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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 behavior of other group members that may not have experienced a stressor themselves.

Hanja Brandl, a behavioral biologist from the Cluster of Excellence Collective Behavior 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. The paper is in the journal Proceedings of the Royal Society B: Biological Sciences.

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 behavior the exposed individuals and—importantly—how these changes in turn affected the behavior 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 behavior 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 behaviors 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.