Tool use: One of the few male spongers with a bright orange sponge held over the end of his beak.
Credit: Ewa Krzyszczyk
LONDON: Some bottlenose dolphins gather up sea sponges in their jaws and use them to uncover fish hidden under the sand, says a new study – but the behaviour is largely restricted to females for unknown reasons.
"This is the first and only clear case of tool-use in a wild dolphin or a whale" said Janet Mann, a biologist from Georgetown University in Washington DC, and lead author of the study published today in the open access journal Plos One.
Sponge tactics
Bottlenose dolphins (Tursiops aduncus) have been known to sometimes play with and carry small cup-shaped basket sponges over the end of their beaks, but researchers have been at a loss to explain precisely why.
In animal species that use tools – such as the chimpanzee – usually all individuals in a population use the tools, which makes it difficult to prove any advantages that the behaviour might confer.
But in one population of dolphins, in Shark Bay, Western Australia, only 41 dolphins, in a population of thousands, are known to use sponges. This allowed Mann and her team to compare, for the first time, the biological fitness of wild animals that use tools against those that do not.
For the study the researchers compared the behaviour and calving success of dolphins that use sponges ('spongers') with those that don't ('non-spongers').
The observational work revealed that spongers would slowly swim along the sand of the sea floor, intermittently disturbing it with the sponge in order to find small burrowing fish such as the spothead grubfish (Parapercips clathrata). They appeared to be "using the sponge to ferret prey from the sea floor" said Mann.
Unexpected discovery
Spongers were also found to spend more time foraging, dived deeper and appear to be more committed to this single foraging tactic, she said. Non-spongers were found to employ a variety of tactics to forage, but none of them included tool use.
The survey also revealed another unexpected discovery: that spongers are nearly all female and the behavioural trait is often passed from mothers to daughters.
The researchers found that many female calves start to learn to use sponges before weaning, while most sons used sponges rarely and even then only after weaning. "While a few males carry sponges, they seem to be slow learners in this regard," noted Mann.
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.
