A castrated male mammal: Teddy, aka Puppers.
A sign at a nearby dog park specifies that only neutered (i.e., castrated) dogs may enter. “Intact” dogs are not welcome. After all, intact dogs might be tempted to mate with other intact dogs and have unwanted offspring. Intact male dogs might also be more likely to engage in aggressive behaviors against other males. The dog world may be a friendlier one without lots of testosterone coursing through males’ bodies. The tamed capacities of castrated male animals have been discovered in many other societies, and not just the local dog park: the Tiv castrated dogs, the Chuuk cats, among many such cross-cultural cases, with the oxen (castrated bulls) large symbols of the neutered male mammal.
What about the effects of castration on male longevity? In the dog world, it has long been appreciated that castrated males outlive their intact brethren. There may be a variety of reasons, with those including effects on behavior, and perhaps also the cardiovascular and immune systems. By definition, castrated males are freed from testicular cancer, and also have lower incidence of prostate cancer.
A new paper is just the third to ask about the effects of castration on human male longevity. In a Korean study of eunuchs, relying on historic data, Kyung-Jin Min and colleagues showed that a sample of 81 eunuchs lived on average around 70 years. That was about 15 years longer than intact men of similar socioeconomic background. Three of those eunuchs lived to 100 years of age, a far higher percentage than found among any general human population. These were eunuchs castrated at young ages either by accident or intentionally (an aid to a desirable court position), but before the effects of puberty had occurred. In the two previous studies on human male castration and lifespan, castrated inmates at a Kansas mental institute lived longer lives than intact inmates, with those differences amplified the earlier they had been castrated. A study of Italian singers found no differences in the lifespans of castrati and non-castrati, however.
Why would castrated men live longer than intact men? From an evolutionary perspective, answering that question beckons to the ultimate in currencies: reproductive success. Evolution favors traits that enhance reproductive success, rather than survival. Gifting males with testes that release testosterone to masculinize the brain, to upregulate oxygen carrying capacities, to put on more muscle—those things may favor increased success in male-male competition, and in turn potential reproductive benefits. Those same effects may also lead males toward fighting injuries that become infected, toward arteries that can be more readily blocked (thanks to the "thicker" blood helping carry oxygen), toward a higher energy budget to support more muscle. What’s good for rough-and-tumble fathering may not be ideal for a long, calm life.
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Natterson-Horowitz B and Bowers K. 2012. Zoobiquity: What Animals Can Teach us About Health and the Science of Healing. New York: Knopf.
Nieschlag E, Nieschlag S, Behre HM. 1993. Lifespan and testosterone. Nature 366: 215.