Dog

According to the Humane Society of the United States, 3–4 million dogs and cats are put down each year in the United States and many more are confined to cages in shelters because there are many more animals than there are homes. Spaying or castrating dogs helps keep overpopulation down. Local humane societies, SPCAs, and other animal protection organizations urge people to neuter their pets and to adopt animals from shelters instead of purchasing them.
Neutering reduces problems caused by hypersexuality, especially in male dogs. Spayed female dogs are less likely to develop some forms of cancer, affecting mammary glands, ovaries, and other reproductive organs. However, neutering increases the risk of urinary incontinence in female dogs, and prostate cancer in males, as well as osteosarcoma, hemangiosarcoma, cruciate ligament rupture, obesity, and diabetes mellitus in either gender.