Born Free: How Captivity-Induced Stress Changes Different Species’ Physiology

Wild animals are adapted to a variety of physical conditions and environments, ranging between extremes of all imaginable sorts. While some of these conditions can be tough, each species is adapted to their particular niche. Nevertheless, sometimes the “normal routine” gets interrupted in a way that triggers a stress response. This response is often adaptive in its own way—it helps the individual to survive that instance. However, long-term exposure to stressors can cause unhealthy weight loss and gain, a suppressed immune system, reproductive failure, and general psychological distress in a wide range of species. Technically, this is known as “chronic stress” and afflicts both non-human and human animals dealing with repeated and prolonged adverse conditions.

In the wild, this is not something animals typically deal with. Animals brought from the wild into captivity, on the other hand, may suffer from chronic stress even if their basic physical needs are met. In part, this may be because wild animals perceive captive environments as threatening in and of themselves. This sense of threat likely stems from being caged, regular human contact or handling, an unfamiliar environment, artificial light conditions, and that all of these are inescapable. Many people who work with animals in these conditions believe that the animals will eventually adapt and their chronic stress will disappear. To test the validity of this belief scientifically, two researchers reviewed the literature on the stress responses of wild animals brought into captivity. Ultimately, they found that while some species may adapt to captivity, many others do not, thus suffering chronic stress throughout their lives.

In their review, the authors looked for scientific articles that met the following criteria:

1. The study had to focus on wild animals that were brought into captivity and included physiological measures (such as stress hormone levels) measured in the days to months since being captured, or compared wild-caught captive animals to their wild-living counterparts.
2. The total time in captivity had to last at least three days.

Importantly, they did not look at wild animals being rehabilitated in captivity (with the intention of being reintroduced to the wild when safe), since it would be impossible to figure out whether these animals were stressed because they were injured or because they were captured. The physiological measures they focused on included immune response, weight loss and gain, and glucocorticoid concentration (a stress-related hormone).

The results are stark. For example, in 40% of the studies that looked at weight, the captured animals eventually regained what weight they had lost, and in one study, even gained more weight than they had started with. In the other 60% of these studies, the animals never regained their lost weight. In the case of glucocorticoid levels, while some studies showed that stress levels returned to animals’ pre-capture baseline, 42% found that captured animals had higher levels than their wild counterparts at the end of the study. 45% of the species studied had elevated glucocorticoid levels fully three months after initial capture, indicating that chronic stress may last a long time for many different kinds of animals. 

Since stress levels are linked with immune responses as well, the researchers also looked at studies that tracked leukocyte counts to see how captivity affected animals’ immune systems. Results for this measure are less clear—24% of the studies showed increased (hyperactive) immune responses, another 24% showed decreased (hypoactive) responses, but the majority (52%) showed no difference. Finally, in looking at the reproductive system, the authors found that fully 74% of studies show that captured wild animals have inhibited reproductive capacity. This is especially species-dependent. For example, in a study on water frogs, reproduction is inhibited after only three days in captivity, while in some fish species, such consequences are only apparent after a year or more. The overall trend, however, is clear: captivity is generally bad for wild animals’ reproductive ability, presenting an important challenge for conservation biologists.

What then to do? The authors conclude with a discussion of what steps people who work with wild animals in captivity can take to minimize the negative impact on these animals. These include, first, adjusting the physical conditions the animals live in, such as shifting from indoor to outdoor cages and tailoring the density of animals in a given holding to species-specific social needs. This applies likewise to the lighting of the animals’ captive conditions. Next, offering “behavioral enrichments,” or hobbies that help animals pass the time in a way that is psychologically beneficial is another important step that helps them avoid developing unhealthy, damaging behaviors. Finally, a more dire step might be treating the animals with medicine that blocks their stress-responses, akin to anti-anxiety or depression medication in humans. While this may be effective, perhaps the best way to minimize the chronic stress of captive conditions faced by wild animals is to ensure that their captive space most closely emulates their natural environment as much as possible. Broadly, this includes a matched physical space, appropriate social interaction, and time to “exercise” their minds and bodies so that they don’t suffer from bad coping habits.

Ultimately, the research demonstrates formally what animal rights proponents have known for a long time: non-human animals are similarly capable of suffering and flourishing in ways that are familiar to us. The authors show that these dynamics are highly species-specific, thus showing that conservationists and others who deal with wild animals in captivity need to attend to the nuanced needs of individual animals. By doing so, they can help mitigate the direst consequences of chronic stress on the animals’ health and help to improve their overall quality of life. In the end, however, the best quality of life for wild animals is outside of captivity.