FRANKFURT/ZURICH. Images of newborn zoo animals regularly attract public attention – yet the impression of cute baby animals is deceptive. Across many species, births are becoming rarer while populations grow steadily older- a development documented by an international study led by the University of Zurich (Switzerland) in collaboration with Goethe University Frankfurt (Germany), Aarhus University (Denmark), Zoo Zurich (Switzerland) and Copenhagen Zoo (Denmark).

For the study, the researchers analyzed the demographic data from a total of 774 zoo mammal populations (361 in North America, 413 in Europe) covering the period from 1970 to 2023. The analysis draws on the global “Species360” database, used by more than 1,200 institutions worldwide. The database records detailed life-history information for individual animals, including age, sex, ancestry, origin, and reproductive status, enabling a systematic assessment of zoo population structures over several decades.

Distorted Age Pyramids
To evaluate population stability, the team examined age pyramids – a standard demographic tool that illustrates the distribution of age groups within a population. Researchers at Goethe University Frankfurt developed a new method for the automated classification of these population pyramids, allowing demographic patterns to be compared across species and regions with greater precision. This approach translates complex population structures into standardized forms – such as pyramids, diamonds, or columns – each associated with different levels of demographic resilience. Prof. Paul Dierkes of Goethe University Frankfurt, who played a key role in developing the method, explains: “Especially for zoos and species conservation, this new methodological approach and the results based on it open up possibilities for clearly communicating demographic developments and making informed decisions.”

But what do the different basic shapes tell us? A classic pyramid shape—with many young and reproductive individuals at the base and progressively fewer older animals—indicates a stable and resilient population. Such populations are better equipped to withstand unexpected shocks, including disease outbreaks. The study shows, however, that an increasing number of zoo populations now display diamond- or column-shaped profiles, characterized by relatively few young animals and a high proportion of older individuals. These structures are considered demographically fragile.

The study also shows that, at the same time, the proportion of actively reproducing females has declined sharply: by 49 percent in North American populations and by 68 percent in European populations. In some cases, populations no longer include any females capable of reproduction. Beyond reducing offspring numbers, this trend can disrupt social structures in many mammal species. Reproduction and rearing young animals are fundamental behavioral components and key elements of species-appropriate husbandry.

Species Conservation Goals at Risk
This development is concerning to the researchers and could threaten the species conservation work of modern zoos. Zoos are internationally recognized partners in global species conservation, particularly through the maintenance of reserve populations of endangered species. In a 2023 position paper, the International Union for Conservation of Nature (IUCN) underscores the importance of zoos, aquariums, and botanical gardens in addressing worldwide biodiversity loss. However, a prerequisite for this role is that the reserve populations kept are stable, capable of reproduction, and viable in the long term. Lead author Prof. Marcus Clauss from the Faculty of Veterinary Medicine at the University of Zurich explains: “This trend must be halted and reversed. Zoos can only fulfill their conservation mandate if they maintain demographically stable and resilient reserve populations. That requires more young animals – and fewer old animals.”

Prof. Dierkes adds that the implications extend beyond conservation breeding: “Zoos also play a crucial role in education and research. They reach millions of visitors each year, raise awareness of biodiversity loss, the causes of species extinction, and the importance of nature conservation. Zoos are therefore important places of learning that strengthen understanding and support for species conservation in society. In addition, zoos enable important scientific studies on the behavior, reproduction, and health of endangered species. These findings help to improve husbandry in zoos and make conservation measures in the natural environment more effective. Declining animal numbers and aging populations would therefore not only significantly impair species conservation itself, but also the educational and research work of zoos. Consequently, population management in zoos should be more focused on demographic sustainability. Only if the current trend toward aging populations can be reversed will zoos be able to permanently fulfill their contribution to international species conservation.”