What is up with female pets?

So my girl friend is an animal nut. She volunteers constantly at the local animal shelter and takes in "rescue" animals alot. Despite spending unthinkable amounts of money on vet bills and such she just can't help herself.

Recently a litter of feral kittens appeared in her garden shed. Luckily the mother cat moved all the kittens but for some reason left one behind. This little gray ball of hissing, spitting terror was pulled out of the garden shed and placed in a kennel cage.

So now the work begins. For instance: The kitten must be a certain weight to be put up for adoption, if it is too small the animal shelter will euthanize it (OK I didn't know this). The kitten must also be tamed, said shelter will not take feral animals and ones who spit and hiss are not adopted (OK makes sense to me). I named the kitten Helga. Why? Well because the first stage of "taming" the feral kitten is to keep it isolated and visit it with food and stuff slowly working your way up to letting it out to play some.

In short the Helsinki syndrome.

Well I must admit she has made stunning progress with this kitten and now it is simply a cute little ball of fur that LIVES to be played with and petted and is in her words "Ready for a home". This after yet another vet bill. The difference in just two weeks is just amazing I never thought that kitten was ever gonna be more than a little Tasmanian devil.

The final step in her Helsinki indoctrination program has been to introduce the kitten to other pets. OK she has a large a number of rescued pets I am not even going into how many but this is when I noticed the most curious thing in the whole process.

All female pets are mean... Not just stand offish but down right mean. This kitten can not get close to another female animal without being slapped or growled at. Yet the male pets are already treating the kitten as a valued companion.

There has to be something here. It is too much coincidence to ignore even the very friendly female border collie wants nothing to do with Helga but the male dog and cat are Helga's constant playthings. The males are downright protective of the kitten yet the females want to kill it when no one is looking.

Is this common in domesticated or wild animals? I have heard of animals adopting other animals but that has always been a female animal thing. It just seems to me that there must be some study out there about this if male animals are more protective than female ones.