Counting placental scars

Muskrat on hind legs
(Seneca Falls, NY)

The course outline said we were going to practice estimating flock sizes as part of our non-invasive techniques unit, but a phone call from NYS DEC Biologist Scott Smith sent my Wildlife Field Techniques course in a whole different direction. Scott is a regular guest speaker each fall in my Wetland Mammals class. I think that is what made him think to call me this past Sunday night asking if I had any students that would be interested in assisting him in counting placental scars on muskrats this week. Instead, I pitched the idea that he bring the 'rats to us and do it as a demonstration with my new Wildlife Field Tech (WFT) class. He agreed and we arranged to receive them on Tuesday and start the thawing process.

Skinned muskrats
(Canandaigua, NY 2/13)

The photos just do not do justice to what 162 skinned muskrats look like. Nor do they even hint at what they SMELL like. They arrived frozen. Each five-gallon bucket seemed heavier than the last. I transferred them to Rubbermaid totes and soaked them overnight to be sure they would be ready for class on Wednesday afternoon. What you see here is the result of careful sorting and thawing: a finished product so to speak. The lids of the aforementioned totes made excellent serving trays. I finished the job with about an hour to spare before class. At this point, students were still expecting a brief lecture followed by time in the field. When class started, I informed them all of the last-minute change of plans. If anyone was disappointed, they hid it well. Of course, at this point none of them had yet to see (or smell) the specimens.

I had prepared a Powerpoint to give the students some context regarding the purpose and methodology of the technique of counting placental scars. As you can see, I chose to title this lecture "Muskrat love". I briefly covered some basic natural history of muskrats including their identification, habitat preference and their sign. Then it was on to vocabulary. Although I do not require my students to always use technical terms, they should be able to recognize them when reading journal articles. Any nursing student would have been at home for the next few minutes as we covered postpartum (the time following birth), parturition (the act of giving birth), uterus ("womb"; where fertilization and fetal development occurs) and placenta (the deciduos organ that supplies nourishment to the fetus). A final term, "placental scar" will take a little more explaining.

Basically, the placental scar is exactly what it sounds like: a scar left on the uterus at the point where the placenta attached. After birth, the placenta is shed and tears away some of the endometrium. As the damage heals, red blood cells pool at the site and a dark spot appears. In some (many?) species, this scar fades. But in muskrats at least, the spots seem to last from year to year. I hesitate to say "last forever", but perhaps that is true.

How would a biologist use these placental scars? Well, each placental scar represents a fetus. Each fetus represents a potential offspring. So biologists count placental scars an indirect measure of fecundity or reproductive effort. In a species like the muskrat, you may be measuring the lifetime effort (since the scars do not fade). In other species (like Arctic foxes) the scars fade and one may be only measuring the size of the most recent litter.



Muskrat tail

By now Scott had arrived and provided some additional insights into the process and how the State will use the data we were about to collect. Scott informed us that all of these muskrats came from a single individual whose trap lines were set in the Montezuma National Wildlife Refuge. I was excited to hear that because MNWR is in my hometown of Seneca Falls. The trapper had skinned each one for the prime fur. We were left with the rest. By now the students were visibly at the point where I could no longer expect them to sit still and listen -- they wanted to "do"! We headed to the shop where I covered a little of the anatomy of the muskrats while Scott made his last preparations.

Front foot of a muskrat

I love the tail of muskrats! They are compressed like the body of an eel and have a really scaly surface. Since I have been into tracking for the past few years, I have been paying more attention to the feet of animals, too. Here is a front foot with the five (yes, five) front toes of a muskrat. Look closely for the stunted inner toe (where your thumb is). I told the students that many tracking field guides provide practical information that may end up being technically incorrect. For example, I have often read that muskrats have four front toes. Well, when you closely examine a detailed track, the fifth toe appears as a dot. And in less than perfect substrate, you wont even see that.... Look closely at the photo of the foot and decide whether you are looking at a right or left front :)






I had one last thing to show the students: the cheek muscles. Just look at the massive masseter muscles! These are the muscles responsible for grinding food, so they are particularly well-formed in plant eaters. Specifically, they are to the right of my thumb and there is a superficial slice present (sorry, didn't even notice that when I was taking the photo).

Masseter muscles of a muskrat

Finally, we begin. Here is Scott ready to show us the proper way to open the muskrat and remove the uterus for inspection on a lightboard. But wait, there is a problem: this one is a male.....

Scott Smith, NYS DEC Biologist

A close up of the male shows the testicles and the musk glands that give this particular species of rat its name:

Male muskrat

The arrow is pointing to a testicle. The other bean shaped item in front of the gonad is the musk gland. We cut one open and managed to detect a slight odor.

The first few females were all young ones with no placental scars as they had yet to breed and produce offspring. Muskrats, like many rodents are heavily r-selected. Their population structure is heavily weighted towards young individuals. The strategy is basically to have many young as most will not survive to adulthood.

Scott set out a scar-less uterus on the light board:

 Here it is close up. The uterine horns show no scars.

Uterine horns of muskrat

"Ah, there we go!", was our first indication that Scott had found our first placental scars of the day. Here is a photo of the uterus removed and on the light board:

Placental scars of a muskrat

 Above, Scott uses his gloved hands to point out the features on the two uterine horns. This uterus is shaped very differently than the ones I remember from tenth grade Health class. The theory is that the two "horns" provide ample space for a large litter. Can you make out the scars on the left side of the photo? There are four distinct dots in the tube-like uterus.

Here is what it looks like when the uterus is in situ:

Placental scars of a muskrat

Wow! Notice above the many scars visible in each horn. We removed it for close inspection:

Placental scars of a muskrat

Again, this probably represents the entire reproductive history of this individual. When the scars remain visible over several years, it is difficult to count them as their will be overlap from litter to litter. You can see some of that in this case.

The students seemed to really enjoy the process:

These animals were harvested for their fur and the carcasses were utilized to help monitor the species and make management decisions for the future. My students were exposed to a new technique and learned other muskrat facts along the way. I didn't want the utilization to end there, so as Scott and the students put the muskrats into the "processed" tote, I went in and removed the heads by hand so we could clean the skulls. I have a thing for skulls, so this was grizzly but pleasant work.

A man that loves his work

But there was one last use for these 'rats. Students helped load all 162 carcasses into my pick up and this morning, with the help of my ATV and trailer, I created a new camera trap set. That's just gotta be better than having them take up landfill space..... stay tuned for that blog post :)