I was watching the great BBC documentary "Frozen Planet" yesterday, and it has a great clip of narwhal pods swimming in narrow channels formed in the arctic ice by the arrival of the spring. Narwhals are fascinating because of their extraordinary asymmetrical canine tooth on the left side of their snout, protruding almost the length of the animal's body in the case of the males, from a weird opening in their upper "lip", not through their mouths. They are actually toothless except for that one long helical tusk that looks like a horn. In the documentary, two pods are swimming along the same narrow channel, one individual at a time, and when they meet, after communicating for a bit, both pods start swimming in the same direction, meaning one pod had to have decided to yield and go with the flow of the other pod. There was no fighting. I watched in amazement. And today I discover a great blog post from Jerry Coyne, on a recent scientific article on the narwhals's tooth. It is full of amazing details, well worth the read. The tooth is a bit of a mystery from an evolutionary point of view: was it sexual selection? Was it an epigenetic by-product of selection?  It apparently can detect water temperature and salinity, and the presence of fish. A truly extraordinary creature.