  We analyse the Fundamental Plane (FP) of 26 strong gravitational lens
galaxies with redshifts up to z~1, using tabulated data from Kochanek et al.
(2000) and Rusin et al. (2003). The lens galaxies effectively form a
mass-selected sample of early-type galaxies in environments of relatively low
density. We analyse the FP and its evolution in the restframe Johnson B and
Gunn r bands. Assuming that early-type galaxies are a homologous family, the FP
then provides a direct measurement of the M/L ratio evolution. If we assume
that the M/L ratios of field early-type galaxies evolve as power-laws, we find
for the lens galaxies an evolution rate dlog(M/L)/dz of -0.62 +/-0.13 in
restframe B, corresponding to a mean stellar formation redshift of <z*>=1.8
(-0.5/+1.4). This formation epoch does not differ significantly from that of
cluster galaxies <z*cl>=2.0 (-0.2/+0.3), obtained by van Dokkum & Franx (2001)
after correction for maximum progenitor bias. However, if we impose the
constraint that lens and cluster galaxies that are of the same age have equal
M/L ratios and we do not correct for progenitor bias, we find that the stellar
populations of the lens galaxies are 10-15 % younger. We find that both the M/L
ratios as well as the restframe colors of the lens galaxies show significant
scatter. About half of the lens galaxies are consistent with an old
cluster-like stellar populations, but the other galaxies are bluer and best fit
by single burst models with younger stellar formation redshifts as low as z*~1.
Moreover, the scatter in color is correlated with the scatter in M/L ratio. We
interpret this as evidence of a significant age spread among the stellar
populations of lens galaxies, whereas the ages of the stellar populations of
the cluster galaxies are well approximated by a single formation epoch.
