Mystery of how sharks smell blood a quarter mile away 'solved'

Scientists claim to have finally solved the mystery of how sharks can smell a drop of blood in the ocean a quarter of a mile away.

A team at Florida University has found that sharks' noses use "smell stereo" to detect tiny delays —— no more than half a second long —— in the time that odours take to reach one nostril compared to the other.

And, according to the scientists, when the hunters experience such a lag, they will turn towards whichever side picked up the scent first, the Daily Mailreported.

The scientists have based their findings on laboratory tests on eight smooth dogfish and one small grey—brown shark.

They fitted headgear consisting of two tubes to the sharks in a 50 litre tank of seawater and then delivered bursts of marinated squid to each nostril in turn, and found that the fish rely on a combination of directional cues, based on scent and the flow of water, to keep themselves orientated and find what they are looking for.

If the delay between the scent reaching one nostril and the other is between a tenth and half a second, the sharks turn their heads to the side where they first smelled squid.

"If a shark experiences no delay in scent detection or a delay that lasts too long —— a full second or more —— they are just as likely to make a left—hand turn as they are to make a right.

"These results refute the popular notion that sharks and other animals follow scent trails based on differences in the concentration of odour molecules hitting one nostril versus the other. It seems that theory doesn't hold water when one considers the physics of the problem," say the scientists.

Added team leader Dr. Jayne Gardiner: "There is a very pervasive idea that animals use concentration to orient to odours. Most creatures come equipped with two odour sensors —— nostrils or antennae, for example —— and it has long been believed that they compare the concentration at each sensor and then turn towards the side receiving the strongest signal.

"But when odours are dispersed by flowing air or water, this dispersal is incredibly chaotic."

The findings have been published in the 'Current Biology' journal.

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