from the ASPCA web site:
Top 10 Reasons to Spay or Neuter Your Pet
1. Your female dog or cat will live a longer, healthier life.
Spaying—the removal of the ovaries and uterus—is a veterinary procedure performed under general anesthesia that usually requires minimal hospitalization. Spaying a female cat or dog helps prevent pyometra (pus-filled uterus) and breast cancer. Treatment of pyometra requires hospitalization, intravenous fluids and antibiotics. Breast cancer can be fatal in about 50 percent of female dogs and in 90 percent of female cats. Spaying your pet before her first heat offers the best protection from these diseases.
2. There are major health benefits for your male animal companion, too.
Besides preventing unwanted litters, neutering your male dog or cat—the surgical removal of the testicles—prevents testicular cancer, if done before six months of age.
3. Your spayed female won't go into heat.
While cycles can vary greatly, female felines usually go into heat four to five days every three weeks during breeding season. In an effort to advertise for mates, they'll yowl and urinate more frequently—sometimes all over the house. Unspayed female dogs generally have a bloody discharge for about a week, and can conceive for another week or so.
4. YouR male dog won't need to roam away from home…
An intact male in search of a mate will do just about anything to get one! That includes digging his way under the fence and making like Houdini to escape from the house. And once he's free to roam, he risks injury in traffic and fights with other males.
5. …and he will be much better behaved to boot!
Neutered cats and dogs focus their attention on their human families. On the other hand, unneutered dogs and cats may mark their territory by spraying strong-smelling urine all over the house. Indoors, male dogs may embarrass you by mounting on furniture and human legs when stimulated. And FYI, a neutered dog protects his home and family just as well as unneutered dog--and many aggression problems can be avoided by early neutering.
6. Spaying or neutering will NOT make your pet fat.
It's no use to use that old excuse! Lack of exercise and overfeeding will cause your pet to pack on the extra pounds—not neutering. Your pet will remain fit and trim as long as you continue to provide exercise and monitor food intake.
7. Spaying or neutering is highly cost-effective.
The cost of your pet's spay or neuter surgery is a lot less than the cost of having and caring for a litter. It also beats the cost of treatment when your unneutered tom escapes and gets into fights with neighborhood strays…or the cost of cleaning the carpet that your unspayed female keeps mistaking for her litter box, or the cost of…well, you get the idea!
8. It's good for the community.
Stray animals pose real problems in many parts of the country. They can prey on wildlife, cause vehicular accidents, damage the local fauna and scare children.
9. Your pet doesn't need to have a litter for your children to witness the miracle of birth.
We've heard this one a lot. But you know what? Letting your pet produce offspring you have no intention of keeping teaches your children irresponsibility. Anyone who has seen an animal euthanized in a shelter for lack of a home knows the truth behind this dangerous myth. There are countless books and videos available to teach your children about birth in a responsible manner.
10. It packs a powerful punch in the fight against pet overpopulation.
Millions of cats and dogs of all ages and breeds are euthanized annually or suffer as strays. These high numbers are the result of unwanted, unplanned litters that could have been prevented by spaying or neutering.