  We combine the globular cluster data for fifteen Brightest Cluster Galaxies
and use this material to trace the mass-metallicity relations (MMR) in their
globular cluster systems (GCSs). This work extends previous studies which
correlate the properties of the MMR with those of the host galaxy. Our combined
data sets show a mean trend for the metal-poor (MP) subpopulation which
corresponds to a scaling of heavy-element abundance with cluster mass Z ~
M^(0.30+/-0.05). No trend is seen for the metal-rich (MR) subpopulation which
has a scaling relation that is consistent with zero. We also find that the
scaling exponent is independent of the GCS specific frequency and host galaxy
luminosity, except perhaps for dwarf galaxies.
  We present new photometry in (g',i') obtained with Gemini/GMOS for the
globular cluster populations around the southern giant ellipticals NGC 5193 and
IC 4329. Both galaxies have rich cluster populations which show up as normal,
bimodal sequences in the colour-magnitude diagram.
  We test the observed MMRs and argue that they are statistically real, and not
an artifact caused by the method we used. We also argue against asymmetric
contamination causing the observed MMR as our mean results are no different
from other contamination-free studies. Finally, we compare our method to the
standard bimodal fitting method (KMM or RMIX) and find our results are
consistent.
  Interpretation of these results is consistent with recent models for globular
cluster formation in which the MMR is determined by GC self-enrichment during
their brief formation period.
