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Franciscana Dolphin: Pontoporiidae

FRANCISCANA DOLPHIN: Pontoporiidae

PHYSICAL CHARACTERISTICS

Franciscana dolphins are also called La Plata dolphins, because the first described specimen, or animal, came from the mouth of La Plata River, Uruguay, in 1884. These dolphins are considered river dolphins, even though they live in the ocean near the shoreline. Originally scientists thought that the franciscana dolphin moved from fresh water to salt water during its lifetime, but now they know that it spends its entire life in the ocean. In the past, franciscana dolphins have been classified in several different dolphin families, but they are currently classified in a family of their own.

The franciscana dolphin is one of the smallest members of the cetacean order. They measure between 4.4 and 5.7 feet (1.3 and 1.7 meters) and weigh between 75 and 115 pounds (34 and 53 kilograms). Females are larger than males. Franciscana dolphins are gray-brown on their back and lighter underneath. Young franciscana dolphins are darker than older animals. Very old animals can appear almost white.

The most notable feature of the franciscana dolphin is its long, slender beak, or snout. They have the longest beak of any dolphin. Their beak may be 15 percent of their body length. Franciscana dolphins have triangular dorsal, or back, fins with rounded tips. Their flippers are broad and short. This dolphin has between 208 and 242 teeth small teeth. The blowhole, or nostril, is a crescent-shaped slit. Unlike the Ganges and Indus river dolphins, franciscana dolphins have good eyesight.

Even though franciscana dolphins can see well, they use echolocation (eck-oh-loh-KAY-shun) to find food and navigate through their environment. The forehead of a dolphin is a lump of fatty tissue called the melon. Echolocation is a sensory system in which dolphins make sounds that seem to be focused through the melon and then sent out into the environment. When the sounds bounce back, the echo is passed through special tissue in the lower jaw to the inner ear. From the time it takes to collect the echoes, their strength, and their direction, dolphins construct a "sound picture" of their environment. This system is extremely sensitive and allows the animal to locate very small objects. Scientists disagree about just how the dolphins actually make the sounds.

GEOGRAPHIC RANGE

Franciscana dolphins are found in the Atlantic Ocean along the coasts of Brazil, Uruguay, and Argentina in South America. Their northern boundary is near Rio de Janeiro, Brazil, and their southern boundary is the Valdes Peninsula in Argentina. Their distribution within this range is uneven. In some places they are rare or absent, and in others they are more common.

HABITAT

Franciscana dolphins are usually found within 33 miles (53 kilometers) of shore in waters no more than 30 feet (10 meters) deep. Often they are found in muddy, murky water with poor visibility. They seem to prefer estuaries, which are places where rivers empty into the ocean and fresh water mixes with salt water.

DIET

Franciscana dolphins eat a wide variety of small bottom-dwelling fish, squid, octopus, and shrimp. Most of the fish they feed on are less than 4 inches (10 centimeters) long.

HOW DO DOLPHINS SLEEP?

Dolphins must rise to the surface to breathe every few minutes. How can they do this and still sleep? The answer is found in the way their brain functions. One half or hemisphere of the brain rests, while the other stays alert and makes sure the dolphin surfaces and breathes. When one half of the brain is rested, it takes over and the other half sleeps.

BEHAVIOR AND REPRODUCTION

Franciscana dolphins usually swim alone or in small groups. Several dolphins may cooperate when feeding. They will swim in a tight circle, surrounding the fish and pushing them together.

Franciscana dolphins are very quiet and shy at the surface. They rarely jump and often only raise their heads out of the water enough to breathe. They are preyed upon by sevengill sharks, hammerhead sharks, and possibly killer whales.

Female franciscana dolphins give birth to one calf after an eleven-month pregnancy. Most calves are born between October and January, spring in the Southern Hemisphere. Newborns are about 28 inches (71 centimeters) long and weigh 16 to 19 pounds (7 to 8.5 kilograms). They nurse, feed on their mother's milk, for about three months. After that, they continue to nurse, but also eat fish until they are completely weaned and not dependent on their mother's milk at about nine months. There is some disagreement about when these dolphins become sexually mature and able to reproduce. Estimates range from two to four-and-a-half years. Their average natural lifespan is about fifteen years.

Franciscana dolphins do not strictly migrate. However, it appears that in areas off the coast of Argentina where there is noticeable seasonal variation in water temperature, they may change their range. This movement does not seem to happen off the coast of Brazil, where water temperatures remain more constant throughout the year.

FRANCISCANA DOLPHINS AND PEOPLE

Franciscana dolphins are shy and rarely intentionally interact with people. However, these dolphins are sometimes caught in fishing nets. In these cases, their oil is used in tanning leather, and their flesh is used as pig feed or shark bait.

CONSERVATION STATUS

The wild population of franciscana dolphins is unknown. Because of this, they are given a Data Deficient conservation status. However, it is estimated that up to 1,500 of these animals are drowned every year by becoming tangled in gillnets and other fishing gear. Scientists believe that as a result, the wild population is decreasing. In addition, because these dolphins live close to shore, they are more at risk for habitat pollution than dolphins that live in the open ocean.

FOR MORE INFORMATION

Books:

Carwadine, Mark, and Martin Camm. Smithsonian Handbooks: Whales Dolphins and Porpoises. New York: DK Publishing, 2002.

Gowell, Elizabeth T. Whales and Dolphins: What They Have in Common. New York: Franklin Watts, 2000.

Mead, James G., and Joy P. Gold. Whales and Dolphins in Question: The Smithsonian Answer Book. Washington, DC: Smithsonian Institution Press, 2002.

Nowak, Ronald. M. "Franciscana, or La Plata Dolphin." Walker's Mammals of the World Online 5.1. Baltimore: Johns Hopkins University Press, 1997. http://www.press.jhu.edu/books/walkers_mammals_of_the_world/cetacea/cetacea.pontoporiidae.pontoporia.html (accessed on July 8, 2004).

Web sites:

American Cetacean Society. http://www.acsonline.org (accessed on July 8, 2004).

Whale and Dolphin Conservation Society. http://www.wdcs.org (accessed on July 8, 2004).

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