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Weird & Wonderful Creatures: Amazon River Dolphins


The Amazon river dolphin (Inia geoffrensis), also known as the boto or pink river dolphin, is the largest freshwater dolphin in the world. It is found in northern and central South America, specifically in the Amazon and Orinoco basins and the Madeira River.

The boto's most striking physical feature is its color. Calves start out a deep grey color, but pale as they get older, and adults are a mottled (blotchy) or solid pink. The pink color seems to come from abrasions, or scrapes, to the skin, and males, who can be very aggressive, often appear much pinker than females. Scientists believe that the murkiness of the water in which the dolphins live may also affect the brightness of their coloring, with clearer waters allowing sunlight to bleach them to a less vivid pink. Some speculate that water temperature may also be a factor in the intensity of the color.

The Amazon river dolphin has some other unusual features. Because its spine isn't fused together, like most other dolphins, it is able to turn its head from side to side. Ranging in length from 7 to 8.5 feet long, it has a long ridge along its back, which ends at its rather short dorsal fin. Its long flippers can operate independently of one another, which allows it to navigate fast-moving water more easily and also allows it to enter flooded forestland in pursuit of prey, but they also cause it to be a slow swimmer.

Botos have good eyesight, but their chubby cheeks seem to affect their ability to see below them, which scientists believe explains why they're sometimes seen swimming upside down. They have bristles on their snout that help them sense prey on the river bottom. They also use echolocation, producing low-frequency clicks and using the echoes to help them navigate obstacles in the water and find prey. The dolphins emit these clicks through the fatty lump on their forehead, called the melon, which helps focus the sounds into a beam. Botos can change the shape of their melon, which scientists think may help them change the echolocation beam's size, direction, and frequency. Echoes are received back in the dolphin's lower jawbone.

The dolphin's snout is long and narrow and filled with two types of teeth: the front are pointy and allow it to catch and hold prey, while the back teeth are flatter for grinding their food. Perhaps because of their two types of teeth, the Amazon river dolphin has a varied diet, eating more than 50 different types of fish, as well as freshwater crabs and turtles.

Botos usually live on their own, although they may also live in pairs (often a mother and her calf, born during the July floods and then staying with its mother for two years) or in small pods, or groups. In fish-rich areas, larger groups may come together to feed.

Due to insufficient data and increasing threats, the Amazon river dolphins numbers are unknown. People have killed them for bait and to prevent their competition with the fishing industry. The dolphins have also died after being caught in fishing nets. In the 1950s–1970s, aquariums in North America, Europe, and Japan paid to capture more than 100 dolphins, but most of the animals died after a short time in captivity. More recently, their habitats have been threatened by damming rivers and by mercury pollution from gold mining. 

You can learn more about dolphin brains and dolphin healing in these Science Update podcasts.

Bats are another mammal that uses echolocation. You can watch a nighttime bat walk in this video.

In this video, you can see how another large river, the Mississippi, has helped shape its ecosystem.

 

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