Again and again in the animal world, males have shorter lifespans than females, an effect scientists attribute in part to the deleterious effects of testosterone.

Now researchers who looked at historical records of Korean eunuchs castrated during boyhood found that the eunuchs lived considerably longer than ordinary, testicled men.

"Our study supports the idea that male sex hormones decrease the lifespan of men," wrote researchers led by biologist Kyung-Jin Min of Korea University in a study published Sept. 24 in Current Biology.

Animal studies have led scientists to propose that evolution may have favored males who invest their energies into youthful reproduction, leaving them less fit later in life.

This "live fast, die young" effect could be mediated by testosterone, a hormone involved in testes growth, muscle development and aggression, but that also seems to have an immune system-weakening effect.

Whether increased testosterone or other male-centric biological traits are linked to shorter lifespans in humans, however, is unknown. Women do tend to live longer than men, but that could be for other reasons, including the longevity-enhancing effects of estrogen, the female sex hormone.

Men might also tend to engage in more high-risk activities. And while castration has been linked to longer lifespans in male lab animals, evidence from castrated people is limited.

For Min's team, historical records of Korean eunuchs, who from the 14th until the early 20th century were used as servants by royalty, allow for a one-of-a-kind real-world test of testosterone's effects.

Lifespan of Korean eunuchs and normal men. Boxes encompass the 25th–75th percentile of the data, with the median shown as a solid horizontal line. Image: Min et al./Current Biology

Working from a genealogical tome called the Yang-Se-Gye-Bo, the researchers were able to calculate the lifespan of 81 eunuchs. They lived an average of 70 years.

The researchers then looked at the lifespans of non-eunuch males from three families who enjoyed comparable social status, including royal family members and even kings. They lived an average of 56 years, or 14 fewer years than the eunuchs.

Several of the eunuchs lived for more than 100 years, a far higher number than would be expected even today.

"The current incidence of centenarians is one per 3,500 in Japan and per 4,400 in the United States," wrote Min's team. "Thus, the incidence of centenarians among Korean eunuchs is at least 30 times higher than that of present-day developed countries."

Interesting as these observations are, they're not conclusive. Given the historical remove, it's hard to rule out confounding factors, such as health traits unique to the comparison families.

"Similar studies on eunuchs in other cultures could be helpful for understanding of aging and reproduction in humans," wrote the researchers.

Citation: "The lifespan of Korean eunuchs." By Kyung-Jin Min, Cheol-Koo Lee, and Han-Nam Park. Current Biology, Vol. 22 No. 16, Sept. 24 2012

Image: Vijay/Flickr