Tool use: One of the few male spongers with a bright orange sponge held over the end of his beak.

Credit: Ewa Krzyszczyk

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Despite the seeming usefulness of the trait, the researchers found no differences in calving success (an indication of evolutionary fitness) between spongers and non-spongers, meaning that it may not be a more successful technique than other foraging methods.

Regardless, sponging appears to be just as good as any other hunting technique, said Mann who has been studying this Indian Ocean sub-population of dolphins for over 20 years. "It turns out the brainiacs of the marine world can also be tool-using workaholics, spending more time hunting with tools than any non-human animal."

Untapped food source

Because there is a wide variety of prey available to bottlenose dolphins, but each prey species is highly adapted to escape predators, individual dolphins specialise in different hunting techniques.

Spongers seemed to have come across an untapped food source, said Mann, who argued that the behaviour may have first come about when the accidental movement of sponges along the sandy bottom produced an unexpected new source of food.

This is a fascinating study, commented Alex Kacelnik, a behavioural ecologist with the University of Oxford, in England, who, studies tool use in New Caledonian crows. The research "is on the right track – [but] the fundamental question of whether tool use is particularly advantageous is as yet unanswered," he said.