American mink (Neovison vison) mating season. I know, I’ve been waiting for it, too. February is celebrated for human romance: fancy dinner dates, shiny gifts, and long walks on the Des Plaines River Trail. But it’s useful to step out of our human-focused perspective once in a while. And thanks to our comprehensive Wildlife Monitoring Program, we know minks live in the Lake County Forest Preserves in northern Illinois. So, let’s examine why humans aren’t the only species that looks forward to February 14.

A mink (Neovison vison) peeks over a fallen tree. Photo © John D. Kavc.
A mink (Neovison vison) peeks over a fallen tree. Photo © John D. Kavc.

Best to start at the beginning. Minks are small, furry, semi-aquatic mammals, meaning they split their time between land and water. In fact, they spend the majority of their lives within 100 feet of rivers, creeks, lakes, ponds, and marshes. Minks weigh only 1.5 to 3.5 pounds on average and belong to the Mustelidae family, which also counts otters, weasels, and badgers among its carnivorous, mammalian members. Minks are mainly nocturnal, but can be crepuscular—one of my favorite words in biology, crepuscular, calling to my mind some sort of flaky pastry—so dawn and dusk are also active times of day for them.

On average, minks are 13 to 18 inches long. White patches on the chest, throat, and chin are common. Photo © John D. Kavc.
On average, minks are 13 to 18 inches long. White patches on the chest, throat, and chin are common. Photo © John D. Kavc.

Monogamous minks might make for good alliteration, but it’s an…inaccurate phrase. Once they’re capable of breeding, usually between 10 and 12 months of age, male minks mate with multiple females (who also have multiple mates) in February and March. This system is polygynandrous—there’s another vocab word!—meaning it involves several partners across a breeding season.

The act of mating is over by the end of March, although when exactly baby minks come into the world is sometimes unpredictable. That’s due to delayed implantation, also called embryonic diapause. Common in the mustelid family, delayed implantation is when a fertilized egg doesn’t develop into a fetus right away. Because of this, the gap between mating and birth runs from a fairly brief 40 days to a nearly-half-a-baseball-season-long 75 days, or a 46 percent difference (!). Nevertheless, a mother mink generally gives birth by May 1. Her litter ranges in size from two to seven individuals—four on average.

Mink toes are webbed at their bases, which helps them swim. Photo © John D. Kavc.
Mink toes are webbed at their bases, which helps them swim. Photo © John D. Kavc.

Mink young, or kits, are born hairless but start growing thick, water-repellent fur at around two weeks. Kits open their eyes for the first time after five weeks. Then they learn to hunt from their mother at six to eight weeks. When they leave home in the fall, minks live the rest of their lives alone. They only interrupt this solitude to mate and to rear their own young. It’s not exactly a monastic vow of silence, though; minks are known to purr on occasion when they’re content.

Walk along a river or stream, and you might unknowingly walk over a mink den. They often burrow into riverbanks or reuse dens from other mammals, such as muskrats (Ondatra zibethicus). If you could poke your head into a mink den, you might find an interior design incorporating several entrances, as if to always have another option. The nest chamber is typically a foot in diameter and insulated with grass and leaves, or even fur and feathers from past prey.

The mink’s menu is heavy on the meat: mice, voles, rabbits, birds, snakes, fish, frogs, chipmunks, crayfish, and even those aforementioned muskrats. They avoid humans, but like other mustelids, minks are pretty ferocious, aggressive predators. In fact, they’re rather well-known to go after bigger prey and succeed. Their favorite killing method is a bite to the neck. In the winter, they hunt through holes in the ice and dig into the mud to capture slow-moving prey. Sometimes they push forward and slide through the snow first, leaving a telltale sign of their hunting grounds.

Devin Hoffer, a master's degree student with the Southern Illinois University Cooperative Wildlife Research Lab, examines a mink slide in the snow. Photo © John D. Kavc.
Devin Hoffer, a master’s degree student with the Southern Illinois University Cooperative Wildlife Research Lab, examines a mink slide in the snow. Photo © John D. Kavc.

Minks, especially males, use strong odors from their scent glands to mark the boundaries of their territories, which they rigidly enforce. If a larger animal attacks them, minks don’t hesitate to defend themselves. Should they need to retreat, minks might use their notable tree-climbing and swimming abilities. Another helpful factor is the relative lack of mink predators, among them coyotes (Canis latrans), snakes, and birds of prey. If a mink outlasts or evades predators and other survival obstacles, it can live for up to about 10 years.

Happily, the Forest Preserves protects thousands of acres of wetlands for our mink friends. I’ve noticed anecdotally that it’s sometimes easier for mammals such as us to appreciate fellow mammals just a bit more than other species. Warm-blooded animals are thicker than water, or something like that. Which is okay with me, as long as we remind ourselves from time to time that other species may require different temperature-regulating methods but they’re just as great, fascinating, and important. The more people connect with any wildlife in any way, the more people get inspired to help it however they can.

True to their semi-aquatic nature, minks split their time between land and water. Photo © John D. Kavc.
True to their semi-aquatic nature, minks split their time between land and water. Photo © John D. Kavc.

As usual, there’s an image in my head. A mink out on its own, away from its mother and siblings for the first time, the first day. It’s fall; that season of seasons. The water’s edge is comfortingly in sight. There are a thousand and one things to do. Find creatures to eat and eat them. Explore the home range. Defend against predators. Stay alert for intruders. Climb trees, if need be, or swim up to 50 feet underwater. So much on the list and on the way. And, come February, mating season.

Tag along on our Walk with a Naturalist program. Stock photo.
Tag along on our Walk with a Naturalist program. Stock photo.

Wildlife can be challenging to spot; sometimes, it helps to have expert eyes. Try our Walk with a Naturalist program at Greenbelt in North Chicago, February 1, 9–10:30 am. Learn about the preserve in winter and restoration work underway. FREE. No registration required. Adults. Meet at the front doors of the Greenbelt Cultural Center.

2 thoughts on “Monogamous minks? Not quite.

  1. 0n our property we have a brush pile adjacent to a vernal pond . On 3/7 as my worked near the pile he saw a critter run into the pond water. We researched and are quite sure by size&color that it is a mi k. We didn’t know minks are here. I surmise the mink has been in the brush pile for the winter..a safe place, mice abound. Or do mink hibernate?

    e

    • Hi, Mary. Yes, minks are fairly widespread through Lake County, but can be quite difficult to spot. They do not hibernate and are active year-round. It’s possible the mink in question is using the brush pile as a den. However, they typically use abandoned dens from other mammals, or dig their own. Thanks for reading!

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