Encased in a neon orange plastic shell, a collection of electronic sensors bobbed along the surface of the Monterey Bay, waiting to be retrieved by Stanford University researchers. A lunchbox-sized speck in the vast waters, it held cargo of outsized importance: the first-ever recording of a blue whale's heart rate.

This device was fresh off a daylong ride on Earth's largest species - a blue whale. Four suction cups had secured the sensor-packed tag near the whale's left flipper, where it recorded the animal's heart rate through electrodes embedded in the center of two of the suction feet. The details of this tag's journey and the heart rate it delivered were published Nov. 25 in Proceedings of the National Academy of Sciences.

"We had no idea that this would work and we were skeptical even when we saw the initial data. With a very keen eye, Paul Ponganis - our collaborator from the Scripps Institution of Oceanography - found the first heart beats in the data," said Jeremy Goldbogen, assistant professor of biology in the School of Humanities Sciences at Stanford and lead author of the paper. "There were a lot of high fives and victory laps around the lab."

Analysis of the data suggests that a blue whale's heart is already working at its limit, which may explain why blue whales have never evolved to be bigger. The data also suggest that some unusual features of the whale's heart might help it perform at these extremes. Studies like this add to our fundamental knowledge of biology and can also inform conservation efforts.

"Animals that are operating at physiological extremes can help us understand biological limits to size," said Goldbogen. "They may also be particularly susceptible to changes in their environment that could affect their food supply. Therefore, these studies may have important implications for the conservation and management of endangered species like blue whales."

Penguins to whales

A decade ago, Goldbogen and Ponganis measured the heart rates of diving emperor penguins in Antarctica's McMurdo Sound. For years after, they wondered whether a similar task could be accomplished with whales.

"I honestly thought it was a long shot because we had to get so many things right: finding a blue whale, getting the tag in just the right location on the whale, good contact with the whale's skin and, of course, making sure the tag is working and recording data," said Goldbogen.

The tag performed well on smaller, captive whales, but getting it near a wild blue whale's heart is a different task. For one thing, wild whales aren't trained to flip belly-up. For another, blue whales have accordion-like skin on their underside that expands during feeding, and one such gulp could pop the tag right off.

"We had to put these tags out without really knowing whether or not they were going to work," recalled David Cade, a recent graduate of the Goldbogen Lab who is a co-author of the paper and who placed the tag on the whale. "The only way to do it was to try it. So we did our best."

Cade stuck the tag on his first attempt and, over time, it slid into a position near the flipper where it could pick up the heart's signals. The data it captured showed striking extremes.

When the whale dove, its heart rate slowed, reaching an average minimum of about four to eight beats per minute - with a low of two beats per minute. At the bottom of a foraging dive, where the whale lunged and consumed prey, the heart rate increased about 2.5 times the minimum, then slowly decreased again. Once the whale got its fill and began to surface, the heart rate increased. The highest heart rate - 25 to 37 beats per minutes - occurred at the surface, where the whale was breathing and restoring its oxygen levels.

An elastic heart

This data was intriguing because the whale's highest heart rate almost outpaced predictions while the lowest heart rate was about 30 to 50 percent lower than predicted. The researchers think that the surprisingly low heart rate may be explained by a stretchy aortic arch - part of the heart that moves blood out to the body - which, in the blue whale, slowly contracts to maintain some additional blood flow in between beats. Meanwhile, the impressively high rates may depend on subtleties in the heart's movement and shape that prevent the pressure waves of each beat from disrupting blood flow.

Looking at the big picture, the researchers think the whale's heart is performing near its limits. This may help explain why no animal has ever been larger than a blue whale - because the energy needs of a larger body would outpace what the heart can sustain.

Now, the researchers are hard at work adding more capabilities to the tag, including an accelerometer, which could help them better understand how different activities affect heart rate. They also want to try their tag on other members of the rorqual whale group, such as fin whales, humpbacks and minke whales.

"A lot of what we do involves new technology and a lot of it relies on new ideas, new methods and new approaches," said Cade. "We're always looking to push the boundaries of how we can learn about these animals."

###

Additional Stanford co-authors include graduate students Max Czapanskiy, James Fahlbusch, William Gough and Shirel Kahane-Rapport and postdoctoral fellow Matt Savoca. Ponganis is senior author of the paper and additional co-authors are from Cascadia Research Collective; the University of California, Santa Cruz; and Scripps Institution of Oceanography. Goldbogen is also a member of Stanford Bio-X.

This research was funded by the Office of Naval Research, a Terman Fellowship from Stanford University and the John B. McKee Fund at Scripps Institution of Oceanography.