Last July, zoo officials in Nuremberg, Germany took the unusual step of killing a dozen healthy, captive baboons. The move, aimed at dealing with overcrowding, sparked protests and worldwide headlines.
But for Swiss veterinarian Marcus Clauss, the zoo’s actions demonstrated a thoughtful reckoning with a slow-moving crisis facing many zoos in North America and Europe. As zoos get better at keeping animals alive, they face a demographic crunch that endangers their effectiveness as tools to preserve endangered species.
Rather than breeding new, young generations of animals, zoos with limited space are restricting reproduction and becoming defacto retirement homes for everything from zebras to wildcats. “This trend must be stopped and reversed,” said Clauss, a veterinarian and researcher at the University of Zurich. “Zoos play a central role in species conservation. They can only fulfill this task if they actually maintain stable and resilient reserve populations. This requires more young animals and fewer old ones.”
It’s not just a problem in Nuremberg. At hundreds of zoos, many species’ populations are aging, creating an imbalance in which middle-aged and older animals outnumber the young, Clauss and other scientists reported this week in the Proceedings of the National Academy of Sciences.
Zoos are, in part, victims of their own success. Many have grown skilled enough in caring for animals that their charges live far longer than they would in the wild, where clashes with other wildlife abound and there are no doctors on call.
As the animals age, programs with only so much room have resorted to birth control to stem the growth. As a result, for many species the number of young animals has shrunk relative to their elders.
In a census of 750 populations of zoo wildlife, the researchers found that 40% in North American and 63% in Europe have been skewing older over the last half century. In 15% and 6% of those populations, there were no juvenile animals at all in recent years. In another sign of the changes, half of the populations in North America and 68% in Europe had falling numbers of female animals reproducing.
The situation threatens zoos’ capacity for acting as a genetic repository, especially for animals dwindling in the wild, the scientists warn. Historically, one mission of many zoos has been to serve as an “ark” of sorts, to house species facing threats of extinction in the wild. But as animals age or when they are treated with birth control, they are often less successful at reproducing in the future. That means zoos with a disproportionately older population are running a higher risk of being unable to recover from losses in case of something like a disease outbreak.
The authors point to three possible solutions: Make more room in zoos for more animals, release some captive animals to make room for younger ones, or euthanasia to manage the demographics of populations.
The first two may be logistically difficult, the authors warn. Zoos face budget and space constraints. Releasing animals into the wild in a humane way isn’t just a matter of opening doors to a cage. It requires entire programs geared toward selecting and preparing animals.
The third option “offers a practical, long-term solution” the authors write. But it faces major moral and political hurdles. As the Nuremberg baboon example illustrates, a zoo choosing to kill animals is a fraught and potentially controversial undertaking. That’s particularly true for charismatic species and mammals, which tend to evoke stronger public reactions.
Clauss sees the Nuremberg case as an example of how zoos can do this well. He praised zoo managers for presenting information to the press and the public that was an “amazing example of compassionate, sober, honest communication.”
It’s easy to feel uneasy at the prospect of killing healthy animals at a place ostensibly dedicated to protecting them. But Clauss says people need to be realistic about the best choices for managing zoo animals, and not put the needs of individual creatures ahead of the needs of an entire species. Otherwise, doing what seems right in the short term might spell trouble for the future.
“If we want to preserve species as entities of living individuals, we have to put priority to a constant production of next generations,” he wrote in an email. “If that means – due to space and resource limitations – that we have to kill healthy older-generation individuals, then we have to (humbly, respectfully, not-gung-ho-like) do that. If we don’t want to do that, we have to honestly accept that that wish will lead to the extinction of many species.”
Meireles, et. al. “Aging populations threaten conservation goals of zoos.” Proceedings of the National Academy of Sciences. Jan. 19, 2026.
Photo: Getty Images For Unsplash+
