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More than 40 whales stranded on a beach in North Andamans, in this photo released by the Andaman and Nicobar Islands department of environment and forests on Oct. 25.
Dozens of whales beached themselves and died on North Andaman Island in the Bay of Bengal this week, the first time that such a large number of whales have died in the area.

Scientists are still trying to figure out why.

Individual whales have occasionally beached themselves in the Andamans, but never before in these numbers, said Samir Acharya, president of Society for Andaman and Nicobar Ecology, a nongovernmental organization based in Port Blair, the capital of the Andaman and Nicobar islands. "This happens once in a while, every year or every second year," he said, but "the numbers this time are really large."

A group of 41 short-finned pilot whales were found near Elizabeth Bay, on the west coast of North Andaman Island, by local fisherman on Oct 21, said Ajai Saxena, additional chief conservator of wildlife in Port Blair, in a telephone interview on Friday. The whales are four to six meters (about 12 to 18 feet) long each and as heavy as four tons, he said.

Officials who conducted a post-mortem investigation on one of the whales did not find any unnatural cause of death, Mr. Saxena said. The 41 whales are being buried in pits on the beach.

Whales migrate in a group, called a pod, to the cold waters of Antarctica because of an abundance of food, Mr. Acharya said, and migrate back to warmer waters during winter to mate and give birth. They use sonar for direction, emitting sounds and using their echos to judge the depth of the water and the direction they are traveling.

Sometimes, Mr. Acharya said, different layers of ocean water, due to differences of temperature and salinity, result in a false echo and confuse the whales, making them think shallow waters are deep.

Military sonar has also been proved to distract whales and drive them off course. In 2000, for example, whales of several species stranded themselves on the beaches of the Bahamas during United States naval exercises. A report by the Natural Resources Defense Council about the incident found that "although the Navy initially denied responsibility, the government investigation established that mid frequency sonar caused the standings."

The United States Supreme Court ruled in 2008 that military training trumped protecting whales, and the Navy continues to use the mid-frequency sonar that is believed to interrupt whales' navigation.

The Indian Navy has a significant presence in the Andaman and Nicobar islands, which are seen as a key part of India's defense against a growing Chinese naval presence in the area. The Indian Navy did not return phone calls and e-mails related to the beached whales, and there is no information connecting naval activities with the giant mammals' unusual activity at this time.