An international study of mammals in zoos shows that limiting reproduction through contraception or sterilization increases average lifespan by about 10 percent. The effects differ between sexes, with males benefiting from reduced testosterone and females from avoiding pregnancy's physical toll. These findings highlight a key evolutionary trade-off between breeding and survival.
Researchers from institutions including the Max Planck Institute for Evolutionary Anthropology analyzed records from 117 mammal species in zoos and aquariums, alongside a meta-analysis of 71 prior studies. Their work, published in Nature in 2025, demonstrates that hormonal contraception and surgical sterilization consistently extend lifespans across diverse groups like primates, marsupials, and rodents.
"Zoos, where reproduction is carefully managed, provide a unique setting to study these dynamics," explained Johanna Stärk, one of the authors. The benefits were particularly evident in species such as hamadryas baboons, where females on hormonal contraception lived 29 percent longer and castrated males 19 percent longer.
For males, only castration—not vasectomy—yielded longer lives, pointing to testosterone's role in accelerating aging. Lead author Mike Garratt from the University of Otago noted, "This indicates that the effect stems from eliminating testosterone and its influence on core aging pathways, particularly during early-life development. The largest benefits occur when castration happens early in life."
Females saw gains from various reproductive suppressions, likely due to spared energy from pregnancy, nursing, and hormonal cycles. However, a review of 47 rodent studies suggested potential later-life health declines, echoing the survival-health paradox in post-menopausal women.
Death patterns also shifted: castrated males faced fewer aggression-related fatalities, while suppressed females had lower infection risks, implying bolstered immunity. Human parallels are tentative; historical Korean eunuch data suggest an 18 percent lifespan boost, though debated, and female sterilizations correlate with a minor one percent reduction.
"This study shows that the energetic costs of reproduction have measurable and sometimes considerable consequences for survival across mammals," said senior author Fernando Colchero. "Reducing reproductive investment may allow more energy to be directed toward longevity."
The authors stress that human factors like healthcare mitigate these costs, underscoring reproduction as a fundamental evolutionary expense.
