• Two unaltered cats and all their descendants can theoretically number 420,000 in just seven years. ¹
• Two unaltered dogs and all their descendants can theoretically number 67,000 in just six years. ¹
• "No homes for littermates" is one of the top ten reasons people relinquish their cats and dogs to shelters. ²
• The top reason both cat guardians and dog guardians give for not having their pets altered is that they simply have not bothered to do it yet. 3
• Twenty percent of cat guardians think their cats are too young to be altered, and 18% say they are not able to afford spay/neuter surgery. ³
• Twenty-one percent of dog guardians want to breed their dogs, and 13% think their dogs are too young to be altered. ³
• An estimated 5 million cats and dogs are killed in shelters each year. ¹ That's one about every six and one half seconds.
• Tens of millions ⁴ of stray and feral cats struggle to survive on their own outdoors. Although some are altered and live in managed colonies, most are not altered and receive no health care. They reproduce at will and many suffer from illness or injury before dying. ⁵
• Over half (56%) of dog guardians and nearly two-thirds (63%) of cat guardians rank pet overpopulation as the most important pet issue. ³
• In a study of relinquishment of cats and dogs in 12 U.S. animal shelters, 30% of the surrendered dogs were purebreds. ⁶
• The same study indicated that 55% of the surrendered dogs and 47% of the surrendered cats were unaltered. ⁶
• It costs U.S. taxpayers an estimated $2 billion each year to round up, house, kill, and dispose of homeless animals. ⁷
• Over 56% of dogs and puppies entering shelters are killed, based on reports from over 1,055 facilities across America. ⁸
• Approximately 71% of cats and kittens entering shelters are killed, based on reports from 1,055 facilities across America. ⁸

References:

¹ The Humane Society of the United States Pet Overpopulation Facts (1999)
² National Council on Pet Population Study and Policy — The Top Ten Reasons for Pet Relinquishment to Shelters in the United States
³ The State of the American Pet—A Study Among Pet Owners. Prepared by Yankelovich Partners for Ralston Purina, October 2000
⁴ Alley Cat Allies
⁵ Alley Cat Rescue
⁶ Journal of Applied Animal Welfare Science, 1998, Volume 1, Number 3, p. 213
⁷ USA Today, June 23, 1998, pg. 1
⁸ National Council on Pet Population Study and Policy — Shelter Statistics Survey (1997 data)