High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! The phenomenon of overpopulation in companion animals refers to the large, growing number of homeless domestic cats and dogs. In the United States alone, between 3 and 4 million cats and dogs are euthanized each year because no one volunteers to adopt them. As a result, most humane societies, animal shelters and rescue groups urge animal caregivers to have their animals spayed or neutered to prevent the births of unwanted and accidental litters.