  Using integral field spectroscopy, we demonstrate that gravity-sensitive
absorption features in the zJ-band (0.9--1.35 \micron) can constrain the
low-mass stellar initial mass function (IMF) in the cores of two elliptical
galaxies, M85 and M87. Compared to the visible bands, the near-infrared (NIR)
is more sensitive to light from low-mass dwarf stars, whose relative importance
is the primary subject of the debate over IMF variations in nearby galaxies.
Our analysis compares the observed spectra to the latest stellar population
synthesis models by employing two different methods: equivalent widths and
spectral fitting. We find that the IMF slopes in M85 are similar to the
canonical Milky Way IMF with a median IMF-mismatch parameter $\alpha_{K} =
1.26$. In contrast, we find that the IMF in M87 is steeper than a Salpeter IMF
with $\alpha_{K} = 2.77$. The derived stellar population parameters, including
the IMF slopes, are consistent with those from recent results in the visible
bands based on spectroscopic and kinematic techniques. Certain elemental
abundances, e.g. Na and Fe, have dramatic effects on the IMF-sensitive features
and therefore the derived IMF slopes. We show evidence for a high [Na/H] $\sim$
0.65 dex in the core of M85 from two independent \ion{Na}{1} absorption
features. The high Na abundance may be the result of a recent galactic merger
involving M85. This suggests that including [Na/H] in the stellar population
model parameters is critical for constraining the IMF slopes in M85. These
results confirm the viability of using NIR absorption features to investigate
IMF variation in nearby galaxies.
