  Collective migration -- the directed, coordinated motion of many
self-propelled agents -- is a fascinating emergent behavior exhibited by active
matter that has key functional implications for biological systems. Extensive
studies have elucidated the different ways in which this phenomenon may arise.
Nevertheless, how collective migration can persist when a population is
confronted with perturbations, which inevitably arise in complex settings, is
poorly understood. Here, by combining experiments and simulations, we describe
a mechanism by which collectively migrating populations smooth out large-scale
perturbations in their overall morphology, enabling their constituents to
continue to migrate together. We focus on the canonical example of chemotactic
migration of Escherichia coli, in which fronts of cells move via directed
motion, or chemotaxis, in response to a self-generated nutrient gradient. We
identify two distinct modes in which chemotaxis influences the morphology of
the population: cells in different locations along a front migrate at different
velocities due to spatial variations in (i) the local nutrient gradient and in
(ii) the ability of cells to sense and respond to the local nutrient gradient.
While the first mode is destabilizing, the second mode is stabilizing and
dominates, ultimately driving smoothing of the overall population and enabling
continued collective migration. This process is autonomous, arising without any
external intervention; instead, it is a population-scale consequence of the
manner in which individual cells transduce external signals. Our findings thus
provide insights to predict, and potentially control, the collective migration
and morphology of cell populations and diverse other forms of active matter.
