Blocking reproduction increases lifespan in both males and females of many different species, a new international University of Otago – Ōtākou Whakaihu Waka-led study has found.
Using data from mammals housed in zoos and aquariums worldwide, the researchers found ongoing hormonal contraception and permanent surgical sterilisation were associated with increased life expectancy.
Lead author Associate Professor Mike Garratt, of Otago's School of Biomedical Sciences, says that while the relative increase in lifespan was similar across sexes, the cause may be different in males and females.
"In males, only castration extends lifespan – not vasectomy – which indicates that the effect comes from removing sex hormones. These hormones may interact with pathways that regulate the biology of ageing, particularly during early-life development, since early-life castration has the strongest effects on lifespan. Health in later life is also increased by castration in laboratory rodents."
"In females, lifespan increases after several different forms of sterilisation, suggesting that benefits arise from reducing the substantial energetic and physiological costs of pregnancy, lactation and caring for offspring, rather than from a single hormonal mechanism," he says.
The study, published in prestigious international journal Nature, included analysis of 117 different species, along with a meta-analysis of other vertebrate sterilisation studies.
The researchers found life expectancy increased by 10 to 20 per cent, depending on the timing of treatment and the environment the animal was exposed to.
Co-author Fernando Colchero, of the Department of Primate Behavior and Evolution of the Max Planck Institute for Evolutionary Anthropology, Germany, says lifespan is shaped by trade-offs with reproduction.
"Reproduction is inherently costly, although environments can soften or exaggerate these costs, particularly human environments, which can buffer or modify them thanks to healthcare, nutrition and social safety."
For males, blocking reproduction also meant they were less likely to die from behavioural interactions involving aggressive behaviour or risk seeking.
For females, they were less likely to die from infection and infectious diseases. This is believed to be related to the high energy costs of reproduction lowering mothers' immune-defence systems.
Associate Professor Garratt says the study supports arguments for the evolutionary benefits of menopause.
"Unlike in males, we found no evidence that the effects of contraception on survival are dependent on the timing of sterilisation, so not reproducing in later adulthood, even after already producing some offspring, might provide later-life survival benefits in females.
"However, while lifespan was increased by a range of sterilisation approaches, in the case of ovary removal, which also removes ovarian hormone production, aspects of health in later life are impaired.
"These findings on lifespan and healthspan shed light on the health-survival paradox observed in post-menopausal women, who outlive men on average but suffer increased frailty and poorer overall health during ageing."
