Sheeps and active matter - CY's LPTM Laboratory witnesses collective intelligence in flocks of sheep

When physics takes the key to the fields

Flocks of sheep reveal collective intelligence.

A small flock of sheep is grazing peacefully on the green hills of peaceful countryside. They are engaged in a very ordinary choreography: regularly, the group moves off, one individual after the other, before coming to a halt again a little further to start grazing again. The movements are as organised as they seem random. Strollers will have noticed this kind of collective coordination in many species of animals, such as birds that suddenly take off in a swarm or fish that elegantly zigzag in tight schools. This collective intelligence is a phenomenon studied by biologists, but mathematical models developed by physicists formalise their observations.

Mathematically describing living organisms.

Fernando Peruani, a researcher at the Laboratory of Theoretical Physics and Modelling (LPTM, CY Cergy Paris University), has made this subject his speciality. His mission is to describe living organisms mathematically, adopting an interdisciplinary approach between physics and biology. After several years of developing tools to describe the movements of bacteria, he found that it was possible to use similar mathematical models to study the movements of groups of animals. His research was recently published in the scientific journal Nature Physics [1].

Active matter physics: a growing field

This approach places his work in the field of active matter physics. This fast-growing discipline emerged some thirty years ago. It was the subject of a EUTOPIA conference (Active Days EUTOPIA - Challenges in Active Matter at the Maison internationale de la recherche, 5-7 December 2022). It aims to use physics to understand living systems at all scales, whether they are cells, bacteria, tadpoles, birds or sheep.

However, while research in this field has so far focused on very brief moments of observation (the moment of the flight of starlings, for example), Prof. Peruani and his team have set out to study the effects of the new technology on the environment. The programme included Newtonian physics, mechanics, Markov chains, stochastic calculus and probabilities. Peruani and his team set out to model the overall behaviour of small groups of sheep on much longer time scales by breaking down phases of movement: grazing (head down), alert (head up), panic (running), etc. This increase in complexity forced them to move away from the models previously proposed by their peers and start the calculations from scratch.

Collaborative research for promising results

Fernando Peruani collaborates with Luis Gómez-Nava, affiliated with the J. A. Dieudonné laboratory at the Côte d'Azur University in Nice, as well as with biologists from Richard Bon's team at the Centre de Recherches sur la cognition animale (Animal Cognition Research Centre) at Toulouse III University. At the LPTM, he also involves students and post-doctoral fellows in developing a theoretical framework for modelling the data.

What can we conclude at this stage? Contrary to popular belief, sheep are not just followers. Not only is every movement of the flock initiated by a group member, but all individuals in the flock can be leaders. This democratic model would benefit the group by simplifying decision-making, optimising the transmission of information and thus facilitating group cohesion. In the long term, it remains to be seen whether the models developed can be applied to larger flocks of sheep. The collective intelligence of sheep has not yet revealed all its mysteries.

[1] Gómez-Nava, L., Bon, R. & Peruani, F. Intermittent collective motion in sheep results from alternating the role of leader and follower. Nat. Phys. (2022). https://doi.org/10.1038/s41567-022-01769-8