Blood in the Tracks

All dates mentioned in this post are based on personal observation and may change by days or even up to weeks depending on the year. It’s nature, not everything is etched in stone. 

Blood itself is a track. But we’ll get to that in a bit. Let’s start with

A little bit of history… or “You got me floating” – Jimi Hendrix

Those of you who are “Mustelid-leaning” (Mustelidae as in the Weasel Family) may have noticed something back in early February (around the 5th—give or take a day or a few). Maybe you had some sudden good luck or felt a slight pep in your step. I didn’t happen to notice anything, but maybe some of you did.



Why all the luck you ask? Well, in early February—after being pregnant for the last ten and a half months (technically)—female Fishers along the coast of Maine finally became attached to their fertilized eggs! You heard right, a female Fisher that mated last March has been walking, bounding, hunting, climbing, and raising last year’s young all while carrying a bunch of fertilized eggs in her uterus (none of them were attached until now).

Anyway, this process known as “delayed implantation” is seen in many species of mammals. Two of my favorite non-mustelid species with delayed implantation are Grizzly Bears and Elephant Seals.

Why delay implantation you ask? It’s in an adaptation to ensure offspring are born at the “right time” and that females have enough reserves to ensure proper pre-birth development. For local Fishers, the delay is over around that first week of February.

What’s more, if you truly “lean Mustelid” you probably felt something back in January when local female River Otters connected to their fertilized eggs for the first time after nine to ten months.  Hardcore “Mustelid-heads” might even have sensed the change back in April 2022, when local Mink ended their delayed implantation which lasts somewhere between three weeks to a month. It’s almost all-year-round, this joy that Mustelids give their biggest fans. Once again, I felt nothing, but I’m only here to observe.

Gestation celebration

Gestation – the process or period of developing inside the womb between conception and birth

No development takes place during delayed implantation, aside from an initial burst of cell division in the first week or so. The egg at this time is what’s known as a Blastocyst and made up of a few hundred cells. But because it hasn’t attached to the uterus, it isn’t getting nourishment directly from the mother, and thus the gestation clock has not officially started. A dramatic pause if you will. Remembering this old nursery rhyme can help –

“It’s only frustration
For fans of gestation,
Until the implantation occurs”

Fishers and Mink have gestation periods of 30-31 days. For River Otters the gestation period is about 58-60 days. I mention Fisher gestation not only because I like you, but also so we can use them as an example and add it up together:

10.5 months delayed implantation + 31 days of gestation = (roughly) 11.5 months of pregnancy

And so, you can see (math don’t lie) that female Fishers are technically pregnant for 11.5 months a year. It’s similar for River Otters, here’s the math:

9 months delayed implantation + 60 days of gestation = (roughly) 11 months of pregnancy

Turns out some female River Otters may mate every other year, and in those cases the length of time for delayed implantation may be longer. But diversity in the length of delayed implantation of fertilized River Otter eggs is not the story here, even though it’s cool.

Where baby Mustelids come from….

So, 31 days after Feb 5, means that Fisher gestation wraps up around March 6, and does anyone know what happens after gestation (hopefully)? Let’s all say it together – BIRTH! That’s right, the end of gestation marks the end of nearly a year-long pregnancy for the female Fisher. And then it’s “Welcome little Fishers!” This happens around the same time River Otters give birth, but you’d have to be a “super-duper Mustelidhead” to pick up on River Otters giving birth in the wild.

Anyway, the mother Fisher delivers a litter with one to six (average two – three) youngsters called “Kits”, like the car from Night Rider.  The Kits are born blind, helpless, and are partially covered with fine hair. They’re able to see at about seven weeks and start to climb at eight weeks and blah, blah, blah. This is not a post about the entire Mustelid life cycles, this is about March…

Den sweet den

We’ve mentioned this before, but just to make sure we’re all on the same page, the den where Fishers give birth is called the natal den and is often in a suitable cavity 20-30 feet off the ground in the trunk of a tree. In my neck of the woods Fishers seem to favor natural cavities in Aspen or Poplars that have opened after dead limbs have decayed into the heartwood. There can be many potential Fisher dens in a single territory. I know of several used dens in my neighborhood alone. They are fun to find, if finding dens in trees is what you think of as fun!

Anyway, regular Fisher dens may be a dime a dozen ($25 a dozen when adjusted for inflation) but I’ve only found a natal den once. And I only found this one because the female who’d given birth in it was seen in the area for five days straight, even with a local dog that would chase it up a tree daily. There are only a few reasons a Fisher would deal with daily harassment like this—food, offspring, or courtship.

One clue that the cavity was “natal” was that the opening had been visibly chewed-up (and likely heavily marked), leaving a message that said either, “Taken” or “Do Not Enter” (in a sassy voice). I have not seen this chewing at any “non-natal” Fisher dens, but other animals, included the dreaded and universally despised Red Squirrel, may sometimes chew at cavities they actively use. Or so I’ve read as I have not seen this either. Anyway…

To confirm Fisher presence, look for scats around the base of said cavity tree. This adds to the evidence of Fisher use—but be warned—non-natal dens may also have scats underneath. Man, so many caveats—anyone confused yet? I kinda am. Bottom line—the combo of active cavity and scat below is something to look for in the middle two weeks of March.

Seeing a Fisher in the flesh generally helps confirm whether it’s a den or not and dens certainly are easier to find if you’ve tracked your local Mustleids in snow. So, early to mid-March is a good time to check out suspected dens in your area.

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River otter

Mid-coast River Otters also give birth around this time, but female otters tend to keep the area around their dens clean and clear, with little to no sign of activity. This may be because their natal dens are in the ground which adds the concern of attracting a slew of predators that don’t climb trees. It’s hard to imagine an animal messing with an Otter or a Fisher and coming out on top. “Death don’t have no mercy” when it comes to Mustelids.

Breeding is the reason for the season*

*Or something like that.

After giving birth the female Fisher (or Otter) stays with the newbies—nursing, resting, snuggling, and staying warm for days to weeks, and sometimes longer. It is at this time (mid-March)—when the newborn Fisher and Otters are tiny and sleepy—that females start to tolerate the presence of males in their area and become receptive to mating again. And thus “estrus season” restarts and it’s the most wonderful time of the year—especially if you are a Mustelid.

Here’s a definition of estrus for ya!

(noun) Estrus, a recurring period of sexual receptivity and fertility in many female mammals; heat.

Full disclosure, between what I have personally observed (which is limited for sure) and have personally read, estrus and mating in the Mustelid world seems like a more wonderful time for the male Mustelid (judgment and anthropomorphizing). But the dynamics and implications of the actual act of “mustelid coitus” (the original “MC”) is not the story here. Thankfully.

And not to be too human-centric (but I will be anyway), Estrus time is also a wonderful time if you like to track Mustelids. And you do like to track Mustelids, even if you don’t know it yet.

This is the story! Finally.

Estrus is the time to say, “I love you.”

Female Mustelids are in heat, or “estrusized”, for relatively short periods of time. For Fishers this is an eight-day window, for River Otters it’s six days of receptiveness (but it can apparently be a series of repeated six-day periods if the female is alternating years of reproduction). Female Mink are also receptive to randy males in March (aren’t male Mink always randy? (stereotype)), even though they gave birth the May before, gotta be a daylight length trigger in there somewhere. Female Mink are receptive for a couple of seven to ten-day intervals during their three-week breeding season. All of this happens in March, and how do we know this? We’ve seen the signs…

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In heat Mink tracks

“Love is in the air”

Just how do female mustelids let males know they are receptive you ask? Estrus is a time of rich, robust, and active marking—when you have such a small period of time of receptivity the word needs to spread fast and loud and large. Estrusized Fishers will use old stumps, downed logs and rocks as locations to mark, where estrusized River Otters increase their use of known latrines. Estrusized Mink may use latrines as well, or simply mark along used trails and corridors. Placement of estrus sign is often on a raised spot, an effort to allow scent marking to be carried by further by breezes and wind. “Love is in the air”, quite literally while also not really being “love” per se.

Scent glands are used to mark, which is very cool but hard to detect unless you are a “way hardcore Mustelid-head”. Urine is another classic form for leaving messages in the animal world, but when Mustelids are “estrusized” the urine changes from a territoriality mark into a “liquid of love”. Sounds gross, right? Not as gross as how porcupines use of urine in courtship, but aftershave and perfume are no less gross, let’s not kid ourselves.

Anyway, the real treat of estrus season is finding blood that has been used for marking. As someone who likes to spend time outside, blood is a fact of life. Often when I see blood on a walk it’s my own, just call me Klumsy Kirk. Blood is exciting when it is associated with a kill site or maybe where a critter was taken to be eaten. Those are fun to find, but clearly associated with death, which once again happens. The little bit of “human” that hasn’t been jaded out of me has the smallest of mixed feelings about these finds. Like finding blood in an animal print and then seeing it over and over in an animal trail. It’s cool but it’s also too bad the raccoon (or whomever) has a cut on their foot. What a softy I have become huh?

So no, this is different blood, different than any other blood you might find in the woods. For an eight-to-ten-day period (or so) blood is the signal for life, for the potential of life. It’s not from death, not from injury, and not after ovulation doesn’t result in an egg being fertilized. They (the royal estrusized female Mustelid ‘They’) leave blood so they can mate and ultimately give birth 11.5 months later. Best use of blood I have heard of. What’s your favorite use of blood? That was rhetorical in case you were wondering.

The blood in these cases can be a simple drop or two, or maybe big enough to make a sploosh sound when it hits the snow. With Fishers and Mink, I have found single drops of blood where courting pairs were in close contact, if not closer (if you know what I mean).

Sometimes the blood is shed in sizable quantities. One March, I’d been following a Mink’s bounding trail for a bit in snow on Lane’s Island (Vinalhaven) when suddenly there were three or four large splashes of blood in the snow. Right in the middle of the trail! The Mink didn’t miss a beat – just kept going – but that was apparently the right spot to release the mark. The mink was by itself, so I think the blood was shed to announce loudly that the local Mink was ready. The one time I found estrusized River Otter blood was at a snow-covered latrine and covered about a six-by-six foot area. That’s a lot of blood, or so it seemed!

To me this is the ultimate getting to know your neighbors and great animal tracking. Better than any kill site, even better than finding dens, not that we need to compare. And the beauty is that, for a short window of time (let’s say March 10-24 to be safe), the bloody evidence of an estrusized Mustelid is laid out and left there for anyone to find. And maybe that anyone is you.  Whatever gets you out there, right?

So where is this all going again?

And in conclusion… I recently told my soulmate Amy that I would gladly give up ice and snow all winter if I was guaranteed snow from March 5th through the 20th. Of course, I would rather have snow before or after as well, but to have snow on the ground during Estrus Season is a dream that doesn’t happen every year by any means.

Now, you’ve been given this information ahead of time, in hopes you will go out and find some “Bloody Mustelid estrus” (BME – pronounced “Be Me”), take some photos and then send them in to me – . When it comes down to brass tacks this is a totally selfish post. You’re all out there and I want to learn from you. But not just about anything… I’m looking for blood (as they say), won’t you help me?

Whatever gets you out there for sure – but why not make it an estrus outing?

And so, a pre-emptive thank you and enjoy the search!

We’ll see you out there!

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