Near-IR line-strengths in elliptical galaxies: evidence for IMF variations?

We present new relations between recently defined line-strength indices in the near-IR (CaT*, CaT, PaT, MgI, and sTiO) and central velocity dispersion (sigma_0) for a sample of 35 early-type galaxies, showing evidence for significant anti-correlations between CaII triplet indices (CaT* and CaT) and log(sigma_0). These relations are interpreted in the light of our recent evolutionary synthesis model predictions, suggesting the existence of important Ca underabundances with respect to Fe and/or an increase of the dwarf to giant stars ratio along the mass sequence of elliptical galaxies.