  Tracking experiments in dense biological tissues reveal a diversity of
sources f or local energy injection at the cell scale. The effect of cell
motility has been largely studied, but much less is known abo ut the effect of
the observed volume fluctuations of individual cells. We devise a simple
microscopic model of `actively-deforming' particles where local fluctuations of
the particle size constitute a unique source of motion. We demonstrate that
collective motion can emerge under the sole influence of such active volume
fluctuations. We interpret the onset of diffusive motion as a nonequilibrium
first-order phase transition, which arises at a well-defined amplitude of
self-deformation. This behaviour contrasts with the glassy dynamics produced by
self-propulsion, but resembles the mechanical response of soft solids under
mechanical deformation. It thus constitutes the first example of active
yielding transition.
