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.

The Cephalic Index

Skulls that are brachycephalic and dolichocephalic, respectively

The cephalic index, also known as the cranial index, is the source of a series of terms that are really rather commonly used in such things as the world of dog fancy. However, it's likely that a lot of people out there don't actually know that the terms mean. It is a way of looking at the anatomy of the skull. The cephalic index is basically a percentage calculated by comparing the length of the skull to the skull's width. Specifically, it's the width multiplied by one hundred and divided by the length. If the index is seventy-five to eighty, the skull is oval in shape, also known as mesaticephalic or mesocephalic. If the index is less than seventy-five, the skull is long and narrow, also known as dolichocephalic. If the index is more than eighty, the skull is short and broad, also known as brachycephalic. If the index is more than eighty-five, the skull is even shorter and broader and can be called extremely brachycephalic or hyperbrachycephalic. In the past, and indeed currently, these designations have been used to describe different racial designations, which usually share very similar characteristics. Some populations have very long skulls, and thus tend to be dolichocephalic. Others have broad skulls and trend toward brachycephalic. Others are mesaticephalic, and still others are hyperbrachycephalic.

Dolichocephalic, index 64

Mesaticephalic, index 79

Hyperbrachycephalic, index 94

Of course, when looking at humans and the cephalic index designations there is one major characteristic that is lacking and is very important when applying the same terminology to dogs or other domestic animals: muzzle. Humans don't have a muzzle, so the cephalic index is almost entirely based on the shape of the brain case. This is far from true when looking at dogs, since dogs have muzzles. As such, the muzzle plays a major role in determining a dog's cephalic index. Generally, dogs with long muzzles will be dolichocephalic, and dogs with short muzzles will be brachycephalic. However, this is not always the case. For example, there are dogs with fairly moderate muzzles that would be classified as brachycephalic because they have very broad skulls.

Technically brachycephalic

In a series of posts to come, I will be discussing more about the cephalic index in dogs and why it can be so important.

Sources are Scientific Electronic Library Online, Encyclopedia Britannica, Encyclo Online Encyclopedia, . Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three, four, five, .