Fewer Offspring, Longer Life: The Hidden Rule of Aging in Mammals

Animals in nature age at very different rates. A female elephant can live up to 80 years, but only gives birth to a few calves during her lifetime. In contrast, a mouse lives only a few years, but can produce dozens of offspring. Evolutionary biology explains this contrast with a fundamental principle: energy is limited, and species must divide it between reproduction and maintaining their bodies. A new large-scale study provides new evidence that this trade-off plays an important role in how long mammals, including humans, live.

An international team of researchers, including scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig, investigated how limiting reproduction affects the lifespan of mammals. The study analyzed records from 117 mammalian species living in zoos and aquariums worldwide and combined them with a meta-analysis of 71 previously published studies. Together, the data showed that long-term hormonal contraception and permanent surgical sterilization are associated with an average increase in lifespan of about ten percent.

The Trade-Off Between Reproduction and Survival

Producing offspring requires enormous biological investment. Pregnancy, breastfeeding, sperm production, courtship behavior, and parental care consume a lot of energy. Even outside the active reproductive phase, sex hormones such as testosterone and estrogen continue to influence growth, behavior, and aging, diverting resources from the body that are needed for long-term health maintenance. “Zoos, where reproduction is carefully controlled, provide a unique setting for studying these dynamics,” says Johanna Stärk, one of the authors. “Animals can be treated with contraceptives or sterilized to prevent reproduction, creating natural comparison groups within the same environment.”

The benefits of reduced reproduction in terms of lifespan were evident in a variety of mammals, including primates, marsupials, and rodents. In some cases, the effects were remarkable. Female baboons given hormonal contraceptives lived 29 percent longer, while castrated males lived 19 percent longer. “This study shows that the energetic costs of reproduction have measurable and sometimes significant effects on the survival of mammals,” says Fernando Colchero, one of the study’s lead authors. “By reducing reproductive investment, more energy can potentially be devoted to longevity.”

Both Sexes Live Longer—But for Different Reasons

While restricting reproduction prolonged the lifespan of both males and females, the biological mechanisms were not the same. Lead author Mike Garratt of the University of Otago explains that only castration, but not vasectomy, prolonged the lifespan of males. “This suggests that the effect is due to the elimination of testosterone and its influence on key aging processes, particularly during early life development. The greatest benefits occur when castration is performed early in life,” he says.

In females, several forms of sterilization were associated with longer life, suggesting that the benefit lies in avoiding the intense physical demands of pregnancy, lactation, and the reproductive cycle. Removal of the ovaries, which stops the production of ovarian hormones, also prolonged life. However, a meta-analysis of 47 laboratory studies in rodents revealed potential disadvantages, as health outcomes may deteriorate later in life. These findings may help explain the “survival-health paradox” observed in postmenopausal women, who often live longer than men but experience higher levels of frailty and chronic disease.

Different Causes of Death by Gender

The study also found notable differences in how males and females died. Castrated males were less likely to die from causes related to aggression or risky behavior. Females whose reproduction was blocked were less likely to die from infections, supporting the idea that the high energy demands of reproduction may weaken mothers’ immune defenses.

Direct evidence from human medicine remains limited. Some historical records, including data from Korean eunuchs from the Chosun Dynasty before the 19th century, suggest that castrated men lived about 18 percent longer than men who were not castrated. However, researchers caution that these records are controversial and should be interpreted with caution.

In women, surgical sterilization for non-oncological reasons (such as hysterectomy or oophorectomy) is associated with a very small reduction in life expectancy, amounting to about 1 percent compared to similar women who have not been sterilized. “Reproduction is inherently costly,” the authors note. “However, the human environment—through health care, nutrition, and social support—can cushion or reshape these costs.”

A Fundamental Evolutionary Trade-Off

Overall, the findings underscore that reproduction represents a significant biological cost for mammals. The balance between producing offspring and maintaining the body represents a central evolutionary trade-off that influences aging and survival. These costs arise from a combination of hormonal effects and the physical risks and stresses associated with reproduction.

Many details remain unclear, and the researchers emphasize the need for further investigation. “Our results show that the costs of reproduction are significant and measurable in a wide range of mammals,” the authors conclude. “Understanding these trade-offs deepens our insight into the evolution of aging and the different ways in which males and females balance survival and reproduction.”