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:

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:

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

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.