Adan; the way his ass got like that, is from numero uno having an incredibly low body fat %. The striation (muscle lines) only appear when there is almost no fat present. Also he probably did a load of glute exercises (squats, jump squats, etc...) But the biggest factor is deffinetly a low bf%, probably around 1-2%, most pro body builders are below 5%, and he is also probably loaded with diaretics to rid his body of water. (water distorts the hard look of the muscle)