In zoos, animals often live to an advanced age and are eventually removed from conservation breeding programs as they lose their ability to reproduce. This poses a threat to efforts aimed at maintaining populations of rare species.

Conservation and Breeding of Rare Species

One of the main goals of modern zoos is to preserve and breed endangered species. To achieve this, groups of animals at risk of extinction are kept in captivity around the world. The more such reserve populations exist, the greater the genetic diversity available for scientists and zoologists to work with. Zoos exchange animals and keep detailed pedigrees to avoid inbreeding and the emergence of weak offspring.

Aging Populations in Zoos

Mammal populations in European and North American zoos are gradually aging. Thanks to excellent care, animals live much longer than they would in the wild. However, as they age, they produce fewer offspring or may lose their reproductive ability altogether. This was reported by an international team of researchers in an article published in the Proceedings of the National Academy of Sciences.

Large-Scale Demographic Study

As part of their research, scientists analyzed demographic data from 774 mammal populations in zoos (361 in North America and 413 in Europe) from 1970 to 2023. The analysis was based on the global Species360 database, which is used by more than 1,200 institutions worldwide. The database contains detailed information on each animal’s life cycle, including age, sex, origin, and reproductive status.

Demographic Pyramids and Their Changes

To assess population stability, the research team studied age pyramids—tools that illustrate the distribution of age groups within a population. A new method for automated classification of these pyramids was developed, allowing for more accurate comparisons of demographic models between species and regions. This method was published in the journal MethodsX.

The classic pyramid shape indicates a stable and resilient population, capable of withstanding unexpected shocks, including disease outbreaks. However, the study found that more and more animal groups in zoos are shifting from a pyramidal structure to a diamond or column shape, characterized by fewer young individuals and a higher proportion of elderly animals. Such structures are considered demographically vulnerable.

Decline in the Number of Breeding Females

The situation is particularly concerning for females capable of active reproduction. Their proportion has sharply decreased: by 49% in North American populations and by 68% in European ones. In some cases, there are no breeding females left in certain populations. Beyond reducing the number of offspring, this can also disrupt the social structures of mammals.

Possible Solutions and Challenges

One possible solution to the problem could be the creation of “retirement homes” for animals. However, organizing such facilities is a complex process: they mainly rely on donations and external funding and can never be self-sustaining. Elderly tigers, zebras, deer, and lions from zoos and circuses are often unsuitable for educational programs, and visits from unfamiliar people can cause them stress. The issue of humane euthanasia for old animals also remains ethically challenging.

The authors of the study did not propose specific solutions but gathered enough data to demonstrate the existence of the problem of aging populations in zoos.