Study sheds more light on sharks' sense of smell

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Sharks are renowned for their keen sense of smell, and research by two American scientists to be published in an upcoming issue of the journal Current Biology provides further insight on how they utilise their extraordinary olfactory abilities to locate their prey.

Sharks have been classically thought to navigate their way to the food source by responding to differences to odour concentrations at their nostrils.  However, authors Jayne Gardiner and Jelle Atema believe that previous experiments to demonstrate this have confounded the effects of odour concentration and arrival time at the nostrils.

Using the smooth dogfish (Mustelus canis) as a model, the authors tested the hypothesis that sharks use a bilateral difference in odour arrival time (and not intensity) to estimate the direction of the signal source by fitting the dogfish with a headstage apparatus fitted with computer-controlled syringe pumps.

This enabled the researchers to separately control the concentration of odour and the timing of its arrival at the nostrils.

The authors subjected the experimental subjects to six different patterns of squid odour pulses.

Four involved timing differences, such that one nostril received an odour pulse 0.1, 0.2, 0.5 or 1.0 seconds ahead of the other nostril, while the other two involved concentration differences.

In these treatments, a full strength and a 100-fold dilution of odour pulses were used, in which they were either delivered simultaneously or with a delay of 0.5 seconds such that the diluted odour pulse was delivered first.

Ambient seawater pulses delivered with a delay of 0.5 seconds were used as a control.

The authors found in the experiments involving the 0.1-, 0.2-, and 0.5-second time delays, the dogfish turned with a significantly greater frequency toward the side receiving the odour pulse first, while there was no significant difference in turning direction with the 1.0-second delay.

When unequal concentrations of odour pulses were used, the dogfish again showed no significant difference in turning direction when they were delivered simultaneously, but once again turned toward the side receiving the first, albeit weaker, stimulus with significantly greater frequency.

The discovery that sharks use odour arrival time and not concentration to steer may explain the evolution of the unusual head shape of hammerhead sharks.  The more widely spaced nostrils seen in hammerhead sharks may allow them to smell better than regular sharks, since the greater time lag between the odours arriving at their left and right nostrils would enable them to track odour patches at a smaller angle of attack.  This is a theory worthy of further testing.

For more information, see the paper: Gardiner, JM and J Atema (2010) The function of bilateral odor arrival time differences in olfactory orientation of sharks. Current Biology doi:10.1016/j.cub.2010.04.053