Certain bottlenose dolphins off the coast of Western Australia have picked up an unusual trick, a new study reports: When they head off to forage for a meal, they first grab sponges and hold them in their beaks as they dive down to the seafloor. These dolphins dive to the bottom of deep channels and poke their sponge-covered beaks into the sandy ocean floor to flush out small fish that dwell there…. Foragers then drop their sponges, gobble up available fish and retrieve the implements for another sweep [Science News]. This complicated procedure is the first confirmed example of tool use by dolphins, researchers say.
Scientists had previously observed some dolphins in Australia’s Shark Bay carrying around sponges, but the purpose was unclear. The new study documents this unusual behavior, which only a subset of a larger dolphin population engages in, and also probes the remaining mysteries. Researchers still aren’t sure why most of the “spongers” are female, and they haven’t determined whether the behavior conveys a real evolutionary benefit, although they have hypotheses to explore on both points.
In the study, published in the journal PLoS ONE, lead researcher Janet Mann explains that while mothers show both their male and female calves how to use sponges, female calves are almost exclusively the only ones to apply this knowledge. “The daughters seem really keen to do it,” says Mann. “They try and try, whereas the sons don’t seem to think it’s a big deal and hang out at the surface waiting for their mothers to come back up” [New Scientist]. Mann speculates that this could be because sponging is a time-intensive and solitary occupation, with more work required per meal; she thinks it’s possible that male dolphins aren’t willing to give up the socializing that could give them access to fertile females.
Despite the isolation, Mann found that spongers give birth to calves at the same rate as non-spongers, suggesting that the work-intensive behavior doesn’t put the tool-using dolphins at a disadvantage. Spongers hunt only in underwater channels with sandy bottoms. Their foraging grounds are a few metres deeper than the sand flats where other dolphins hunt, meaning they spend more energy on feeding. “But ultimately they’re doing as well as others,” says Mann. Mann believes they may be exploiting an otherwise unused larder, as the density of females in channels is lower than out on the sand flats [New Scientist]. But even if the spongers are profiting from exploiting a forgotten niche in the ecosystem, researchers don’t think that gives them an evolutionary edge: If it did, all the other dolphins in Shark Bay would have caught on to the behavior by now, Mann says.
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Image: Ewa Krzyszczyk