All euthanized pets came from just one litter. Spayed and neutered pets are better, more affectionate companions. Spayed and neutered pets live longer, healthier lives. Spaying eliminates the possibility of uterine or ovarian cancer. Spaying before the first heat cycle greatly reduces the chance of breast cancer.
Neutering reduces the incidence of prostate cancer and disorders.
So is 1 out of 4 animals brought to shelters. There are simply too many cats and dogs - mixed and purebred.