where C 1 and C 2 represent the two sets of contacts. The Jaccard matrix is of dimension N f rames x N f rames . J = 1 if S1 and S2 have identical contacts. On the contrary, J = 0 if S1 and S2 have no common contacts. Hierarchical clustering using the Ward.D2 method 63 as implemented in R was performed on the dissimilarity matrix 1 -J to identify clusters. The choice of the optimal number of clusters was guided by the analysis of the topology of the clustering dendograms and the assessment of cluster size and stability in time when varying the number of clusters as proposed in ref.