Well these things may er describe erm  sexual behaviour and how to facilitate it, but they can't explain it and nor are they really fundamental, because after all okay in mammals males may be the sex defined by the X chromosome, but in birds it's the other way round in birds it's the female  chromosome and all, in other words, all bird embryos start, start out as male and differentiate into females if they're going to, whereas in mammals all embryos start out as female and differentiate into males, in so far as those terms have any sense.