  The evolution of on-site number fluctuations of ultracold atoms in optical
lattices is experimentally investigated by monitoring the suppression of
spin-changing collisions across the superfluid-Mott insulator transition. For
low atom numbers, corresponding to an average filling factor close to unity,
large on-site number fluctuations are necessary for spin-changing collisions to
occur. The continuous suppression of spin-changing collisions is thus a direct
evidence for the emergence of number-squeezed states. In the Mott insulator
regime, we find that spin-changing collisions are suppressed until a threshold
atom number, consistent with the number where a Mott plateau with
doubly-occupied sites is expected to form.
