Sniffing around to find a way home

Long hot sunny days spent lazing around our Kiwi beaches are a true sign that summer is here. However, for some of us the second we dip our toe in the ocean, that haunting theme tune to Jaws begins playing in the back of our minds. There have already been several shark sightings this summer as the warmer weather coincides with many shark species, including thresher, mako and bronze whaler, moving inshore to pup and feed.

One question that has puzzled scientists is how do sharks know which direction to go? Because some sharks can home in on electric signals generated by their prey using electroreceptors in their skin, some think they may also be able to pick up on the direction and intensity of the earth's magnetic field, similar to how sea turtles navigate. Although there is some behavioural evidence that suggests sharks that migrate along long trans-oceanic paths travel in straight lines consistent with Earth's magnetic field lines, the theory is still being researched.

This week Californian scientists decided to sniff out a new theory by plugging the noses of leopard sharks with petroleum oil-covered cotton balls to prevent them being able to smell. The nose of a shark is made up of a pair of nares, nostril-like holes on the snout. Each nare has two openings, one for water to enter and one for water to exit. The shark sniffs by sucking in water, which passes over a series of sensory cell-filled skin folds known as olfactory lamellae. These send signals to the smell-processing olfactory bulb in the shark's brain.

Plugging the nostrils stops the sharks from being able to smell, but as they are cartilaginous fish that breathe by flowing water over their gills, it doesn't put them in any danger of suffocating. As some species of shark have noses so sensitive they can sniff out blood at concentrations of one part per million, the experiment was designed to test whether sharks can navigate by smelling chemical gradients of plankton or amino acids from prey that live close to their home and then use this information to find their way back when relocated in the open ocean several kilometres out to sea.

The researchers found sharks with blocked noses ended up swimming around in circles and didn't seem to have a sense of which way was home, making it only 37.2 per cent of the way back to shore after four hours of swimming. The freely sniffing sharks took a direct path towards the shore and made it 62.6 per cent closer to home within the same timeframe, indicating that either smell is used for navigation, or sharks with cotton-filled nostrils are too stressed to go home. Although only leopard sharks were tested, many coastal migrating sharks have larger olfactory bulbs than expected for their size, suggesting smell is really important.

If you are worried that a local shark is likely to sniff you out during your sunset paddle, then you are probably right. With only 13 fatal shark attacks documented in New Zealand over the past 170 years, chances are it sniffed you miles away and is swimming off in the opposite direction. Data shows that us ocean lovers should be much more scared of drowning than being mauled by a shark, but if you do see one and it has cotton wool up its nose, don't panic, it's probably just for science.

Dr Michelle Dickinson, also known as Nanogirl, is an Auckland University nanotechnologist who is passionate about getting Kiwis hooked on science.