  We use our new stellar population models, which include effects from variable
element abundance ratios, to model the Ca4227 absorption line indices of
early-type galaxies (Trager et al.), and to derive calcium element abundances.
We find that calcium, although being an alpha-element, is depressed with
respect to the other alpha-elements by up to a factor 2. This confirms
quantitatively earlier speculations that early-type galaxies are calcium
underabundant. We find a clear correlation between alpha/Ca ratio and central
velocity dispersion, which implies that more massive galaxies are more calcium
underabundant. Interestingly this correlation extends down to the dwarf
spheroidal galaxies of the Local Group for which alpha/Ca ratios have been
measured from high-resolution spectroscopy of individual stars (Shetrone et
al.). The increase of the calcium underabundance with galaxy mass balances the
higher total metallicities of more massive galaxies, so that calcium abundance
in early-type galaxies is fairly constant and in particular does not increase
with increasing galaxy mass. This result may be the key to understand why the
CaII triplet absorption of early-type galaxies at 8600 A is constant to within
5 per cent over a large range of velocity dispersions (Saglia et al.; Cenarro
et al.). The origin of the calcium underabundance in early-type galaxies
remains yet to be understood. We argue that formation timescales are
disfavoured to produce calcium underabundance, and that the option of
metallicity dependent supernova yields may be the most promising track to
follow.
