Preventing reproduction increases lifespan in many vertebrate species, according to research involving a University of Alberta evolutionary biologist.
Using data from zoos around the world, Shinichi Nakagawa and an international team of researchers found that sterilization and contraception increased life expectancy in both males and females by 10 to 20 per cent — but for different reasons.
"This is the first demonstration that an external intervention, other than dietary restriction, can prolong vertebrate life," says Nakagawa, professor in the Department of Biological Sciences and Canada Excellence Research Chair in Open Science and Synthesis in Ecology and Evolution. "Surprisingly, there's not much difference in the greater lifespan of males and females."
Nakagawa and his colleagues, led by Michael Garratt of the University of Otago in New Zealand, completed a comprehensive literature review of other vertebrate sterilization studies, along with a meta-analysis of data from 117 zoo species. The results were published this week in Nature.
According to the study's authors, only castration early in life extended lifespan in males, as opposed to vasectomy, indicating that the reduction of hormones such as testosterone made the difference by decreasing aggressive or risk-seeking behaviour.
In females prevented from reproducing — either by ongoing hormonal contraception or permanent surgical sterilization — increased life expectancy resulted from reducing the energy and physiological costs of pregnancy, lactation and caring for offspring, rather than from a change in hormones.
Although the study focused on zoo animals, there are implications for humans as well, say the authors, because they share similar physiological systems.
"Reproduction is inherently costly, although environments can soften or exaggerate these costs — particularly human environments, which can buffer or modify them thanks to health care, nutrition and social safety," says co-author Fernando Colchero of the Department of Primate Behaviour and Evolution at the Max Planck Institute for Evolutionary Anthropology.
Females who did reproduce were more likely to die from infection and infectious diseases resulting from a decrease in their immune defences.
The study also suggests that menopause in women has evolutionary benefits, say the authors, extending life expectancy by preventing pregnancy in later adulthood. But they also note that ovary removal can impair health in later life because it eliminates the production of ovary hormones.
