Most Mammals Live Longer in Zoos Than in the Wild

A study recently published in Scientific Reports confirms that mammals generally have a longer life expectancy in zoos than in the wild. More than 80% of the mammal species analyzed—and all carnivores—lived longer in captivity than in the wild. The effect was most pronounced in smaller animals with short life spans and high mortality rates in the wild. Living in zoos had less effect on longevity and mortality rates in species with naturally longer life spans and lower mortality rates.

Longevity is one indicator researchers use to measure the welfare of captive animals. The authors note that zoos play important roles in education, research, and conservation of endangered species. “When assessing the justification of holding nondomestic species in zoos, the welfare of the individual animals housed in captivity is a critical ethical issue that has to be weighed against these aims,” they write.

The investigators constructed life tables for 59 mammalian species using data from the Species360 database of captive animals (formerly the International Species Information System) and from published studies of free-ranging animal populations. They analyzed four survival metrics: longevity, baseline yearly mortality, age at onset of senescence, and rate of senescence. They defined longevity as the age at which 90% of the members of the original cohort had died. They considered the age at onset of senescence to be the point of lowest annual mortality between the age of sexual maturity and the age at which 90% of the animals in a cohort had died.

“Our findings indicate that, in general, a life in zoos allows mammals to live longer,” write the authors. “However, our data suggest that the species-specific pace of life influences the extent to which a given species may benefit from captivity.”

For most species (84%), longevity was higher in zoos than in the wild. The onset of senescence was the same or later in zoos than in the wild for about 69% of species. These effects were greatest in species with a faster pace of life (shorter life span, higher mortality rate, and earlier onset of senescence). In zoos, say the authors, these species are protected from predators, competition for food, and harsh climates. Some species with a slower pace of life actually experienced earlier senescence in captivity.

Baseline mortality and the rate of senescence were also lower overall in zoos than in the wild, especially for species with a faster pace of life. However, some species with a slow pace of life had higher baseline mortality and senescence rates in zoos. “With regard to long-lived species that generally have lower mortality rates in the wild, there is less that zoos can protect them from. As such, the effect is not as great and, indeed, in some cases is even reversed,” said Prof. Marcus Clauss, of the University of Zurich, Switzerland, in a press release. The authors suggest that an earlier onset of breeding could contribute to a small reduction in longevity and increase in senescence rate in some large zoo animals like female elephants.

“These findings emphasize that husbandry efforts to optimize the longevity of species with a slower pace of life should be intensified,” say the authors. They point out that because the analysis included only animals that had already died, their results reflect historical husbandry practices and not recent improvements in living conditions for long-lived species. “For example, there has been tremendous effort in building new elephant enclosures in a great number of zoos in the last decade, and large-scale studies have been performed on the potential to increase captive elephant welfare. However, the benefits of such efforts on survival measures will not be detectable before many years from now,” they write.

All 15 species of carnivores included in the study had longer life spans in captivity than in the wild. “It seems that even for predators, life in the wild is not necessarily without its perils,” said Clauss. However, carnivores in captivity are prone to behavior problems. Because longevity is only one measure of animal welfare, conclude the authors, their results should not be taken as a conclusive judgment on the ethics of animal captivity.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.