Drawing on data from more than 100 mammal species, an international team of researchers examined the trade-off between living a long life and having babies.
For centuries, zoos and aquariums around the world have monitored, fed and protected animals from the dangers their wild counterparts face. And now, scientists, including from UNSW Sydney, have discovered that inside these enclosures lies an accidental experiment in the biology of ageing.
An international study published today in Nature examined the records of 117 captive mammal species, from lions to rats. The data revealed a pattern: when animals stop reproducing - whether through hormonal contraception or surgical sterilisation - they tend to live longer.
Much longer.
"The increase in life expectancy averaged between 10 and 20% across species. These findings offer some of the strongest evidence yet for one of evolutionary biology's central ideas: reproduction shortens lifespans," says UNSW coauthor Dr Malgorzata Lagisz.
Evolutionary theory predicts the energy invested in producing and raising young is energy taken away from repairing cells, fighting infections or maintaining the body's long-term health. Yet, in mammals, evidence for this has so far been inconsistent - mostly because of a lack of large and comparable datasets across species.
Zoo datasets changed that. Here, well-documented birth and death data sit alongside data on hormonal contraception or surgical sterilisation, which is administered to prevent surplus animals, maintain genetic diversity, manage behaviour or for health reasons.
By tapping into these extensive life-history records, researchers could build a picture of longevity across mammals of all sizes, diets and life strategies.
Their results suggest hormones may interact with pathways that regulate the biology of ageing. But while the relative increase in lifespan was similar in males and females, there were different underlying reasons.
"In males, only castration extends lifespan - not vasectomy - which indicates that the effect comes from removing sex hormones," says lead author Associate Professor Mike Garratt from the University of Otago.
A vasectomy prevents reproduction but leaves testosterone production intact, while castration removes both the testes and the hormones they produce.
Dr Lagisz says this distinction matters.
"Sex hormones shape the development of individuals," Dr Lagisz says. "The biggest longevity boosts occurred when males were castrated early in life."
Comparing the sexes
Without testosterone, males likely engaged in fewer risky, aggressive behaviours that often lead to injury or early death. But, Dr Lagisz says, the influence of sex hormones reaches deeper - into the cellular and molecular machinery of ageing itself.
In females, the pattern was different. While contraception or surgical sterilisation consistently extended female lifespans across species, timing didn't matter. Early-life and later-life interventions both produced similar benefits.
Here, the reason is energetic.
"Pregnancy, lactation and repeated reproductive cycles are not only metabolically taxing but can weaken the immune system," Dr Lagisz says.
Without such physiological demands, females retain more resources to fuel their immunity and tissue repair - and are less likely to die from infection and infectious diseases.
"It's not just one hormonal mechanism," Dr Lagisz says. "It's the cumulative cost of females making and supporting offspring."
What about us?
Though the study mainly drew upon zoo data, it also incorporated meta-analyses of lab studies, wildlife research - and human datasets. The same broad pattern emerged across all: halting reproduction is associated with an increased lifespan.
"Some evidence from historical human populations showed that castrated men were likely to outlive their counterparts," Dr Lagisz says.
The study doesn't suggest that either sterilisation or contraception extends life for people, and the researchers caution against applying the findings directly to health decisions.
"Human longevity is shaped by a wide range of factors that don't apply to zoo or wild animals," Dr Lagisz says.
"The biology connecting reproduction and ageing is deeply conserved, yet humans experience these costs differently.
"Our access to health care, medicine, nutrition and social support dramatically buffers the physical hardships of reproduction."
Our access to health care, medicine, nutrition and social support dramatically buffers the physical hardships of reproduction.
Rethinking menopause - and the limits of a long life
The study also provided insights into menopause and why it exists. Halting reproduction can enhance survival - even after childbearing in earlier years. This makes the evolution of menopause in humans easier to explain, as it may offer later-life survival benefits.
But longevity gains don't necessarily equal better health. For example, in some animals, removing ovaries led to a longer lifespan - but poorer health in later life.
"This may also help explain the 'survival-health paradox' in human women, where, despite living longer than men, post-menopausal women often experience more frailty and chronic illness," Dr Lagisz says.
Another key finding was that reproductive costs alone still can't explain why one sex lives longer than the other across species.
In some species, females with heavy reproductive burdens still outlive the males. In other species, males live longer.
"This suggests sex chromosomes or age-related hormonal changes also play major roles," Dr Lagisz says.
And external environments and factors such as predation or health care can soften - or exaggerate - any costs.
While reproduction is influential, it is still only one piece of the puzzle.
"Our findings don't suggest that animals - or people - are better off never having babies," Dr Lagisz says.
"But they do show us how the demands of producing offspring can shape biology across the animal kingdom - and help explain our own."
