WASHINGTON (Reuters) - Using a bright orange electrocardiogram machine attached with suction cups to the body of a blue whale, scientists for the first time have measured the heart rate of the world's largest creature and came away with insight about the renowned behemoth's physiology.

The blue whale, which can reach up to 100 feet (30 meters) long and weigh 200 tons, lowers its heart rate to as little as two beats per minute as it lunges under the ocean surface for food, researchers said on Monday.

The maximum heart rate they recorded was 37 beats per minute after the air-breathing marine mammal returned to the surface from a foraging dive.

"The blue whale is the largest animal of all-time and has long fascinated biologists," said Stanford University marine biologist Jeremy Goldbogen, who led the study published in the journal Proceedings of the National Academy of Sciences.

"In particular, new measures of vital rates and physiological rates help us understand how animals work at the upper extreme of body mass," Goldbogen added. "What is life like and what is the pace of life at such a large scale?"

Generally speaking, the larger the animal, the lower the heart rate, minimizing the amount of work the heart does while distributing blood around the body.

The normal human resting heart rate ranges from about 60 to 100 beats per minute and tops out at about 200 during athletic exertion. The smallest mammals, shrews, have heart rates upwards of a thousand beats per minute.

The researchers created a tag device, encased in an orange plastic shell, that contained an electrocardiogram machine to monitor a whale's heart rhythm swimming in the open ocean. The device had four suction cups to enable them to attach it to the whale non-invasively.

The researchers obtained nine hours of data from an adult male whale about 72 feet (22 meters) long encountered in Monterey Bay off California's coast.

"First we have to find a blue whale, which can be very difficult because these animals range across vast swaths of the open ocean. By combining many years of field experience and some luck, we position a small, rigid-hulled, inflatable boat on the whale's left side," Goldbogen said.

"We then have to deploy the tag using a six-meter (20-foot) long carbon-fiber pole. As the whale surfaces to breathe, we tag the whale in a location that we think is closest to the heart: just behind the whale's left flipper," Goldbogen added.

Baleen whales such as blue whales, despite their immense size, feed on tiny prey. As filter-feeders, they take huge amounts of water into their mouths and strain out prey including shrimp-like krill and other zooplankton using baleen plates made of keratin, the same material found in fingernails.

During feeding dives, the whale exhibited extremely low heart rates, typically of four to eight beats per minute and as low as two. After surfacing to breathe following foraging dives, the whale had heart rates of 25 to 37 beats per minute.

(Reporting by Will Dunham; Editing by Sandra Maler)

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