Heat cycles bring hormonal changes that can lead to personality changes. Neutering refers to the surgical removal of a male companion animal's testicles. Castration is another term for the same thing. Males who have been neutered cannot manufacture sperm. In reality, neutering a male is similar to spaying a female.
Female dogs go into heat every six months usually starting at six months of age.
Dogs and cats are pregnant for 63 days. Nursing dogs and cats CAN get pregnant. In 6 years, one female dog and its offspring can be the source of 67,000 puppies. In 7 years, one female cat and its young can produce 420,000 kittens. 70,000 puppies and kittens are born every day in the USA alone.