Wednesday, May 27, 2009

Like Taking Candy From A Baby

...err..well, not exactly. But it is a much easier way for a 45 ton sperm whale to get a quick bite to eat. This amazing footage of sperm whales (Physeter macrocephalus) shaking cod fishing lines 108 m deep off the coast of Sitka, Alaska, is the first ever of sperm whales feeding - albeit not on its usual prey, the massive squids of the deep sea.

Most of the time, sperm whales dive to a staggering 900 to 2600 feet (that's up to half a mile or so deep) to catch their meals. At such depths, with no light and such a wide expanse, filming feeding is nigh impossible. But we do know that eating the massive, deep-sea squid is no piece of cake - scars on the heads of the whales tell tales of arduous battles between predator and prey. It's no wonder, then, that some sperm whales have found an easier route to food, thanks, begrudgingly I'm sure, to cod fishermen off Alaska. The long lines of black cod (Anoplopoma fimbria) just waiting to be plucked are a tempting target for a lazy but quick-witted sperm whale. This video captures this incredible feeding behavior, long thought to exist but never before shown.

ResearchBlogging.orgBut the video does much more than just show how sperm whales steal the fishermen's catch. It reveals fascinating details about sperm whale echolocation and bioacoustics.

What we do know about marine mammal sound production and function is most often from studies shallower species. The trouble is, sperm whales are unique when it comes to head anatomy. Unlike other odontocete species, they don't have a "melon" structure (in yellow). Instead, they have two triangular sacs, one atop the other. The lower sac, referred to as "junk", is filled with a very dense oil and is likely derived from the melon. But the upper sac is unlike any other melon. It's filled with a lighter, waxy substance called "spermaceti". While this strange two-sac complex seems to have functions in diving and fighting, it's also presumed to play a large role in sperm whale acoustics, as it replaces the melon found in other, similar whales.

Previously, recordings of sounds had been correlated to total body length that was estimated when the animal surfaced. From these data, scientists had come up with two different equations that estimate sperm whale body length from what is called the "inter-pulse interval" (IPI), or the time between clicks. But no studies were able to compare the size of the whale, its IPI and the size of its spermaceti organ and junk sac, as the sizes of these organs wasn't directly measured at the same time as the length and sounds. Other studies have tried to model how the spermaceti and junk organs relate to sound, finding equations which estimate their length based on the distance between an initial and a full sound of a click, presumed to relate to how the sound bounces off and around the two sacs.

The video, caught by scientists from Scripps and the University of Alaska, was used to examine the relationship between sperm whale's anatomy and the sound it produces in a paper published in The Journal of the Acoustical Society of America. Because the hungry thief was oriented perpendicularly to the camera as it contacted the longline and scientists knew how far away it was from the camera, they were able to estimate the total length of the whale as well as the size of its spermaceti organ. They wanted to compare the sizes with the sounds produced to see if they matched what scientists had previously predicted.

The real-time audio and video gave the scientists a unique opportunity to see if models of acoustic creation, made from examinations of dead whales and recorded sounds without direct video to compare to, were accurate. By comparing the actual sizes with the predicted sizes based on the sounds produced, the researchers were able to see how well the current model of sound production in sperm whales actually describes reality.

They found that when it comes to body length, the models which utilize sound to estimate length are spot on. The animal's body length was estimated to be between 13 and 16 meters by both sound models using two IPI choices, which fell well within range of the roughly 15 m estimate given by four anatomical models: two which utlize the snout, one using the distance from the blowhole to the eye and one based on the size of the teeth.

But the acoustic models were not nearly as accurate when it came to the sound-related organs, the spermaceti and the junk. By sound alone, the researchers estimated the spermaceti to be about 5.5 m long, but this number is a good 2.1 m longer than the visual estimate. The acoustics also estimated the junk to be close to 3.1 m long - which is odd, considering the 5.5 m estimate for the spermaceti. Biologically, that difference is too large - the two organs are always much more similar in length.

Combined, what these data mean is that while IPI can be used to determine a sperm whale's size, the assumption that the sound the animal produces can be used to determine the length of the spermaceti organ is off. This, in turn, means that the way in which we think sound travels and propagates from a sperm whale's head may be off as well, and that there's something going on that scientists are missing about how the sperm whale produces and receives sound. So a simple, neat behavioral video has opened the door to a much better understanding of sperm whale anatomy and acoustics.

Mathias, D., Thode, A., Straley, J., & Folkert, K. (2009). Relationship between sperm whale (Physeter macrocephalus) click structure and size derived from videocamera images of a depredating whale (sperm whale prey acquisition) The Journal of the Acoustical Society of America, 125 (5) DOI: 10.1121/1.3097758