Zoos also have an ageing problem

Targeted culls are necessary to create space for young animals and thus guarantee their continued reproduction. "If I could permanently build new enclosures, this measure would probably not be necessary," Clauss told the German Press Agency. "But if I keep all the animals in a world with limited enclosure space until old age, I have no room for the next generation."

The researchers looked at 774 mammal populations from ungulates to primates to carnivores in European and North American zoos over a 53-year period. Across all species, they saw an increase in populations with more older than younger animals. The average age has risen steadily. The proportion of females still reproducing has fallen in Europe to 69 percent and in North America to 49 percent of the species kept in zoos.

Acceptance among the population is a problem

Experts warn that an ageing population is less resistant to animal diseases, for example. Smaller groups with fewer than 150 animals and little reproduction could disappear completely in the event of an outbreak. "The danger is not that all zoo animals will be gone in ten years' time," said Clauss. "But the trend is so striking that a new strategy is needed."

More needs to be done to ensure that targeted killing for the benefit of the next generation is socially acceptable. "The figures show that this option is not chosen often enough," says Clauss. In Germany, for example, there were protests in the summer of 2025 because twelve baboons were killed at Nuremberg Zoo due to a lack of space.

According to Zurich Zoo, the first choice is always to place older animals elsewhere, but the number of places is limited. Targeted killings are therefore necessary. "Culls are always carried out in accordance with the law, in accordance with animal welfare and after thorough consideration of all options."