Credit: A. MARTIN

Lustily trashing some hollow-stemmed matting grass, this pink Amazonian river dolphin (Inia geoffrensis) is working it down the waterway in Mamirauá, a flooded rainforest reserve in Brazil. It's almost an exculsively male practice, this brandishing of grasses, sticks or clay. It is thought to be a courtship ritual, although no researcher has seen the dolphins mating, despite more than 13,000 hours of observation. If so, it places dolphins, with humans, chimps and birds, among the few animals that use objects as a flirting device.