Sexing Red foxes (Vulpes vulpes) based on skull morphometrics

In a 1996 paper to the journal Biology and Environment, University College Dublin zoologist John Lynch studied the skulls of 210 foxes in the collections of Ulster Museum in Belfast. Lynch came up with a mathematical discriminant function to predict the sex of a fox based on its cranial morphometrics. In other words, he devised an equation that can give you a good idea of whether the fox skull in front of you came from a male or female animal.

Lynch presented this equation:

Fox Sexing Calculation

We don't need to worry about the maths of it (which is just as well, as I'm no mathematician!), all we need are two measurements to get us our D value. We need the total length of the skull and the constriction in the top of the skull just behind the orbital processes; both in centimetres. Here's what I mean:

Fox Skull Morphometrics

So, if we take these two measurements and plough them into the equation, we get the following:

Fox Sex Calculation - Worked

According to Lynch's data, a D (sex indicator) value of greater than zero (i.e. a positive number) indicates a male, while less than zero (a negative number) indicates a female. Therefore, in our example, we most likely have the skull of a vixen. This method isn't foolproof, but Lynch was able to correctly sex 86% of vixens and 87% of dogs.