Grey seals are vulnerable to typical predators for a pinniped mammal. Otoliths cannot be used alone to reliably estimate cetacean or pinniped diets, however. All told, the walrus is the third largest pinniped species, after the two elephant seals. Unlike terrestrial mammals, pinniped milk has little to no lactose. Harbour seals are the most abundant pinniped in the eastern North Pacific Ocean. "Puijila darwini" was a semi-aquatic carnivore which represents a morphological link in early pinniped evolution. The name pinniped refers to mammals with front and rear flippers. Monk seals have one of the highest documented rates of entanglement of any pinniped species. For example, killer whales are known to intentionally beach themselves to catch pinniped prey. The parasite's spread threatens the recovery of this highly endangered pinniped. Three pinniped species breed on the islands. The breeding cycle of the Australian sea lion is unusual within the pinniped family. In comparison to other pinniped species, ringed seals haul-out with a shorter duration year round. Curiously, crabeater seals have been known to wander further inland than any other pinniped. It is also the only exclusively freshwater pinniped species.