  Understanding the response of complex materials to external force is central
to fields ranging from materials science to biology. Here, we describe a novel
type of mechanical adaptation in cross-linked networks of F-actin, a
ubuiquitous protein found in eukaryotic cells. We show that shear stress
changes its nonlinear mechanical response even long after that stress is
removed. The duration, magnitude and direction of forcing history all impact
changes in mechanical response. The `memory' of the forcing history is
long-lived, but can be erased by force application in the opposite direction.
We further show that the observed mechanical adaptation is consistent with
stress-dependent changes in the nematic order of the constituent filaments.
Thus, this mechano-memory is a type of nonlinear hysteretic response in which
an applied, "training" strain modifies the nonlinear elasticity. This
demonstrates that F-actin networks can encode analog read-write
mechano-memories, which can be used for adaptation to mechanical stimuli.
