Bottlenose dolphins have been observed “crater-feeding” – diving deep into sandy seabeds to retrieve concealed prey.
Now it is thought that, while they use their familiar echo-location ability to detect edible creatures buried as deep as 30m under the sand, once their own rostrum and eyes are buried in sediment they switch to using a previously unknown ability to detect weak electrical fields. This allows them to zero in with impressive accuracy on their unseen prey.
Both bottlenose (Tursiops truncatus) and Guiana dolphins (Sotalia guianensis) are now known to employ for this purpose a line of sensitive dimples on their rostrums that would have held whiskers when they were first born. The whiskers fall out as they grow, leaving behind the “vibrissal pits” that serve as sensors.
University of Rostock researchers worked with captive bottlenose dolphins Dolly and Donna at Nürnberg Zoo to find out exactly how sensitive these electro-receptors were.
All marine creatures generate weak direct (DC) electrical currents, as well as alternating (AC) pulses when moving parts of the body, such as the gills when breathing. The two dolphins were trained to rest their rostrums against a metal bar with electrodes in the water, and gradually learnt to swim away from it within five seconds of detecting an electrical field, to earn rewards.
The strength of the field was reduced progressively over time. Both dolphins proved 90% accurate in detecting DC fields under 125 microvolts a centimetre. Dolly’s detection rate reduced to 50% only once the signal was as low as 5.4 microvolts, while Donna continued to detect signals down to 3 microvolts with 80% accuracy. It was only at 2 microvolts that her success rate dropped to 3%.
With AC currents, Dolly and Donna were able to pick up on signals as weak as 28.9 and 11.7 microvolts respectively.
When the electric fields were at their weakest, Dolly was seen wiggling her rostrum as if searching for the current, which matches a behaviour commonly seen during crater-feeding.
Further experiments are needed to prove beyond doubt that dolphins use their electro-receptors for the purpose of foraging as well as helping them to navigate using Earth's magnetic field.
Dr Tim Hüttner, biologist and research assistant at the zoo, was lead author of the study with Prof Guido Dehnhardt, chair of sensory and cognitive ecology at the university. Their research is published in the Journal of Experimental Biology.
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