  Various formation channels for the puzzling ultra-compact dwarf galaxies
(UCDs) have been proposed in the last few years. To better judge on some of the
competing scenarios, we present spectroscopic [Fe/H] estimates for a sample of
26 compact objects in the central region of the Fornax cluster, covering the
magnitude range of UCDs and bright globular clusters. We find a break in the
metallicity distribution of compact objects at M_V ~ -11 mag (~3*10^6 M_sun):
for M_V<-11 mag the mean metallicity is [Fe/H]=-0.62 +/- 0.05 dex, 0.56 +/-
0.15 dex higher than for M_V>-11 mag. This metallicity break is accompanied by
a change in the size-luminosity relation for compact objects, as deduced from
HST-imaging: for M_V<-11 mag, r_h scales with luminosity, while for M_V>-11
mag, r_h is almost luminosity-independent. We therefore assume a limiting
absolute magnitude of M_V=-11 mag between UCDs and globular clusters. The mean
metallicity of five Fornax dE,N nuclei included in our study is about 0.8 dex
lower than that of the UCDs, at 4.5 sigma significance. Because of this large
metallicity discrepancy we disfavor the hypothesis that most of the Fornax UCDs
are the remnant nuclei of tidally stripped dE,Ns. Our metallicity estimates for
UCDs are closer to but slightly below those derived for young massive clusters
(YMCs) of comparable masses. We therefore favor a scenario where most UCDs in
Fornax are successors of merged YMCs produced in the course of violent
galaxy-galaxy mergers. It is noted that in contrast to that, the properties of
Virgo UCDs are more consistent with the stripping scenario, suggesting that
different UCD formation channels may dominate in either cluster.
