Dolphins hunt wearing fake noses — but only the clever ones
Researchers have tracked the strange technique of 'sponging' for about 35 years, watching the animals as they draw the fish out, drop the sponges that protected their beaks from the rocks and devour their prey.
Now, though, scientists are beginning to understand why this behaviour, which is seen in no dolphins elsewhere in the world, is confined to only about 5 per cent of dolphin population even within Shark bay. Ellen Jacobs, of Aarhus University in Denmark, and her colleagues wanted to find out how the sponges affected the dolphins' echolocation which is famously their strongest sense.
The team collected sponges from Shark Bay and used a CT scanner to put them into a computer model and simulate how dolphins' echolocation clicks travelled through them. They found that sponging is a game of trade-offs.
Although the sponges help to flush fish from their hiding places, they also distort the clicks. Worse still, every time a dolphin drops a sponge, when they put it back on its orientation may mean a different distortion. The researchers likened the effect to putting on glasses with the wrong prescription. Sponging, in short, is mentally taxing.
'It could help explain why not everybody sponges,' Jacobs said. 'Sponging is kind of a weird thing to do. The sponging dolphins — they are a little socially isolated, they hang out with each other more than they do with other individuals. And they spend a lot more time foraging than non-spongers. So it's not an efficient technique in terms of time usage. So why would you do it?'
• What's that Flipper? Scientists listen in on incredulous whistling dolphins
Sponging has not spread sideways across the dolphin population of Shark Bay, but is passed down by mothers. 'We think that this distortion could be playing into why you only become a sponger if your mother was also a sponger,' said Jacobs, whose study was published in the journal Royal Society Open Science.
Dolphins are not born knowing the method, so they learn it from their mothers. Researchers are divided on whether calves learn simply by watching or whether mothers actively teach the technique. Either way, though, sponging takes a long time to master. Jacobs said that dolphins could hunt their own fish at four years old, but may still be improving at sponging well into their twenties.
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