Species

The Boto (also known as the Amazon river dolphin or pink dolphin) is confined to freshwater and occurs only in the Amazon and Orinoco river basins of South America. There are botos in Brazil, Venezuela, Colombia, Peru and Bolivia. In Mamirauá males reach a maximum length of 249 cm and a weight of 185 kg, but adult females do not exceed 216 cm and 142 kg. The Boto has long, forceps-like jaws and many small teeth designed for grasping fish, its main prey. It has unusually strong jaws and, uniquely among dolphins, cusped rear teeth which are capable of crushing armoured fish and even turtles. The Boto is slow-swimming and flexible, with wide flippers, enabling it to forage among the tangled undergrowth of the rainforest when water levels rise and flood the Mamirauá Reserve and similar low-lying habitat.

Although famous for its pink colour, not all botos are pink. Youngsters start life a dark grey, and become lighter with age. Truly pink dolphins are almost always adult males, and we now know that the pinkness is caused by scarring. Many adult males are almost completely covered with scars caused by fighting with other males, so they appear pink. Some old females may also appear pink, but most are a light grey above and pinkish below.

All river dolphins have a diet dominated by fish, and botos are no exception. They eat many different species and sizes of fish, and their diet varies seasonally as the river level rises and falls.

Our research has shown that male and female botos are segregated for much of the year. Adult males dominate the main rivers, while females and their calves prefer the more protected waters of the floodplain. All are forced onto the rivers at low water, and it may be that this is when most mating activity occurs. This sexual segregation is unusual in cetaceans (whales, dolphins and porpoises), and is most pronounced in the sperm whale (Physeter macrocephalus). It may be no coincidence that, as in botos, male sperm whales are much larger than females, and have a heavy burden of scarring from fighting with other males.

The Tucuxi is one of the smallest of all dolphins (maximum around 152 cm). In body shape it resembles the more familiar Bottlenose Dolphin (Tursiops truncatus), and is classed in the family of marine dolphins Delphinidae. The Tucuxi occurs only in the Amazon river, and has a close relative called the marine tucuxi (Sotalia guianensis) in coastal waters of eastern South and central America. The two species have only recently been recognised as distinct. Tucuxis are very different from botos in many respects – they are faster, travel in tight groups, avoid shallow waters and are much more wary of human presence. They share with Botos a diet of fish, and also a tendency to become entangled in fishing nets.

Tucuxis occur in or near Mamirauá year-round, but they may be absent from the flooded forest for months on end during the low-water period. They often appear within sight of our research base apparently out of nowhere, and may disappear again as rapidly. They surface very quickly, so photographing them is a particular challenge, and can be seen pursuing fish at great speed just below the water surface.

© Projeto Boto 2006. Site designed by Ed Parnell.