When tracking down the tantalizing smell of prey, a shark relies mostly on which nostril first detects the scent rather than on the strength of the odor, a study has found.
The findings, published online Thursday in the journal Current Biology, suggest that sharks with more widely spaced nares may be better able to judge the location of their prey.
Scientists had long thought that the concentration of the smell determined how a shark would react. "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 the study's lead 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 whiffs 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 found, instead, that 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 introduced squid-scented seawater into one nostril — followed by a burst 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 was stimulated first, the animal would turn in that direction.
Even when the smell hitting the second nostril was 100 times stronger than the scent administered to the first, the shark would turn in the direction of the first nostril — showing that timing, not strength, was key, Gardiner said.
The sharks were able to sense a difference between 0.1 second, 0.2 second 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."