Q: Should we neuter Max, our 3-year-old Jack Russell terrier? Our veterinarian described the behavioral benefits of neutering, but those are not an issue for us. Max is healthy and lots of fun, and we just want him to live forever.  

A: Keeping Max around as long as possible is a good enough reason to have your veterinarian neuter him, because many studies show that sterilized dogs live longer than what veterinarians call “intact” dogs.  

The first study to examine the association between canine sterilization and life span was published in 1982. Researchers found that intact male dogs lived an average of 4.8 years, while neutered males lived 9.9 years. Intact females lived 4.7 years, and spayed females lived 8.6 years.  

In another study involving 721 dogs, the intact dogs lived an average of 9.6 years, while those that had been sterilized averaged 11.9 years.  

A third study reviewed the records of 70,574 dogs constituting 185 breeds; 43.6% were intact, and 56.4% were sterilized. The intact dogs lived an average of 7.9 years, while the sterilized dogs survived 9.4 years.  

In this large study, the causes of death differed based on sterilization status. Intact dogs were more likely to die of infectious disease, trauma, vascular disease or degenerative disorders, while sterilized dogs more often died of cancer or immune-mediated disease.  

The researchers noted that intact dogs probably die more often from infectious diseases because their immune systems are suppressed by progesterone and testosterone, reproductive hormones secreted by their sex organs. More often, I see their traumatic deaths: Intact males get a whiff of females in heat, escape their enclosures and get hit by cars far more often than neutered males.  

When gender was factored into this study, sterilization increased life expectancy by 13.8% in males and 26.3% in females. The greater sterilization benefit enjoyed by females is likely due to the high prevalence of fatal uterine infection and mammary cancer in intact females.  

Incidentally, it’s not just neutered male dogs that enjoy long lives. The report’s authors cite a study of historical Korean eunuchs and a 1969 study of intellectually disabled men whose testicles had been removed to prove that castrated men experience the same extension of life span.