Blue Whale Song Mystery Baffles Scientists

All around the world, blue whales aren’t singing like they used to, and scientists have no idea why.

The largest animals on Earth are singing in ever-deeper voices every year. Among the suggested explanations are ocean noise pollution, changing population dynamics and new mating strategies. But none of them is entirely convincing.

“We don’t have the answer. We just have a lot of recordings,” said Mark McDonald, president of Whale Acoustics, a company that specializes in the sonic monitoring of cetaceans.

McDonald and his collaborators first noticed the change eight years ago, when they kept needing to recalibrate the automated song detectors used to track blue whales off the California coast. The detectors are triggered by songs that match a particular waveform. Every year, McDonald had to set them lower.

Since then, he and Scripps Institution of Oceanography researchers Sarah Melnick and John Hildebrand have gathered thousands of blue whale recordings made since the 1960s, spanning populations from the North Atlantic to the South Pacific to the East Indian Ocean. Their analysis, published in October in Endangered Species Research, shows that the songs’ tonal frequency is falling every year by a few fractions of a hertz.

“It’s a fascinating finding,” said John Calombokidis, a blue whale expert at the Cascadia Research Collective. “It’s even more remarkable, given that the songs themselves differ in different oceans. There seem to be these distinct populations, yet they’re all showing this common shift.”

According to McDonald, the first explanation to come to mind involved noise pollution caused by increased shipping traffic. Ambient ocean noise has increased by more than 12 decibels since the mid-20th century. But if whales were trying to be heard above the din, they’d sing at higher rather than lower pitches, said McDonald.

It’s also possible the whales are responding to changing dynamics in how sound travels through water that’s become warmer as Earth heats up, absorbing more carbon dioxide and growing more acidic than before. “But those factors are so small, and this is such a huge shift in frequency,” said McDonald.

Another explanation involves the recovery of blue whale populations, which were nearly hunted to extinction during the first half of the last century. It’s only since hunting ceased that they’ve been recorded. Maybe songs were higher-pitched when recording started, because the whales had to sing extra-loud in order to reach their scattered brethren. Now that there are more, they can lower their voices and their pitch.

But even in populations that escaped the carnage relatively unscathed, where population densities have remained steady, songs are getting lower.

“That’s the first place to look for an answer, but it doesn’t fit more-localized patterns. The population of blue whales off the U.S. west coast hasn’t shown a dramatic upwards trend in numbers, but its pitch is declining,” said Calombokidis.

Those whales are the best-studied of all blue whale populations, and their song pitch has dropped by 31 percent since the late 1960s.

Because only male blue whales sing, the answer may involve mate choice and sexual selection. The researchers hypothesize that as larger, ostensibly more virile whales tend to produce deeper songs, other males may be trying to emulate them, just as human guys might lower their voices when trying to impress a woman.

That the largest animals in the world could feel the need to inflate their size is an appealing idea, but Calombokidis warned that very little is known about how blue whales use their songs, or how social dynamics could affect them. “We need a better understanding of the songs, and a better understanding of their reproductive habits,” he said.

Hal Whitehead, a Dalhousie University biologist who specializes in cetacean communication, emphasized that whale song is a cultural affair. Humpback whales are known to learn from each other, and whales have extraordinarily large and complex brains. They appear to share many social and cognitive traits with people.

“The exciting possibility, I think, is that they’re all listening to each other,” said Whitehead. “This is a worldwide cultural phenomenon, and that’s very cool.”

Source: http://www.wired.com/wiredscience/2009/12/blue-whale-song-mystery/

Rip Currents Pose Greater Risk To Swimmers Than To Shoreline, Study Suggests

ScienceDaily — Rip currents — powerful, channeled currents of water flowing away from the shore — represent a danger to human life and property. Rip currents are responsible for more than one hundred deaths on our nation's beaches each year, according to the United States Lifesaving Association, and if rip currents persist long enough they can cause beach erosion.

Henry Bokuniewicz, Professor in the School of Marine and Atmospheric Sciences at Stony Brook University, and Ph.D. candidate Michael Slattery found that rip currents at East Hampton Village Beach lasted on average a little over one minute, not long enough to substantially alter the shoreline. They will present their findings October 14th at the American Shore and Beach Preservation Association's 2009 National Coastal Conference, "Integrating Coastal Science & Policy."

With funding from the East Hampton Beach Preservation Society and the Halpern Foundation, Dr. Bokuniewicz and graduate student Michael Slattery set up a video camera to record an image of a half mile stretch of the East Hampton Village Beach every 20 seconds. In the images, rip currents can be detected as a gap in the line of incoming waves. They collected over 500 hours of video images and observed hundreds of rip currents in this short stretch of coast.

The monitoring showed that the rip currents were not associated with man-made structures and they were short lived, with the most persistent rip currents lasting no more than a few minutes. "Most rip currents we observed did not last long enough to change the character of the shoreline, although they could pose a risk to swimmers unfortunate enough to encounter them," said Dr. Bokuniewicz.

Besides gathering statistics on the occurrence of rip currents, Dr. Bokuniewicz and Michael Slattery are studying the wave patterns that lead to rip currents. Rip currents are generated by a combination of waves, including, long, low, barely perceptible waves that appear along the ocean shoreline, called "infragravity waves." Infragravity waves cannot be measured directly and computer models are inadequate for predicting them. Bokuniewicz and Slattery are using a novel approach to study these waves; they deploy seismometers to measure the noise created by breaking waves.

"It appears that very slow, long-period changes in the amount of wave noise are precursors to the generation of rip currents," said Dr. Bokuniewicz. "We are hopeful that seismometers can be used to measure wave patterns that we can't easily observe in any other way. In the future, we hope to utilize this method to monitor and ultimately forecast wave conditions that cause rip currents."

Source: http://www.sciencedaily.com/releases/2009/10/091013132131.htm

Wide Heads Give Hammerhead Sharks Exceptional Stereo View

Scalloped hammerhead, Sphyrna lewini. (Credit: Courtesy of Flickr, via Wikimedia Commons)

ScienceDaily — Hammerhead sharks are some of the Ocean's most distinctive residents. "Everyone wants to understand why they have this strange head shape," says Michelle McComb from Florida Atlantic University. One possible reason is the shark's vision.

"Perhaps their visual field has been enhanced by their weird head shape," says McComb, giving the sharks excellent stereovision and depth perception.

However, according to McComb, there were two schools of thought on this theory. In 1942, G. Walls speculated that the sharks couldn't possibly have binocular vision because their eyes were stuck out on the sides of their heads. However, in 1984, Leonard Campagno suggested that the sharks would have excellent depth perception because their eyes are so widely separated. "In fact one of the things they say on TV shows is that hammerheads have better vision than other sharks," says McComb, "but no one had ever tested this."

Teaming up with Stephen Kajiura and Timothy Tricas, the trio decided to find out how wide a hammerhead's field of view is and whether they could have binocular vision and publish their results on November 27 2009 in the Journal of Experimental Biology.

Hammerheads come in all shapes and sizes so McComb and Kajiura, opted to work with species with heads ranging from the narrowest to the widest. Fishing for juvenile scalloped hammerheads off Hawaii and bonnethead sharks in the waters around Florida, the team successfully landed the fish and quickly transported them back to local labs to test the fish's eyesight.

The team tested the field of view in each shark's eyes by sweeping a weak light in horizontal and vertical arcs around each eye and recorded the eye's electrical activity. Comparing the hammerheads with pointy nosed species, the team found that the scalloped hammerheads had the largest monocular visual field, at an amazing 182 deg., and the bonnethead had a 176 deg. visual field, which was bigger than that of the pointy nosed blacknose and lemon sharks, at 172 deg. and 159 deg., respectively.

Having collected the animals' monocular visual fields, the team plotted the visual fields of both eyes on a chart of each fish's head to see whether they overlapped. Amazingly, they did. The scalloped hammerhead had a massive binocular overlap of 32 deg. in front of their heads (three times the overlap in the pointy nosed species) while the bonnet head had a respectable 13 deg. overlap. And when the team measured the binocular overlap of the shark with the widest hammerhead, the winghead shark, it was a colossal 48 deg. The hammerheads' wide heads certainly improved their binocular vision and depth perception.

Finally, the team factored in the sharks' eye and head movements and found that the forward binocular overlaps rocketed to an impressive 69 deg. for the scalloped hammerheads and 52 deg. for the bonnetheads. Even more surprisingly, the team realised that the bonnethead and scalloped hammerheads have an excellent stereo rear-view: they have a full 360 deg. view of the world.

"When we first started the project we didn't think that the hammerhead would have binocular vision at all. We thought no way; we were out there to dispel the myth," says McComb. But despite their preconceptions, the team have shown that the sharks not only have outstanding forward stereovision and depth perception, but a respectable stereo rear view too, which is even better than the TV shows would have us believe.

Source: http://www.sciencedaily.com/releases/2009/11/091127101034.htm