How a shark's nose knows

New insight into how sharks sniff out prey may help explain the evolution of widely spaced nostrils, such as those of hammerheads. According to a linkurl:study;http://www.cell.com/current-biology/abstract/S0960-9822%2810%2900591-9 published in the latest issue of Current Biology, sharks navigate through odor trails by sensing time delays in the arrival of odor signals from one nostril to another. Presumably, sharks with more widely spaced nostrils can sense more subtle changes in the direction of the odors.

Hammerhead shark
Image:flickr/graspnext

"It's one step toward solving the puzzle" of how sharks track their prey through the tumultuous ocean environment, said linkurl:Jayne Gardiner,;http://shell.cas.usf.edu/motta/Jayne.htm a biology PhD candidate at the University of South Florida and co-author of the study. Odors diffuse through water chaotically -- twisting and swirling much like smoke from a chimney. They form eddies and patches of concentrated scent followed by odorless pockets. The survival of top marine predators hinges upon being able to efficiently follow these complex odor trails to their sources. But the mechanisms by which they stay on course have eluded scientists. "It is a common idea that animals in general, because they have two nostrils or two antennae, compare odor concentrations when tracking a scent," Gardiner said. But this is unlikely, she said, because odor concentrations are not evenly distributed throughout water due to the turbulent nature of their dispersal. Instead, she set out to find whether sharks determine where the trail is by using their nostrils to gauge differences in the time it takes the odor to reach one nostril versus the other. Gardiner and marine biologist linkurl:Jelle Atema;http://www.bu.edu/biology/people/faculty/atema/ from Boston University fitted the small shark species, Mustelus canis, with head gear that delivered computer-regulated puffs of squid scents directly into their nostrils via tubes. They found that when one nostril received the smell fractions of a second ahead of the other, the sharks would turn in the direction of the nostril that picked up the scent first. "Even if they got a weaker odor cue in advance of a stronger one, they would turn in favor of the first, albeit weaker, cue," Gardiner said. But when both nostrils received cues at the same time -- even if there was a 100-fold concentration difference between left and right -- the sharks were as likely to turn in either direction. "Their brain interprets it as a head-on encounter," Gardiner said. Turning in either direction will still keep them in the trail. "They're not responding to that concentration difference at all," Gardiner added. "It really is timing of odor arrival that matters to these animals." If sensing these time delays are really what sharks use as a compass, then this may give hammerheads an advantage over their pointy-nosed counterparts, since the wide spacing of their nostrils may make them more adept at picking up subtle directional changes in odor trails, especially when swimming at higher speeds. "It might suggest that this is one of the reasons sharks started getting flatter heads with nostrils wider apart," she said. "It gives them better spatial and temporal acuity when tracing odors." linkurl:Marc Weissburg,;http://www.biology.gatech.edu/people/marc-weissburg?id=marc-weissburg professor and co-director of the Center for Biologically Inspired Design at Georgia Tech, who did not participate in the study, said the research "adds to our suite of mechanisms that animals can use to navigate through [scent trails] to find a source." He cautioned, however, that it may be premature to rule out concentration differences as an important guiding mechanism for sharks. "They proved that sharks have a mechanism to encode these time differences," Weissburg said. "What's unclear is how reliant these animals are on this particular mechanism when they are naturally foraging." Indeed, the next step for Gardiner and her colleagues is looking at how sharks use differences in odor arrival time when exposed to a continuous set of odor encounters -- as opposed to instant puffs of squid -- much like they would in the wild. J.M. Gardiner and J. Atema, "The Function of Bilateral Odor Arrival Time Differences in Olfactory Orientation of Sharks," Current Biology, 20:1-5, 2010. DOI 10.1016/j.cub.2010.04.053
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[April 2010]*linkurl:Shark Rx;http://www.the-scientist.com/article/display/53521/
[September 2007]*linkurl:On the Trail of an Odor Map;http://www.the-scientist.com/article/display/15004/
[25th October 2004]