Complex sentence: Lions are the only members of the cat family to display obvious sexual dimorphism--males are more robust with broader heads and have a prominent mane which grows downward and backward and may cover most of the head, neck, shoulders, and chest.
Split into simpler sentences: Lions are the only members of the cat family to display obvious sexual dimorphism. Males are more robust than females, have broader heads and a prominent mane, which grows downward and backward and covers most of the head, neck, shoulders, and chest.