New Zealand's most famous sheep has castration to partially thank for his fame, scientists say - suggesting it could also be an effective, if rather drastic, way to help Kiwi men live longer lives.
When wool-heavy Shrek was caught in April 2004 after six years on the run, he was already 10 years old -"roughly the maximum age of the most long-lived sheep on a commercial farm", according to Tim Hore, a senior lecturer in the University of Otago's Department of Anatomy.
"I think at least part of Shrek's fame was simply that he lived so long - something which almost certainly wouldn't have happened if he was not castrated."
In a new study, Dr Hore and colleagues have found removing male hormones from sheep delays the aging process.
It's common in mammals for females to have a higher life expectancy than males. It's true for sheep and it's true for humans too - at birth, the current life expectancy for Kiwi women is 83.5, but for men it's just 80.
"Both farmers and scientists have known for some time that removing male hormones by castration improves longevity compared to their intact counterparts; however, this is the first time anyone has looked at DNA to see if it also ages slower,” said the study's first author, University of Otago anatomy PhD student Victoria Sugrue.
They teamed up with University of California geneticist Steve Horvath, who invented the epigenetic clock in 2013 - a way of measuring biological age by looking at DNA.
"We have looked at over 200 species so far and discovered surprising commonality in which animals age," he said. "But the sheep study was unique in that it specifically isolated the effects of male hormones on aging."
The epigenetic clocks of castrated males' had a slower 'ticking rate' - their DNA not aging as fast as males with all their bits still intact.
"Despite being male, the castrates had very feminine characteristics at specific DNA sites," said Dr Hore.
"Interestingly, those sites most affected by castration also bind to receptors of male hormones in humans at a much greater rate than we would expect by chance. This provides a clear link between castration, male hormones and sex-specific differences in DNA aging."
The research has an immediate practical use for Kiwi farmers, they say - helping to identify which sheep might live longer and be more productive, or differentiate mutton posing as "succulent New Zealand lamb".
A previous study on castrated men in South Korea found they lived up to 19 years longer than others in their social class, BBC News reported in 2012. It wasn't clear from that research what was behind it, with some experts crediting lifestyle choices rather than something biological.
The University of Otago study was published Wednesday in journal eLife.