  We present velocity dispersion measurements of 20 Local Group stellar
clusters (7 < log(age [yrs]) < 10.2) from integrated light spectra and examine
the evolution of the stellar mass-to-light ratio, Upsilon_*. We find that the
clusters deviate from the evolutionary tracks corresponding to simple stellar
populations drawn from standard stellar initial mass functions (IMFs). The
nature of this failure, in which Upsilon_* is at first underestimated and then
overestimated with age, invalidates potential simple solutions involving a
rescaling of either the measured masses or modeled luminosities. A range of
possible shortcomings in the straightforward interpretation of the data,
including subtleties arising from cluster dynamical evolution on the present
day stellar mass functions and from stellar binarity on the measured velocity
dispersions, do not materially affect this conclusion given the current
understanding of those effects. Independent of further conjectures regarding
the origin of this problem, this result highlights a basic failing of our
understanding of the integrated stellar populations of these systems. We
propose the existence of two distinct initial mass functions, one primarily,
but not exclusively, valid for older, metal poor clusters and the other for
primarily, but not exclusively, younger, metal rich clusters. The young
(log(age [yrs])< 9.5) clusters are well-described by a bottom-heavy IMF, such
as a Salpeter IMF, while the older clusters are better described by a top-heavy
IMF, such as a light-weighted Kroupa IMF, although neither of these specific
forms is a unique solution. The sample is small, with the findings currently
depending on the results for four key clusters, but doubling the sample is
within reach.
