BEIJING, Sept. 24 (Xinhuanet) -- Small, gray marine mammals called beaked whales have been at the center of an argument about the U.S. Navy's use of high intensity sonar since several washed ashore with bleeding around their brains and ears during naval exercises in the Bahamas seven years ago.

"They appear to be the most susceptible group of cetaceans to impacts from Navy sonars," said Robin Baird, a marine biologist based in Olympia, Wash., whose team recently spent three weeks off Hawaii's Big Island studying whales.

Environmentalists have filed lawsuits challenging the Navy's plans for sonar training exercises, claiming the underwater noise harms whales and arguing there's enough evidence to require the Navy to take more aggressive measures to protect the animals.

Many scientists suspect it is the beaked whales' unique ability to swim at great depths for long periods that makes them more vulnerable to sonar.

One unverified theory is that the loud sonar noise startles the whales, prompting them to surface unusually rapidly and causing injuries similar to the bends in human divers.

Scientists in the Bahamas ran controlled experiments this month to see how beaked whales and other marine mammals respond to different sounds. The Navy is almost entirely funding the study, about 3 million U.S. dollars this year.

"We still know almost nothing about the reactions of marine mammals to underwater sound," Brandon Southall, the Bahamas study's principal investigator, told reporters in a recent conference call. "Our field is very much in its infancy."

Adm. Robert F. Willard, U.S. Pacific Fleet commander, said the Navy is willing to post whale lookouts on its ships and limit sonar use when the animals get too close. But he said there's no scientific basis for more stringent measures demanded by some environmentalists, including designating entire areas as non-sonar zones.

(Agencies)