Spay and neuter programs reduce companion animal overpopulation, preventing the suffering of millions of animals born into homelessness and euthanized in shelters annually.
The animals most affected by inadequate spay and neuter are those born into homelessness and euthanized before adoption. The welfare of these animals — the fear, cold, hunger, and disease of life as strays, followed by euthanasia in shelters — is the primary welfare cost of overpopulation. Intact female animals cycle repeatedly, with each pregnancy carrying maternal welfare risks and producing offspring whose welfare outcomes are uncertain. The cumulative welfare benefit of population control through sterilization has been the single largest animal welfare intervention of the modern era.