Stress is contagious – for animals, too

Animal habitats are currently changing extremely rapidly and extensively due to urbanization and climate change. As a result, animals are increasingly exposed to stressors. However, until now, little research has been conducted on how individual stress levels affect the group and the behaviour of other group members that may not have experienced a stressor themselves. Hanja Brandl, a behavioural biologist from the Cluster of Excellence Collective Behaviour at the University of Konstanz, and her colleague Damien Farine (now at Australian National University) conducted experiments with 96 zebra finches. Their aim was to find out whether and how the stress responses of individual birds affect other members of their group.

In three rounds of experiments lasting four weeks each, some members of groups of zebra finches were exposed to different disruptions resulting in stress. Cameras then recorded in fine detail how stress affected the behaviour the exposed individuals and—importantly—how these changes in turn affected the behaviour and reproduction of other members of the group that had not experienced the disruptions. In addition to this, the researchers measured the level of the stress hormone corticosterone in the animals' tail feathers.

Stress is contagious

"Our experiments show that stress responses can spread beyond individuals to other members of their social group, even to those who are not exposed to stress themselves", Hanja Brandl says. "In groups where a larger proportion of birds experiences stress, we observed this effect even more strongly". The social environment impacted both the activity and the social behaviour of the zebra finches that had not experienced stress. Their response was similar to that of the zebra finches that had experienced stress.

The researchers observed that unexposed birds in groups that had a large proportion of its members experience stress moved less. "For birds in the wild, this reduced level of activity could, for example, mean that they explore their environment less and limit their range of movement, which, in turn, means that they could have access to fewer resources", Brandl explains.

The experiment also showed that the presence of stress-exposed group members altered the social behaviours of other group members. Individual birds that were not exposed to the stressor maintained fewer social bonds and tended to rely more heavily on existing relationships. "This means that stress in a group could impact its social cohesion. At the same time, a reduction in weak social ties would, in turn, reduce the risk of stress transmission", Damien Farine says. Thus, stress transmission can have profound effects on both the dynamics of social groups as well as the fitness of individual members.

You can read the study "Stress in the social environment: behavioural and social consequences of stress transmission in bird flocks" online at: https://doi.org/10.1098/rspb.2024.1961

Key facts:

• The study "Stress in the social environment: behavioural and social consequences of stress transmission in bird flocks" was recently published in the Proceedings of the Royal Society B: https://doi.org/10.1098/rspb.2024.1961.
• The experiments were conducted by the Cluster of Excellence Collective Behaviour at the University of Konstanz where Hanja Brandl is a researcher. Damien Farine was a member of the Cluster of Excellence and a principal investigator at the Max Planck Institute of Animal Behavior during the study, and now works at the University of Zurich and the Australian National University in Canberra, Australia.
• Funding for the study was provided by the Cluster of Excellence Collective Behaviour at the University of Konstanz as well as the Swiss National Science Foundation and the Max Planck Society.

Caption zebra finches: Zebra finches are social animals, copyright: Hanja Brandl

Caption enclosure: Three breeding pairs of zebra finches each shared an enclosure, whereby the stress level of each group varied: In some groups, one of the three pairs was exposed to disruptions causing stress, in other groups it was two pairs, and in still others it was none of the pairs. After each round of the experiment, there was a six-week break. Three cameras were mounted inside each enclosure that used high-resolution tracking to record the movements and interactions of the zebra finches.

Copyright: © Abschnitt eins / Cluster of Excellence Collective Behaviour

Caption portrait: Hanja Brandl is a researcher in the Cluster of Excellence Collective Behaviour at the University of Konstanz, copyright: Elisabeth Böker