  Dynamical mass estimates of ultra-compact dwarfs galaxies and massive
globular clusters in the Fornax and Virgo clusters and around the giant
elliptical Cen A have revealed some surprising results: 1) above about 10^6
M_sun the mass-to-light (M/L) ratio increases with the objects' mass; 2) some
UCDs/massive GCs show high M/L values (4 to 6) that are not compatible with
standard stellar population models; and 3) in the luminosity-velocity
dispersion diagram, UCDs deviate from the well defined relation of `normal'
GCs, being more in line with the Faber-Jackson relation of early-type galaxies.
In this contribution, we present the observational evidences for high
mass-to-light ratios of UCDs and discuss possible explanations for them.
