  Cells in an epithelial monolayer migrate in collective packs of sizes
spanning multiple cell diameters, but the physical mechanisms controlling the
pack size remain unclear. Here, we measured cell velocity and traction with
single-cell resolution, enabling us to identify both a persistence time of
traction and a correlation of traction between neighboring cells. A
self-propelled Voronoi model matched the experiments only when it accounted for
alignment of traction between eighboring cells. Hence, traction alignment, in
addition to persistence, affects the pack size in collective cell migration.
