Determining The Difference Between Red Fox and Eastern Coyote Skulls
Found skull, 14 cm long. 2023.01.02 Antler Woods
The other day I went out to track White-tailed Deer (Odocoileus virginianus) and anyone else I might encounter. I went to the swampy Eastern White Cedar (Thuja occidentalis) woodlands where I found a beautiful pair of antlers a couple of years ago in hopes that while out tracking the Deer I might come across another antler. Instead, when I walked into the low thickety edge of the darker Cedar woods, I found some bones and eventually a skull. While examining the skull a couple thoughts came really quickly. The first thought was “byron’s rule” which states that it is always a Raccoon (Procyon lotor) until proven otherwise. It didn’t take long to notice that the rostrum (sometimes referred to as the muzzle), the nasal part of the skull, was too elongated to be a Raccoon. I then moved on to the second thought; this animal was still certainly a carnivore, based on the location of the eyes and the structure of the teeth (not pictured). So I got to wondering Red Fox (Vulpes vulpes) and Eastern Coyote (Canis latrans). But a new question came up. How do we distinguish between Red Fox and Eastern Coyote skulls? I had some more quick ideas, but I decided to take a lot of photos and a couple of measurements to help some better i.d. when I got home.
Red Fox (Vulpes vulpes) on left and Eastern Coyote (Canis latrans) skull on right, dorsal view, both of which I had at home.
Size
Back in the woods, I figured one way to tell the difference was definitely size. I had noted that the length of the skull I found was 140 mm (skulls are measured in millimeters) which was the same as the Red Fox skull I measured when I got home. The Coyote skull I also had at home and compared with came in at 194 mm, much larger. So size seems to be a pretty reliable means to differentiate these two animal skulls, but what if the skull was from a juvenile, not yet fully grown? Are there additional factors which can help tell the two species apart?
Teeth
The dental formula is the same for both species, 3 incisors on the top and 3 on the bottom, canines 1 and 1, premolars 4 and 4, and finally molars 2 on top and 3 on the bottom for total 21 on one side of the skull. We then double that number for a total of 42 teeth in the whole mouth (remember that we count the teeth on the cranium first, then on the mandible, reading one side of the skull, and then we double for the total amount of teeth). So that wouldn’t help to distinguish the two species.
Post-orbital processes
Another possible differentiating feature could be the pointiness of the post-orbital processes (the sticky-outty bits behind the eye which support where the eye would be). In the two skulls I have at home the Red Fox has sharp post-orbital processes which point towards the posterior (the back) of the skull, while the Coyote’s post-orbital processes appear to be more rounded, but this could be because of wear or the animals younger age. Post-orbital processes tend to grow longer and stick out even more as an animal gets older so maybe the Coyote was younger?
Unknown skull I found, dorsal view.
Unknown skull I found, ventral view.
Red Fox (Vulpes vulpes) on left and Eastern Coyote (Canis latrans) skull on right, ventral view.
Projections over pterygoid region
Something Mark Elbroch notes in his book Animal Skulls (Stackpole Books, 2006) is that the skulls of some Red Foxes may show small pointy projections over the pterygoid region. These little pokey bits of bone stick out from the back of the palate, out over the rim of the hole in the middle of the skull between the eyes. You can see the gallery to the right and notice how two of the skulls are similar, the one I found in the forest, and the Red Fox skull I had at home, while the Coyote skull lacks this projection. So, in considering the size, and this projection, I am leaning towards Red Fox skull.
Sutures
One more thing I noticed, and this isn’t mentioned anywhere in the literature so I don’t know how reliable it is for identification, is that the orientation of the sutures (the nearly perforated lines between different bones of the skull) are at different angles for the Red Fox and for the Coyote. The sutures occur where three bones of the skull, the maxilla, the nasal bones, and the frontal bone, come together. These sutures are near the posterior of the rostrum. It seems to me that the angle of the sutures for the Coyote are at a 45° angle, while the Red Fox angle seems to be about 90° angle, similar to a capital “L”. Scroll through the images to the right to see the sutures highlighted. I consulted with the Animal Skulls book as well, and while there is nothing mentioned in the text, this feature does appear in the illustrations of the skulls. I have also found this to be true through a google image search of many Red Fox (1)(2)(3)(4) and Coyote (5)(6)(7)(8) images (note image 8 has both Coyote and Red Fox skulls). I thought this was a pretty interesting observation and I hope to get more chances to observe this on other skulls I find.
The size : 140 mm = Red Fox
Pointiness of the post-orbital processes : pretty pointy = likely Red Fox
Projections over the pterygoid area : visible = Red Fox
Angle of sutures at posterior of nasal bones : 90° = Red Fox
I think the math adds up to Red Fox. How cool to find a mystery skull and the practice paying close attention and observation to determine who’s skull it was in the end!
I wonder at how these differences between the Red Fox skull and the Coyote play out in their daily lives? I can see how the smaller skull of the Red Fox relates to a more consistent diet of smaller food items, but what do the longer post-orbital processes relate to? Do Red Foxes have larger eyes? And the projections over the pterygoid area and the sutures at the posterior end of the nasal bones, what do those imply? I have noticed that the 45° angle sutures are similar to those found on Red Wolves (Canis rufus) and Algonquin Wolves/Eastern Timber Wolves (Canis lycaon) which is kind of cool because Coyotes are much more closely related to these Wolves than the Red Fox is. It’s thought that Coyotes only broke off evolutionarily from these two Wolves about 300,000-150,000 years ago, not much time to change how the skull grows and is shaped, which is really exciting to note when holding the skulls in your hand and getting to see the relationships written in the bone.
Noticing the differences can sometimes feel like Aristotelian compartmentalization and overly logical, putting things into boxes, but I think noticing difference isn’t just reducing and compartmentalizing, but instead can reveal individuality, subjectivity, identity and on larger scales, help to understand the natural history and experiential differences between animals involved. To disregard differences and to pretend they do not exist means we aren’t being good trackers and we aren’t paying attention. This disregard of variety and variance can be associated with not heeding to the diverse experiences of life and lifeforms, but instead holding up human centric preconceptions. I guess what I am getting at is that instead of pretending that we do not notice difference, or that assuming noticing difference is bad, we need to be practicing examining difference, exploring what makes us and others distinct, and remembering that distinction and diversity are awesome and beautiful.
Big gratitude to the Red Foxes and Coyotes for teaching me so much.
To learn more :
Animal Skulls by Mark Elbroch, Stackpole Books, 2006.