Contraception, Castration Boost Mammal Lifespan

Comparative study across 117 mammals shows that blocking reproduction increases life expectancy

To the Point

• Extended lifespan: Blocking reproduction extends lifespan by approximately ten percent across many mammals, sometimes more.
• Reproductive block: Male longevity rises only with castration (testosterone effect), while female lifespan benefits from any reproductive block, likely due to lower energy costs of pregnancy and lactation.
• Causes of death: Castrated males face less death from aggression/risk-taking while females with blocked reproduction have fewer deaths from infections.

Why do lifespans vary across different species? A female elephant can live up to 80 years, but usually only has a few calves during her lifetime. A mouse, on the other hand, lives for a few years at most, but can theoretically give birth to more than a hundred offspring. Evolutionary theory explains such differences by a fundamental trade-off: species must balance energy between reproduction and maintaining their own bodies. A new large-scale study now provides new evidence for these trade-offs across many species of mammals, including humans.

An international research team, including scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig, examined how different forms of reproductive suppression affect longevity in mammals. Using data from 117 mammal species housed in zoos and aquariums worldwide, combined with a meta-analysis of 71 published studies, the researchers found that ongoing hormonal contraception and permanent surgical sterilization increase life expectancy by on average ten percent.

The reproduction - survival trade-off

Reproduction is costly. Pregnancy, lactation, sperm production, mating behaviours, and parental care all demand substantial energy. Even when an animal is not actively breeding, sex hormones such as testosterone and estrogen continue to influence growth, behaviour, and ageing, consuming resources that could otherwise support bodily maintenance. "Zoos, where reproduction is carefully managed, provide a unique setting to study these dynamics", says Johanna Stärk, one of the authors. "Animals may receive contraception or sterilization to prevent breeding, creating natural comparison groups within the same environment."

Lifespan-enhancing effects were found in many mammalian groups-from primates to marsupials to rodents. Some species showed particularly striking effects: for example, female hamadryas baboons receiving hormonal contraception lived 29 percent longer lives, while castrated males lived 19 percent longer. "This study shows that the energetic costs of reproduction have measurable and sometimes considerable consequences for survival across mammals," says Fernando Colchero, one of the study's senior authors. "Reducing reproductive investment may allow more energy to be directed toward longevity."

Both sexes live longer - but for different reasons

Although both sexes lived longer when reproduction was blocked, the underlying causes differed. Lead author Mike Garratt of the University of Otago explains that only castration-not vasectomy-extends male lifespan. "This indicates that the effect stems from eliminating testosterone and its influence on core ageing pathways, particularly during early-life development. The largest benefits occur when castration happens early in life," he says.

In females, multiple forms of sterilization increased lifespan, suggesting that the advantage comes from relieving the substantial physiological costs of pregnancy, lactation, and reproductive cycling. Ovary removal, which eliminates ovarian hormone production, still extends lifespan, although a meta-analysis of 47 laboratory rodent studies indicates potential trade-offs: later-life health may be impaired. These findings may help explain the "survival-health paradox" seen in post-menopausal women, who typically outlive men but experience higher frailty and chronic disease burden.

Patterns of mortality also differed by sex. Castrated males were less likely to die from behavioural causes linked to aggression or risk-taking. Females with blocked reproduction were less likely to die from infection, consistent with the idea that the high energy costs of reproduction may lower the mothers' immune-defence systems.

Insights from humans

Data on the effects of castration and sterilisation in humans is rare. Some historical data, such as those of Korean Eunuchs in the pre-19th century Chosun Dynasty, suggest that castrated men lived, on average, 18 percent longer than non-castrated men. However these historical records need to be interpreted carefully, as their accuracy is debated. Among women, surgical sterilization for benign reasons (such as hysterectomy or oophorectomy) is associated with a small decrease in lifespan, only about 1 percent relative to comparable non-sterilized women. "Reproduction is inherently costly," the authors note. "However, human environments-through healthcare, nutrition and social support-can buffer or reshape these costs".

The study makes it clear that reproduction in mammals involves considerable biological costs-a fundamental evolutionary trade-off between reproduction and survival. These costs arise from an interplay of hormonally controlled processes and the manifold risks and stresses associated with reproduction. However, the exact mechanisms involved remain unclear and require further research. "Our findings show that the costs of reproduction are substantial and measurable across a vast range of mammals," the authors conclude. "Understanding these trade-offs deepens our insight into how ageing evolves and how males and females balance survival and reproduction differently."