[Emedgella]

If you do not spay an animal, and she is not able to become pregnant, she will constantly go into heat for nothing. Some animals are susceptible to what is called phantom pregnancy, where the physical symptoms of pregnancy occur. It's really not kind to keep a 'whole' animal unless you are a knowledgeable breeder.

I'm not sure animals' desire to have babies is very much like the human desire. One hopes we don't want babies just in order to have something to care for and to carry on our genes: we want babies who will be part of our family. Given that most animals don't seem to remember their children once these are grown, I don't think the same is true of them!