Male beaked whales of most species have prominent teeth, or small tusks, which are visible even when the mouth is closed. (In one weird species, they actually curve in enough to restrict how far the mouth can open). Why do they exist? They are not used for feeding and don't have any other critical use, though some males scar each other with them. Some cetologists now speculate they evolved distinctive placement and form, varying between species, because that helped females distinguish between species that are often so externally similar that even experts need have trouble telling them apart.
COMMENT: Why tusks as a differentiator? Why not color, or dorsal fin shape, or something else more obvious? And do they really differentiate visually anyway? This article captures some of the debate on this. It also gives me an opportunity to mention two of the cetologists quoted here, Drs. Robert Pitman and Merel Dalebout, and give them a nod of recognition (not that they need it) because they were happy to answer questions from some science writer they probably never heard of while I was writing Shadows of Existence: Discoveries and Speculations in Zoology (2006).