Q&A: Getting a grip on wildlife stress

For wildlife, as well as for humans, stress can be helpful. A rush of adrenalin can help an animal avoid becoming prey. But over long periods of time, stress can harm animals, wearing them down and making them vulnerable to difficult conditions around them.

Wildlife and humans have something else in common when it comes to stress, researchers found. Just as it can be stressful to be around someone else who is stressed out, wildlife can transmit stress, too. In a recent study published in the Proceedings of the Royal Society B, Hanja Brandl, a postdoctoral researcher at the University of Konstanz and the Max Planck Institute of Animal Behavior, both in Germany, explored how animals transmit stress to one another.

We caught up with her to talk about her research. Her responses were edited slightly for brevity and style.

Hanja Brandl is a behavioral ecologist and collective behavior researcher at the University of Konstanz.
Credit: Elisabeth Böker

We all understand what stress means for humans. What is stress in wildlife?

From the physiological aspect of what happens in the body, stress in humans and wildlife is very similar, at least among vertebrates. The stress response starts in the brain, when a human or other animal recognizes a stimulus in its environment and categorizes it as threatening. The brain can then trigger a stress response, a complex cascade in the body that is designed to help us cope with the situation better and increase our chances of survival.

The stress response can, for example, help a caribou being hunted by wolves escape faster as energy is being mobilized in the body and the adrenalin rush allows animals to continue fighting even when already wounded. A stress response is a generally a healthy response for animals to deal with risks and unexpected disturbances in their environment that is evolutionarily very deeply rooted.

How is stress harmful for wildlife?

Stress can generally become harmful when animals experience it continuously over longer periods, when they experience it repeatedly or when there is a mismatch between the stressor and the response to it. Continuous stress can lead to wear-and-tear of the system and might make animals more prone to disease or have other negative effects on their condition. Snowshoe hares, for example, reduce their reproduction due to stress in years with high numbers of predators. The stressed mums even pass on some of their stress to their offspring that will themselves be more stress reactive and reproduce less.

Repeated stress exposure, for example, when animals experience frequent disturbances, can also make them more sensitive to future stress encounters by lowering their threshold. This sets them up for similar consequences as that of chronic stress. Finally, the response of animals to human-induced stressors will often not be appropriate. It will trigger a costly physiological process without generating any benefits because there is no actual threat—maybe just loud noise—or it is a stimulus that they have no appropriate response for in their repertoire. Think of a deer in the headlights.

Your research focused particularly on the transmission of stress. What does that mean? How do animals transmit stress?

Transmission of stress describes the phenomenon when we become stressed by interacting with another stressed person. This might be when we talk to a friend or family member experiencing stress or even just observe someone in a stressful situation. It can happen that we get almost as stressed, without being directly exposed to any stressor ourselves.

In animals this has so far not been investigated very much, but since animals are often very social, and the stress response acts in the same way as in humans, I suspect that stress transmission is also very common in wildlife and might lead to stress propagating through animal groups. We have good evidence for stress transmission in mice and rats already, and as we newly discovered, also birds seem to be sensitive to stress transmission. But we don’t know much yet about how this happens in animals. In some species, odors and pheromones might play a role.

Vocal signals or subtle changes in body movement or breathing rates also might be involved in transmitting stress.

What effects do humans have on stress in wildlife?

Human interventions can be very pervasive and cause manifold disturbances in the natural habitats of animals. Human presence or anthropogenic noise can, for example, trigger anti-predator behavior, which will not provide any benefits to the animal in that situation. Animals suffer from anthropogenic light pollution, which has been shown to reduce their sleep. Wildlife that is disturbed often can become more alert and wary, which may reduce time for feeding and other important behaviors. Trying to avoid busy areas and human structures reduces the habitat they have available, which can also lead to reduced food intake, disturbed reproduction and can increase stress levels. But not every human intervention in an animal’s habitat will cause stress, and not every individual responds the same way to disturbances.

It seems like, just as with humans, stress can be helpful or harmful for animals. When is stress beneficial and when does it become harmful?

Broadly speaking, the stress response is helpful whenever it helps animals survive better. There is some physiological cost to it. But if the energy and other bodily resources that become mobilized during the stress response help to better get through a sudden cold snap, food shortage or to win a fight, the benefits are considerable. The more this balance shifts to low or no survival benefits, while maybe incurring costs of chronic and repeated stress exposure, the more it can go into the direction of being harmful.

How does understanding wildlife stress affect conservation or management efforts?

A proper understanding of the specific needs and disturbance factors for different species is crucial for directed conservation efforts and to manage a sustainable co-existence of humans and wildlife. Not every animal has the same movement patterns and space needs, for example. Some species will suffer more if their habitat is strongly fragmented and their social population structure gets disrupted. Other species might be more tolerant and have other needs.

Also, when animals move more and more into areas of human settlements, which sometimes leads to conflicts, understanding what pressure drives them to do so will often provide a key to finding solutions.