  We study the stellar populations of 1,923 elliptical galaxies at z<0.05
selected from the Sloan Digital Sky Survey as a function of velocity
dispersion, sigma, and environment. We construct average, high signal-to-noise
spectra and find the following: (1) lower-sigma galaxies have a bluer optical
continuum and stronger (but still weak) emission lines; (2) at fixed sigma,
field ellipticals have a slightly bluer stellar continuum, especially at
wavelengths \lesssim 4000 \AA, and have stronger (but still weak) emission
lines compared to their group counterparts, although this environmental
dependence is strongest for low-sigma ellipticals. Based on Lick indices
measured from both the individual and average spectra, we find that: (1) at a
given sigma, elliptical galaxies in groups have systematically weaker Balmer
absorption than their field counterparts, although this environmental
dependence is most pronounced at low sigma; (2) there is no clear environmental
dependence of <Fe>, while the alpha-element absorption indices such as Mgb are
only slightly stronger in galaxies belonging to rich groups. An analysis based
on simple stellar populations (SSPs) reveals that more massive elliptical
galaxies are older, more metal-rich and more strongly alpha-enhanced. We also
find that: (1) the SSP-equivalent ages of galaxies in rich groups are, on
average, ~1 Gyr older than in the field, although once again this effect is
strongest at low sigma; (2) galaxies in rich groups have slightly lower [Fe/H]
and are marginally more strongly alpha-enhanced; and (3) there is no
significant environmental dependence of total metallicity, [Z/H]. Our results
are generally consistent with stronger low-level recent star formation in field
ellipticals at low sigma, similar to recent results based on ultraviolet and
infrared observations. (Abridged)
