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Wednesday, April 3, 2013

Are Striped Skunks Symmetrical?

Are striped skunks symmetrical? That might sound like an odd question but I have a very good reason for asking. One way camera traps are used in research is to determine the presence of species in an area. At its simplest, think of this as marking off species on a checklist each time one is
Three striped skunks at bait
photographed. At a more advanced level, multiple cameras are used to determine the presence of a species over a wider area and perhaps a range of habitats. In this case, the researchers are striving to calculate more than the mere presence of a species but maybe its relative abundance or more. And in species where individuals can be distinguished, camera traps can be used to conduct a mark-recapture study to make an estimate of population size.


 Much of the published literature I have seen involving individually recognizable animals involves cat species. Think of tigers, snow leopards and cheetahs as animals with distinctive and unique markings. In North America, our distinctively marked species are fewer and when we further narrow the field to the portion of New York where I camera trap, the list is downright pathetic. White-tailed deer (antlered individuals only!), bobcats (still a real rarity around here) and striped skunks are the mammals that come to mind. Perhaps there are others, but these are the ones I am confident in.
I have written about striped skunks before. In this post from last year, I describe numerous individual skunks photographed on my property. One limitation in determining individuals is that some skunks
Bipedal stance of striped skunk
would be photographed in their left profile while others would be oriented with their right profile to the camera. I could pair camera traps and therefore capture images of both sides of  every skunk that passes by but if I were able to safely say that striped skunks had symmetrical body markings (meaning that their left sides matched their right sides), then I would not need to pair cameras and could compare skunk photos regardless of the side captured by the cameras. Make sense?
Since this original idea came to me, I have conducted an incomplete review of the literature and not found anything written on this subject. Perhaps I am not searching properly or perhaps no one has found the need to write about this. But just maybe I have come up with an idea that is worth answering that has not been formally investigated. I thought an easy way to look at many skunks at once would be to attend fur auctions where trappers bring furs of all kinds to be sold. Alas, that plan has yet to make it to the front burner. And another fur season is past. Fortunately, an opportunity presented itself recently and I was able to obtain 162 muskrat carcasses to use as camera trap bait. I chose one large pile rather than several smaller ones and thought this would be the perfect first test of the brand new Reconyx camera I have access to through an undergraduate research grant. With literally thousands of striped skunk images over the last five weeks, I have some evidence to help me answer my question.

First, here is a look at two very white striped skunks. This photo is from a Cuddeback camera I also placed at the bait pile:
Striped skunks at a bait pile
 Let's call these SK01 and SK02. I only have a single image of these skunks during this visit, so there is nothing to help answer our symmetry question, but clearly we have two individuals so our population is at least two.
This next photo is only two days later and is also of multiple skunks but looky looky it appears to be two DIFFERENT skunks (probably three different skunks as that one in the back just seems much blacker than the two skunks in the first image), SK03 and SK04. I will not go through all the skunk photos in this entry and in fact have a student who is trying to tackle that instead. 

Striped skunks and opossum at a bait pile
I have the Reconyx camera set up to take a burst of three photos together and then to take another series of three as fast as the camera can rearm itself. This makes for many many images of the same individual and for my purposes here, it often provided images of rights and lefts of the same individual.


 Let's start with the photo on the right. As this particular stinker walks towards the camera, we get a sense that he/she is symmetrical. There is a bit of a notch in the shoulder area of both sides and the tail is uniformly white. However, we are missing the whole hindquarter. So what we really need are profiles. I will provide you two individuals in this post just to get us started, but before I do you should stop for a moment and guess at my results: symmetrical or no? I will assume those who would answer "Who cares?" stopped reading long ago....










EXHIBIT A: Again, remember that I know this is the same skunk in both profiles because of the almost continuous nature of the photos from the Reconyx camera.


Note the thickness of the stripe on this skunk's right shoulder or front quarter. Now scroll down and see the left shoulder of this same skunk....













Yipes! Look how thick the white mark is on that side. I was a bit surprised as I really thought my efforts were going to show that these guys were symmetrical. Let's look at one more:















EXHIBIT B: Again a right and a left profile from the same individual


Sigh... Again, there appears to be a marked difference. I have considered the idea that in THIS case, the difference may only be perceived due to body positioning, but I don't think that is the answer. Note that the photos are 12 minutes apart. Well, in that entire time this individual was feeding on the bait. I have many photos of this guy and I think these two represent the profiles the most accurately of them all.

A few words of caution: First, two samples is not enough to draw huge conclusions. And second, the asymmetrical skunks may stand out and were easier to find in my cursory review of the thousands of pieces of data from this camera. But one thing seems certain: Not all striped skunks are symmetrical. That makes the job of estimating their population harder. However, I am a man who tries to look at the bright side of things. I now know I need to photograph skunks from both sides in order to obtain the data necessary to make a proper population estimate. I believe I have just found the justification to ask the grantors for a SECOND Reconyx camera :)








2 comments:

  1. Really, really interesting post JVN. Are "the researchers" your wildife tech students? Or you?

    Also, I've seen a study on IDing individual BLACK BEARS...as they stand up to rub and bite on trees, a camera trap was trained at the side most frequently marked. The researchers were using chest blazes as ID. Still a stretch, but cool.

    ReplyDelete
    Replies
    1. Yes, the student I mention is in that class. I have not seen the black bear study you reference but I do have a thesis that was done in Ecuador on Andean bears.

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