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Many animal rights groups recommend that pet owners spay or neuter their dogs in order to help control the size of the pet population. However, a recent study indicates that there is a benefit to pets' lives themselves. A study performed by researchers from the College of Veterinary Medicine found that pets live an average of two years longer if they have been fixed, and die from other causes as well.

The study was based on the examination of data from the Veterinary Medical Database. By looking at 40,139 death records between the years 1984 and 2004, the researchers determined that intact dogs lived for an average of 7.9 years. Sterilized dogs, on the other hand, lived an average of 9.4 years. Dogs who had not been fixed were more likely to die from trauma or infection, while dogs who had been fixed were more likely to pass away from cancer or autoimmune disorders.

"There is a long tradition of research into the cost of reproduction, and what has been shown across species is if you reproduce, you don't live as long," Dr. Kate Creevy, an assistant professor of internal medicine at the College of Veterinary Medicine, said in a statement. "The question that raises is why would you die younger if you have offspring?"

This is not the first time that studies have pointed to a longer lifespan for people who cannot have children, though dogs are the closest species to humans that have been studied. Last year, a study found that eunuchs outlived typical men by as much as 20 years.

Researchers suggest that reproductive hormones, particularly progesterone and testosterone, may suppress the immune system, making people who have children more likely to die earlier than their peers.

"The overall average life span is likely shorter than what we would observe in private practice, because these were dogs seen at teaching hospitals, but the difference in life span between sterilized and intact is real," Creevy stated. "The proportionate effects on causes of death are translatable to the global dog population, and it will be interesting to see if explanations for these effects can be found in future studies."

The study was published in the journal PLoS One.