Modern zoos take on important tasks in species protection. Among other things, they care for, breed and preserve animal species that are threatened with extinction in the wild. But many animal populations in European and North American zoos are aging and producing too few offspring, according to a new study. As a result, it is no longer guaranteed that these species will reproduce sufficiently in the long term. Age development therefore endangers the stability of these so-called reserve populations in zoos.
Zoos like to advertise with photos and videos of newborn zoo animals. The cute young animals attract attention – and the impression that there are regular offspring in the zoo. But is that true? Researchers led by João Pedro Meireles from the University of Zurich have now checked this. To do this, they analyzed the demographic development of a total of 774 mammal populations that were kept in zoos in North America and Europe between 1970 and 2023.
Less reproduction, fewer offspring
They received the data for this from the international database “Species360”, in which, among other things, age, gender, ancestry, origin and reproductive status are recorded for each individual animal. Meireles and his colleagues used this to analyze the age structure of zoo populations over several decades. In order to assess how stable a population is, they used a new evaluation method to create visually more comparable demographic profiles.
This analysis showed that there are fewer and fewer offspring of mammals in zoos and many populations are now aging. Apparently the better conditions in zoos are to blame for this. To be considered stable, the populations would have to have many reproductive young animals and fewer older individuals – a pyramid-shaped age structure. In fact, there are significantly fewer young animals at the base of the pyramid and more older animals. This makes the demographic profile of most groups more like a column or diamond shape, the study found. Such populations are less prepared against unexpected events such as disease outbreaks or other crises and are therefore considered less resilient.
In many zoos, the proportion of such diamond or columnar populations has increased over the decades, the analyzes showed. At the same time, the proportion of actively reproducing females fell – by 49 percent in North American and 68 percent in European populations. In some animal groups there were recently no longer any females capable of reproducing. This not only results in a lack of offspring, but also affects the social structure of many species.
Species protection goals at risk
The researchers are concerned about their findings. The aging animal populations endanger the species conservation work of modern, scientifically managed zoos, which play an important role in the global fight against the loss of biodiversity. This means they can no longer breed sufficiently stable reserve populations to save species threatened with extinction in the wild. In addition, the reproduction and rearing of young animals are among the basic needs of animals and are elementary components of species-appropriate husbandry.
The findings are a wake-up call: “This trend must definitely be stopped and reversed. More young animals and fewer old animals are needed again,” says senior author Marcus Clauss from the University of Zurich. But how can this be achieved? “These results open up opportunities for zoos and species conservation to clearly communicate demographic developments and make well-founded decisions,” says co-author Paul Dierkes from the Goethe University Frankfurt. According to him, more sustainable population management should help combat declining animal numbers and aging populations.
Sources: Goethe University Frankfurt am Main, specialist article: PNAS, doi: 10.1073/pnas.2522274123 and MethodsX, doi: 10.1016/j.mex.2025.103591