  We present evidence from cosmological hydrodynamical simulations for a
co-evolution of the slope of the total (dark and stellar) mass density profile,
gamma_tot, and the dark matter fraction within the half-mass radius, f_DM, in
early-type galaxies. The relation can be described as gamma_tot = A f_DM + B
for all systems at all redshifts. The trend is set by the decreasing importance
of gas dissipation towards lower redshifts and for more massive systems.
Early-type galaxies are smaller, more concentrated, have lower f_DM and steeper
gamma_tot at high redshifts and at lower masses for a given redshift; f_DM and
gamma_tot are good indicators for growth by "dry" merging. The values for A and
B change distinctively for different feedback models, and this relation can be
used as a test for such models. A similar correlation exists between gamma_tot
and the stellar mass surface density Sigma_*. A model with weak stellar
feedback and feedback from black holes is in best agreement with observations.
All simulations, independent of the assumed feedback model, predict steeper
gamma_tot and lower f_DM at higher redshifts. While the latter is in agreement
with the observed trends, the former is in conflict with lensing observations,
which indicate constant or decreasing gamma_tot. This discrepancy is shown to
be artificial: the observed trends can be reproduced from the simulations using
observational methodology to calculate the total density slopes.
