Sex and gender is something we keep arguing about, and one of the central arguments of those who think it's immutable is that XY = boy and XX = girl.

I could talk all day about how that varies in humans, but I actually feel like talking about how sexual differentiation works in things that are not like us. Suffice to say that you can be an XY girl, an XX boy, and...well...all kinds of variants.

But what about other animals? Mammals work much like we do, but what about birds? Reptiles? Insects?

As a note, we're going to use the correct biological definitions of male and female here.

A male animal produces smaller and typically more active gametes such as sperm.

A female animal produces larger and typically more passive gametes such as eggs.

You may have heard some variants of this from transphobes...they're using it wrong, but this is the basic biological difference.