Monday, June 2, 2008

Talk: Bill Minkel

Bill Minkel: The Amazonian pink river dolphin

[ Bill had great photos of these dolphins; you might want to do some surfing on Google Images to see what they look like! ]

The Amazonian pink river dolphin (henceforth referred to as a "boto," the local native name; it has many other names) is quite unlike more familiar dolphin species. It has a very wide range, extending into Peru, Bolivia, Venezuela, and Ecuador, and is found in both the Orinoco and Amazon basins. It is found only in rivers; it does not go into salt waters. There are three subspecies which derived from a common ancestor perhaps 8-12 million years ago (longer ago than when salt water dolphins evolved, interestingly); one subspecies is found in the Orinoco, one in the Amazon basin, and one in a tributary of the Amazon above a 500 meter high waterfall barrier that has acted as a cause of allopatric speciation. There are various similar dolphin species around the world that may be distantly related.
The boto has a very flexible body and neck due to unfused vertebrae, unlike mostly marine dolphins. The boto also has very wide pectoral flipper and flukes, and has a very long humerus in its pectoral fin that allows its fin to describe figure eights that allow extremely precise motion. Their dorsal fin is less tall than in marine dolphins, perhaps advantageous for maneuvering in complex river environments. Their rostrum (their "nose") is quite elongate compared to marine dolphins, and vibrissal hairs on their rostra may be used as vibrational sensory structures, or may be vestigial. They have a large, highly flexible melon on their forehead used to focus sounds for echolocation; their echolocation abilities are quite impressive. Their eyes are very small but their vision is good. Their dentition is heterodont (different teeth are different shapes), unlike most marine dolphins, to cope with a wide variety of food; they eat many kinds of fish, some freshwater crustaceans, and even river turtles (whose shells they break with their posterior teeth).
Boto behavior can be playful or aggressive; particularly in mating season, males get aggressive, and they often bear rake scars from the teeth of other males. They porpoise ("dive" upwards into the air) less often than marine dolphins typically do, and are rather less friendly than other dolphins. Their mating pattern is similar to lions: many times in a short period of time (perhaps every four minutes for an hour or so). Boto nurse for about a year and stay with their mother for about two and a half years. Pairs seen together are almost always a mother and calf, but they will occasionally group together for feeding.
Boto live roughly twenty years in the wild (up to 28 or so), but only a few years in captivity. The males grow to about eight feet and 700-800 lbs.; females are 6-6.5 feet long and therefore less massive. Their color varies considerably, but seems to generally change from gray when they are young to pink when they are older. The pink color is often patterned, not uniform, and can be used to identify individuals.
The boto characteristics are probably less derived than those of marine dolphins; they appear to represent an earlier branch in odontocete evolution. The odontocetes are thought to have evolved due to geologic and oceanic changes (particularly sea level changes) since the middle Miocene (~15 mya). At that time, ocean levels were much higher (perhaps 150 meters higher!) than today, and much of the Amazon was open water. At that time, the drainage from the continent seems to have been northward, out of the Orinoco; the northern Andes that now cut the Amazon basin off from the Orinoco had not yet uplifted significantly. The common ancestor of the boto subspecies presumably lived in that area, and as uplift and sea level lowering occurred, populations were isolated and diverged. About 10 mya the Amazon started to drain chiefly to the east, and the areas occupied by the subspecies were probably completely separated.
In native Brazilian folklore, the boto is an incantado, an enchanted being. They inhabit the enchanted world below the surface of the water, but they can transform into humans, especially at night, as botos incantados to walk among people. Male incantados are often described as handsome young men dressed all in white, good dancers and drinkers, on the lookout for young women. Female incantados seduce men to come and live with them in the enchanted world under the surface where life is beautiful and easy. Botos incantados are often used as scapegoats for human activity that is uncomfortable, embarrassing, or inexplicable.
Today botos are doing reasonably well. They are listed as threatened, but their current distribution does not appear to differ significantly from its estimated past distribution. Some loss due to net entanglement has been observed, and they are sometimes used as fish bait; they are also threatened by planned hydroelectric dam development. But today, at least, the current status of boto populations is good.

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