This week let's learn about some unusual cetaceans, river dolphins! An Amazon river dolphin and the nose of another Amazon river dolphin: Another Amazon river dolphin. Note the teeny eye and disk-like teeth: A South Asian river dolphin. Note the almost nonexistent eye: A Chinese river dolphin in better days: Show transcript: Welcome to Strange Animals Podcast. I’m your host, Kate Shaw. This week we’re going to learn about an animal I’ve wanted to cover for a long time but never got around to, the river dolphin. All whales and almost all dolphins live in the ocean. They can survive for a short while in fresh water but need saltwater to thrive. But as the name suggests, the river dolphin lives in rivers, usually in fresh water and sometimes in the brackish water where rivers empty into oceans. Brackish just means a mixture of fresh and salt water, so it’s saltier than fresh water but not as salty as ocean water. When I was a kid I thought it meant ocean water with a lot of seeweed and dead leaves in it, because I thought the word brackish had something to do with bracken, which is a type of fern. There are only a few species of river dolphin alive today. They live in warm water and have very little blubber as a result. They primarily use echolocation to navigate since river water tends to be muddy and hard to see through. Their flippers are broad and most have long snouts and flexible necks, or at least flexible compared to other cetaceans. All river dolphins are endangered due to pollution, habitat loss, and accidental drowning in fish nets, with the Chinese river dolphin only having been declared extinct in 2006. River dolphins evolved from dolphins that once lived in the ocean, but most aren’t closely related to the marine dolphins alive today. Researchers think that when modern dolphins and other toothed whales evolved, they outcompeted their more primitive cousins, who moved into freshwater habitats as a result. A few years ago, fossils of an extinct river dolphin that grew more than nine feet long, or almost three meters, were found in Panama. It lived around 6 million years ago in the Amazon River, but researchers don’t think modern river dolphins are closely related to it. In other words, freshwater dolphins have evolved repeatedly in different parts of the world to fit an available ecological niche. In the case of the newly discovered fossil river dolphin, Isthminia panamensis, it probably lived in the warm, shallow Caribbean Sea between North and South America before the Isthmus of Panama formed. It took millions of years for the isthmus to form, with undersea volcanos first emerging from the ocean around 15 million years ago to form islands, then the land itself being pushed upward as the Pacific Plate slid underneath the Caribbean Plate. Researchers think the isthmus became fully formed around 3 million years ago, separating the Atlantic Ocean from the Pacific and connecting North and South America. Because we have Isthminia panamensis’s fossil from the Amazon River, we can hypothesize that by around 6 million years ago, there wasn’t enough of the original Caribbean Sea habitat to support the dolphins and they had already moved into the Amazon River and adapted to life in freshwater. The river dolphin isn’t the only cetacean that lives in freshwater. There’s a species of porpoise called the finless porpoise that lives around Asia in shallow coastal waters, but often spends at least part of the time in mangrove swamps and in rivers not too far from the sea. Porpoises and dolphins look very similar but belong to different families, which means they aren’t actually very closely related at all. Like, seriously not related at all. Like, the difference between horses and cows. The finless porpoise can grow almost seven and a half feet long, or over two meters, and is called finless because it doesn’t have a dorsal fin. Instead,