People need to collaborate effectively when hunting together. Researchers at the University of Konstanz have just discovered that this cooperation also affects their heart rates. The study observed a special group of fishers that hunt along with dolphins in Brazil.
There are a wide range of dynamics in play during a hunt – for people as well as for animals. How does communication within such groups work? What factors influence how the hunt progresses? And how does each participant's body react to the situation? This last question was the focus of a research team from the Cluster of Excellence "Centre for the Advanced Study of Collective Behaviour" at the University of Konstanz led by biologist Hanja Brandl. By studying a group of Brazilian fishers, they documented that, while working together to catch fish, the fishers' heart rates synched up.
Collaboration with dolphins
The fishers studied were from a particular group in southern Brazil that collaborates with dolphins to catch fish. The people stand in a line near the shore, ready to throw out their nets. The dolphins drive the fish towards the people and then suddenly dive underwater. This is the signal that the fishers must respond to by throwing out their nets. The dolphins benefit too, since this action drives the fish swarm apart and makes the fish easier for the dolphins to catch.
For several years already, this special cooperation between animals and people has been the focus of research. Since not only people and animals have to work in close collaboration, but also the group of fishers, this joint hunt is particularly well-suited for Hanja Brandl's research project. "The fishers have to coordinate themselves and respond to the dolphins' behaviour at just the right moment if they want to be very successful at catching fish", the biologist explains.
In order to record the heart rates and movements of the fishers, the researchers gave 24 of them chest sensors that took both electrocardiogram (ECG) and GPS measurements as they fished. Later analysis of the heart rate variability (HRV) gave the researchers insight into the synchronization of the fishers in relation to their location. "The HRV shows variation in the time between heartbeats. It is considered to be a sensitive indicator for physiological excitement and emotional states", Brandl explains. "We observed that the HRV was highly synchronous during positive social interactions of the fishers." That means, the closer the fishers' proximity to each other while fishing, and the longer they worked together, the stronger the correlation between their heart rates. This was especially true for situations in which the fishers engaged in watching and waiting together. When one individual fisher threw out his net or the group became larger in size, HRV synchronization decreased.
No increase in the catch rate
The study's results underscore how cooperation truly goes "more than skin deep": "When people work closely together, not only do their movements and decisions synchronize, but also their bodily rhythms. Such physiological synchronization processes are considered to be a key mechanism in collective behaviour. They mirror how strongly individuals coordinate their behaviour, as well as their inner states, with others", Brandl says.
Closeness to or interactions with the dolphins, however, had no measurable effect on the people studied. "We thus conclude that synchronization during fishing is mainly shaped by internal group dynamics and less by external influences", Brandl adds. At the same time, she highlights a particular fact: "Surprisingly, a very high level of synchronization was negatively associated with the amount of fish caught. When fishers' heart rates were more synchronous, they caught fewer fish." One potential explanation is that too much physiological coordination – such as shared tension or excitement – reduces the flexibility required for the team to succeed. "This indicates that strong synchronous behaviour may not always have an advantage but can depend on the context", Brandl concludes.
Key facts:
- Original publication: Hanja B. Brandl, João V. S. Valle-Pereira, Jens C. Pruessner, Alexandre M. S. Machado, Fábio G. Daura-Jorge, Mauricio Cantor, Damien R. Farine (2025): Physiological synchrony among human fishers during collective hunting with wild dolphins, Biol. Lett. 212025042, DOI: 10.1098/rsbl.2025.0421
- The Cluster of Excellence "Collective Behaviour" at the University of Konstanz is a global hotspot for the study of collective behaviour across a wide range of species and across scales of organization.