Animal protection advocates call attention to pet overpopulation. According to the Humane Society of the United States, animal shelters care for about 6 to 8 million dogs and cats each year, but approximately 3 to 4 million are euthanized.[27] A fertile cat or dog can produce up to two litters per year containing four or more kittens or puppies per litter.[28] Not every pet entering a shelter is a stray. Many pets are surrendered every day because of reasons such as moving, cost, allergies, too many pets, no time for pet, illness, and personal problems.[29] Local humane societies, Societies for the Prevention of Cruelty to Animals (SPCAs), and other animal protection organizations urge people to spay or neuter their pets, as well as to adopt animals from animal shelters instead of purchasing them from breeders or pet stores.