Friday, July 20, 2012

How to Calculate your Dog's Cephalic Index

Dogs have heads that vary a lot in size and shape
For those who don't know what the cephalic index is, please refer to my first post on the subject. As I mentioned there, the index is a comparison of the length and width of a skull. To calculate what index, you must first get some measurements. The two measurements you will need need to be taken from the absolute broadest part of the skull for the width and the absolute longest part of the skull for the length. To find these, it's easiest to look for bony protrusions.

For the skull width, gently feel between the dog's eye and ear. You should find a bar-shaped bone: the zygomatic arch. This projects out from the skull over the jaw muscles on either side of the head. Since dogs have rather strong jaw muscles, this will always be the widest part of the skull. If you're curious, check it out. Feeling under the ears will reveal a smaller skull width than what is seen at the zygomatic arch. To measure the skull width, don't simply lay a tape measure from zygoma to zygoma. Since the skull isn't flat, this will give you a falsely large measurement. This is why calipers are usually used to make these measurements. You can jerry-rig a set of calipers using two straight objects and a ruler, but this is difficult and there are easier ways to do it, which I will discuss later.

For the skull length, find the body protrusion on the back of the skull: the occiput. It should be fairly simple to find, sitting between the ears and often very close to where the head meets the neck. The prominence of this can vary a lot by breed, so if you have a dog with a very round skull you may not be able to find it. If this is the case, take the measurement from the farthest protruding part of the skull you can find. Measure from this point to the tip of the nose.

What would be the easiest way to get these measurements? Well, a lot of dogs would probably not be very happy with you messing with their head, so an alternate method of finding these measurements is using pictures. If you take a photograph of your dog's head from above, you can use this to measure the CI. Units don't matter, so you can use one measurement to figure out the other. Here's me doing this with Ebon:

Width is one unit and length is two and eight-one hundredths units. So, after calculating the CI ((1 x 100)/2.08), I found out that Ebon has a cephalic index of just slightly over forty-eight. This is really rather small, definitely less than seventy-five, and makes him dolichocephalic (for those that are curious, my dad's greyhound, Siggy, has a CI of 44.6). Now, I'll calculate the index for a bulldog:

The length is one unit and the width is one and fifty-five-hundredths. After calculating the CI ((1.55 x 100)/1), I found that this bulldog has a cephalic index of one hundred fifty-five. This is rather ridiculously large since the scale usually only goes to one hundred, and may be off as the dog's head is somewhat tilted. Either way, this dog is most definitely hyperbrachycephalic as his CI is far over eighty-five.

So, can you calculate your dog(s)' cephalic index? If you do, let me know what you come up with.

The First and third images are from Wikimedia Commons. The first is under a Creative Commons license and the third is copyright free. The second image is mine.

1 comment:

  1. Hi,
    I just discovered your blog a few weeks ago and really enjoy it. I used your post on Merle verses Ticking/Roan to explain to a woman that even though her dog was listed at the shelter as an Australian Cattle Dog cross, her blue merle dog was more likely to be a Australian Shepherd mix.

    I decided to measure my collies (Fawkes & Lucy) to calculate their CI. I have a set of calipers and was easily able to get accurate measurements of width but it couldn't open wide enough to get length. That said, with their shallow stops, it's not difficult to get a fairly accurate length measurement with just a ruler. Fawkes' head is 9.9cm wide x 27cm long, Lucy's is 9.5cm wide x 24cm long. If I did the calculations right, Fawkes' CI is 37 and Lucy's is 40. I think it's safe to say they are dolichocephalic.