I just got into a book I received some time ago:  "101 Questions Your
Cat Would Ask Its Vet (If Your Cat Could Talk)".  It's by a British vet
- Bruce Fogle - and contains a number of cat facts I'd never
encountered, even though I've been owned by cats for most of my adult
life, and have read quite a few books about them.

I had read that cats have a double uterus (which supposedly accounted
for the fact they can have litters fathered by more than one tom).  But
I didn't know that, unlike all other mammals, a female domestic cat does
not ovulate until AFTER she has mated!   Being "in heat" for a female
cat is not the same as for a female dog (which is why there's no vaginal
discharge, only behavioral clues to her condition).  Usually it takes
several matings before she is fully fertile, so if she is running free,
and has "accomodated" several toms (and has all that mingling of sperm
swimming about looking for an egg), who can tell which is the father of
how many of her kittens?  (I suppose DNA testing might provide an answer
- assuming all the possible fathers were available - but I doubt whether
anyone would want to go to that kind of trouble and expense!)

It's a delightful (and informative) book, written with humour by someone
who REALLY understands cats - I highly recommend it.