The dusky smooth-hound or smooth dogfish shark (Mustelus canis) used in this study.
Credit: Wikimedia
WASHINGTON, D.C.: A shark's nostrils don't use concentrations of odours to guide it to its next meal, but instead it has an impressive sense of timing, scientists said.
Until now, it was thought that sharks used the differences in odour concentrations to guide them toward their prey. "There is a very pervasive idea that animals use concentration to orient to odours," said Jayne Gardiner, the lead researcher on the study published in Current Biology and a researcher at the University of South Florida in Tampa.
Gardiner and her colleagues found that sharks will turn in the direction of whichever nostril picks up a scent first rather than use scent concentrations to guide them, as previously thought.
Concentration hypothesis flawed
Using the concentrations of scent molecules as guides can be misleading since concentrations can spike even when they're spread a long way from their sources.
"The physics of odor dispersal makes it difficult or practically impossible for most animals to use concentration differences in large-scale odor plumes," said Jelle Atema, a co-author of the study.
"Our surprise was that they can use time differences, but do so only within a one second 'window.'"
Nostrils detect time lag of scent
So for sharks to turn in the direction of a scent, there must be no more than a half a second to a second lag for the smell to reach the other nostril or the shark is just as likely to turn left as right. "The response of turn into the first hit direction keeps them connected to the odor plume, a very important part of tracking the plume to the source," said Atema.
The researchers controlled the delivery of odor to the nostrils by using a headstage apparatus that placed tubing just inside each nostril, said Gardiner. The two sides of the headstage were connected to two computer-controlled syringe pumps that delivered precisely timed odor puffs.
The small arrival time differences of the odor helped the animal steer into a patch of scent, said Gardiner. Using each subsequent scent encounter, the animal was able to orient patch to patch and move up the odor plume.
Sharks sit on edge of scent stream
"I think the delay works only with the same level of concentration. If the two are combined then you can detect direction," said Terry Peake, a biologist at the Shark Research Institute in Australia, adding: "I have noticed sharks sit on the edge of a scent stream rather than in the middle of it."
It could be why the hammerhead evolved the way it did, suggested Gardiner. Their wide flat heads allow for maximum distance between the animal's nostrils, giving it an advantage in more easily pinpointing the source of a scent.
"The new findings on the fact that sharks can detect delays in the time that odors reach one nostril versus the other explains why sharks in general, and hammerhead in particular, have well spaced nostrils. It's a small but important progress in our knowledge of [the] shark sensory world," explained Alessandro De Maddalena, a biologist and shark expert at the Italian Ichthyological Society.
