Exaggerated secondary sexual characteristics are apparently costly and seem to defy natural selection. This conundrum prompted Charles Darwin to propose the theory of sexual selection. Accordingly, exaggerated secondary sexual characteristics might be ornaments on which female choice is based and/or armaments used during male-male competition. Males of many cichlid fish species, including the adaptive radiation of Nicaraguan Midas cichlids, develop a highly exaggerated nuchal hump, which is thought to be a sexually selected trait. To...