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
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