From Why do pets end up in animal shelters?
According to the study:
The majority of the surrendered dogs (47.7 percent) and cats (40.3 percent) were between 5 months and 3 years of age.
Approximately half of the pets (42.8 percent of dogs; 50.8 percent of cats) surrendered were not neutered.
Many of the pets relinquished (33 percent of dogs; 46.9 percent of cats) had not been to a veterinarian.
Animals acquired from friends were relinquished in higher numbers (31.4 percent of dogs; 33.2 percent of cats) than from any other source.
Close to equal numbers of male and female dogs and cats were surrendered.
Most dogs (96 percent) had not received any obedience training.[/b]