Some elements of the images are that they are taken with their bodies oiled down so that their skin is reflective on the surface...second they are shot with lights that are behind or to the side of them so that you get highlights across the body and any high ridge on the torso or arms etc...third the skin is colored with a curve adjustment layer that is pulled down in the green channel and pulled down a lot in the rgb channel so that the skin looks very dark magenta or brown. To get true brown you will have to add in small amounts of blue and red also in the curves and adjust them so the color looks right to you. The adjustment layer is transparent and so it does not cover or blur or degrade the quality or texture of the skin. They may have also gotten an airbrushing of dark tan coloring from the staff makeup artist as well before the shoot.
These are the aspects that I see...im sure there are others.
Best of luck.