In 2013, a study conducted by researchers at the University of Georgia was published, based on data from the Veterinary Medical Data Base, including all canine patients who died at veterinary teaching hospitals in North America in a 20-year span from 1984 to 2004. Based on the age at death, the study found that spaying or neutering increased longevity by nearly 14 percent in male dogs and 26 percent in females. The average age of intact dogs at death was 7.9 years, while that of sterilized animals was 9.4 years. "The Atlantic" magazine said that's a decade longer in dog years.

The University of Georgia study found that spayed or neutered dogs were far less likely to die from trauma incurred in such events as motor vehicle accidents or fights than sterilized animals. Intact canines were more likely to die from infectious disease. However, sterilized dogs were more likely to die from various types of cancer -- with important exceptions -- and from immune-related disorders. Because cancer affects older animals, it's possible that the increased age of sterilized dogs means they are more likely to develop cancer. It's also possible that the increased cancer risk is due to taller growth for dogs spayed or neutered before puberty, as a link seems to exist between excess growth and cancer.

While a sterilized dog runs a greater risk of developing osteosarcomas and other cancers, spaying or neutering your pet greatly reduces, if not completely eliminates, the risk of developing testicular, prostate, mammary, uterine or ovarian cancers. Non-spayed female felines and canines have a 25 percent chance of developing mammary cancer. Approximately 60 percent of intact male dogs and cats develop enlarged prostates, which can lead to cancer.

It's not just that spayed or neutered pets are less inclined to roam, thus lowering the odds that they'll get hit by motor vehicles; they're also less aggressive -- so even if they do get loose outdoors, they're less likely to fight. Meanwhile, frequent pregnancies take a toll on a female animal's body. If a female pet is spayed, you don't have to deal with the bloody menstrual cycles of female dogs. Urine-marking is far less prevalent in neutered dogs and cats.