LOS ANGELES — When tracking down the tantalizing smell of prey, a shark relies more on the distance between its nostrils than the strength of the odor, a new study has found.
The report, published online Thursday in the journal Current Biology, provides a better sense of how these streamlined predators are able to pinpoint distant prey.
Scientists had long thought that the concentration of the smell determined how a shark would react to it. "People have always just kind of had this idea that it was concentration … it's always been the assumption that this is how it works," said lead study author Jayne Gardiner, a marine biologist at the University of South Florida in Tampa.
But in practice, that could be a poor tactic for the predators, Gardiner said — because plumes of scent billowing through the water can create uneven patches of concentration that could send a hungry shark off in the wrong direction.
Gardiner and co-workers showed that, instead, sharks take note of which nostril smells the odor first. The scientists showed this by harnessing sharks in a tank and fitting tubes to the animals' nostrils. Then they puffed a burst of squid-scented seawater into one nostril — followed by a puff of squid scent into the other just a fraction of a second later.
When the right nostril received the scent first, the shark would turn to the right. When the left nostril got it first, the animal would turn to the left.
Even when the smell hitting the second nostril was 100 times stronger than the smell administered to the first nostril, the shark would still turn in the direction of the first nostril — showing that timing, not strength, was key, Gardiner said.
The two puffs of scent had to be close together in time: The sharks were able to sense a difference between 0.1 seconds, 0.2 seconds and half a second, and when the puffs of squid scent were separated by one second, Gardiner said, the animals seemed to assume that the two puffs had different sources.
"It's a very neat study," commented Stephen Kajiura, a shark sensory biologist at Florida Atlantic University in Boca Raton. And, he added, "it provides another reason why the hammerhead (shape) may be advantageous."