In bipedal mammals, the iliac crests are parallel to the vertically oriented sacroiliac Joints.

http://en.wikipedia.org/wiki/Pelvis

 In bipedal mammals, the iliac crests are parallel to the vertically oriented sacroiliac joints, where in quadrupedal mammals they are parallel to the horizontally oriented sacroiliac joints. In heavy mammals, especially in quadrupeds, the pelvis tend to be more vertically oriented because this allows the pelvis to support greater weight without dislocating the sacroiliac joints or adding torsion to the vertebral column.