  Understanding and controlling collisions is crucial to the burgeoning field
of ultracold molecules. All experiments so far have observed fast loss of
molecules from the trap. However, the dominant mechanism for collisional loss
is not well understood when there are no allowed 2-body loss processes. Here we
experimentally investigate collisional losses of nonreactive ultracold RbCs
molecules, and compare our findings with the sticky collision hypothesis that
pairs of molecules form long-lived collision complexes. We demonstrate that
loss of molecules occupying their rotational and hyperfine ground state is best
described by second-order rate equations, consistent with the expectation for
complex-mediated collisions, but that the rate is lower than the limit of
universal loss. The loss is insensitive to magnetic field but increases for
excited rotational states. We demonstrate that dipolar effects lead to
significantly faster loss for an incoherent mixture of rotational states.
