A shark's sharp teeth aren't the only reason we find them so scary---their ability to smell blood in the water, even from a long distance, is also a big factor. We know they'll find us. But how do they know what direction to swim in order to find a wounded fish (or person)? Conventional wisdom says that they follow trails of scent based on differences in the concentration of odor molecules detected by each nostril. Not so, say marine biologists in a new study in the journal Current Biology. They say it's all in the timing.
The researchers collected eight smooth dogfish (Mustelus canis), a small shark species, off the coast of Massachusetts for their experiment. Each of the sharks was placed in a tank and exposed to various patterns of odors, some based on timing differences (one nostril would be exposed to the scent before the other) and others involving differences in odor concentration (one nostril would be exposed to a higher concentration of scent). The scientists then recorded how quickly the shark turned and in which direction.
The sharks turned in the direction of the nostril that first received the odor, regardless of the concentration of that odor, when the timing difference was half a second or less. When the delay was a full second or they received the odor in both nostrils at the same time, the sharks didn't know which way to turn and showed no preference for any direction.
Using scent concentration to determine direction would not be useful, the researchers note, because odors in air or water are not dispersed uniformly. These patterns can be quite chaotic and, if an animal were trying to determine direction from them, misleading.
They also noted that if other shark species also depend on timing differences to sniff out their prey, this may explain the evolution of the hammerhead shark's distinctive shape. Having more widely spaced nostrils---the hammerhead's are at the far right and left of its flat head---may give those species a better sense of smell.
(And did you know that sharks use math to hunt?)