Solved: Mystery of how sharks can smell a drop of blood from quarter of a mile away



They can smell a drop of blood in the ocean a quarter of a mile away.

Now the mystery of exactly how sharks use their astonishing sense of smell to find prey has been solved.

Researchers have shown that their 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 one.

Powerful noses: Sharks use 'smell stereo' to detect odours

When the hunters experience such a lag, they will turn towards whichever side picked up the scent first, a new study has shown.

The findings - published in Current Biology - help solve one of the long-standing mysteries of sharks.

Scientists at the University of South Florida made their discovery after lab tests on eight smooth dogfish and one small grey-brown shark.

Lead researcher Dr Jayne Gardiner 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.

She 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 the 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,' added a spokesman for the scientists.

'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.'

Dr Gardiner said: '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.'

As well as shedding light on sharks, the findings might also lead to underwater robots that are better equipped to find the source of chemical leaks, like the oil spill that is now plaguing the Gulf Coast, according to the researchers.

'This discovery can be applied to underwater steering,' said Dr Gardiner.

'Previous robots were programmed to track odours by comparing odour concentrations, and they failed to function as well or as quickly as live animals.'

With the oil spill in the Gulf of Mexico, the main oil slick is easily visible and the primary sources were easy to find, but there could be other, smaller sources of leaks that have yet to be discovered. An odour-guided robot would be an asset for these types of situations.

The smell organs in the noses of some sharks are able to detect one droplet of blood in one million drops of sea water. They are often attracted to chemicals found in the guts of animals and loiter near sewage outlets.