  We establish that the rheological curve of dry granular media is
non-monotonic, both in the presence and absence of external mechanical
agitations. In the presence of weak vibrations, the non-monotonic flow curves
govern a hysteretic transition between slow but steady and fast, inertial
flows. In the absence of vibrations, the non-monotonic flow curve governs the
yielding behavior of granular media. Finally, we show that non-monotonic flow
curves can be seen in at least two different flow geometries and for several
granular materials.
