Extending physical chemistry to populations of living organisms. First step: measuring coupling strength

Abstract

For any system, whether physical or non-physical, knowledge of the form and strength of inter-individual interactions is a key-information. In an approach based on statistical physics one needs to know the interaction Hamiltonian. For non-physical systems, based on qualitative arguments similar to those used in physical chemistry, interaction strength gives useful clues about the macroscopic properties of the system. Even though our ultimate objective is the understanding of social phenomena, we found that systems composed of insects (or other living organisms) are of great convenience for investigating group effects. In this paper we show how to design experiments that enable us to estimate the strength of interaction in groups of insects. By repeating the same experiments with increasing numbers of insects, ranging from less than 10 to several hundreds, one is able to explore key-properties of the interaction. The data turn out to be consistent with a global correlation that is independent of distance (at least within a range of a few centimetres). Estimates of this average cross-correlation will be given for ants, beetles and fruit flies. The experimental results clearly exclude an Ising-like interaction, that is to say one that would be restricted to nearest neighbours. In the case of fruit flies the average cross-correlation appears to be negative which means that instead of an inter-individual attraction there is a (weak) repulsive effect. In our conclusion we insist on the fact that such "physics-like experiments" on insect populations provide a valuable alternative to computer simulations. When testable group effects are predicted by a model, the required experiments can be set up, thus permitting to confirm or disprove the model.