Delayed Implantation (And Other Weasel-like Behavior)

Delayed Implantation (And Other Weasel-like Behavior)

Illustration by Adelaide Tyrol

In the English language, someone who overeats is called a “pig”; a coward gets labeled a “chicken.” If you raise pigs or chickens, you know that these barnyard associations are deftly accurate. Pigs are voracious eaters, chickens are laughable cowards. It all makes perfect sense.

Not every animal association works, though. Take weasel. In our collective vernacular, “weasel” is often synonymous with “rat.” The word is used to denigrate, to refer to some unsavory character as sneaky and small. If you were casting a weasely mob boss in a Hollywood picture, you’d use actor Steve Buscemi, right? That’s the face of a weasel.

But anyone who has experience with real weasels knows that the conventional use of the term is way off. In the animal kingdom, weasels are little warriors, perhaps the bravest, most ferocious animals in all the forest. To really make the association work, we should be using “weasel” to mean a combination of little and bad-ass. The 96-pounder on your local high-school wrestling team would be a good weasel. New England Patriot’s wide-out Wes Welker. Napoleon Bonaparte, in a negative sense. You get the idea.

In Vermont and New Hampshire, we have two species of weasels: the long-tailed (Mustela frenata), and the short-tailed (Mustela erminea). An adult, female short-tailed weasel is surprisingly small: about 9 inches long from her nose to the tip of her longish tail, which makes her body about the size of the tube of toothpaste sitting on your bathroom sink. Long-tailed weasels are larger than that, and males of both species are considerably larger than their female counterparts. Both species of weasel are ferret-brown in summer and snow-white in winter, a color change that’s prompted by changes in seasonal light.

Weasels generally feed on mice and rats but are fearless enough to kill rabbits, small woodchucks, even baby pigs. As anyone who’s ever raised chickens knows, they think nothing of brazenly walking into a chicken house and ripping the throat out of a full grown Rhode Island red; in fact, it’s not uncommon to confront a weasel in the chicken coop and have her stare brazenly back at you.

Weasels, like many members of the mustelid family, have a reproductive quirk commonly referred to as delayed implantation (DI). Males and females mate in early summer, but the resulting fertilized egg doesn’t implant itself in the uterus right away – instead, it just hangs out in the female’s body until she’s healthy and ready to give birth the following spring. Once the egg implants, the pregnancy lasts less than a month.

DI is a mysterious thing. In some species, it makes perfect sense: if a hibernating female black bear, for instance, hasn’t put on enough winter weight, her embryo won’t implant, thus potentially saving her life. But it’s less clear what evolutionary advantage weasels get from DI. Considering how small the animals are, how short lived they are (most won’t make it to age 2), and considering how their chief prey species (rodents) are notoriously boom and bust, it seems that a more reactive birthing regime that allows for breeding to be ramped up in boom times would make more sense. And, in fact, some species of weasel have lost the DI trait, and do reproduce “normally.” Least weasels, for instance, a species you’ll find north and west of us, might have one litter per year when food is scarce and two or even three per year when prey is abundant. They’re turning excess rodent meat into more babies in a very direct way.

So why haven’t our Northeastern species of weasel evolved away from DI? Maybe because DI gives males access to concentrated populations of females who are all ready to breed at the same time – a distinct reproductive advantage in its own right.  In the case of short-tails, males sometimes mate with young females while they’re still in the nest. Long-tails aren’t quite that sexually precocious, but females do become sexually reproductive at around 2 months of age.

At this time of year, look for weasels wherever there are mice – your wood pile would be a good place to start. If you don’t have cats, you may find weasels in the walls of your house.

If you’re lucky enough to have weasels in your walls, you could trap them with rat traps, but why would you? They’ll do a better job killing your mice than any product you can buy. Leave them be, and in the spring they’ll move out and give birth in some hollow log on the back 40. You won’t hear from them again until the mice move back into your house the following fall.

Dave Mance III is the editor of Northern Woodlands magazine.

  1. Ann → in Palmerston, Ontario
    Dec 31, 2012

    Myself & several family members have seen a weasel in our barn, it is killing all our barn cats, who are pets, & are well feed & looked after. In the last 2 years, we have gone from 20 cats to 5; they have all had their necks ripped open, or are very wet & show bite marks at the neck. This is very sickening to all of us, we have tried live traps, different baits, plugging all the rat holes ( by the way, we have no rats or mice since the weasel moved in 2 years ago). Any suggestions? We really like our barn cats.

  2. dave → in corinth
    Jan 03, 2013

    Hi Ann,

    If it were me, I’d be inclined to keep the weasels. They’ll do a better job killing the mice and rats than the cats will. Plus, barn cats can do a number on an ecosystem—they’re callously killing mice and birds and they don’t really need to because you’re feeding them. I’ve always seen barn cats as exotic invasive species—it doesn’t really seem fair to the wild animals in your ecosystem; the songbirds or the predators.

    Having said that, I’m not your mother. So if you really want to kill the weasels, use rat traps—you can buy them in the hardware store. Put a trap under a bushel basket or in a shoe box so it won’t catch a cat. Cut a weasel-sized hole in the enclosure so a weasel can get in. Bait with something fresh and bloody. 

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