In 1984, researchers spotted dolphins doing something unusual in Shark Bay, Western Australia. When the animals got hungry, they ripped a marine basket sponge from the sea floor and fitted it over their beaks like a person would fit a glove over a hand. The scientists suspected that as the dolphins foraged for fish, the sponges protected their beaks, or rostra, from the rocks and broken chunks of coral that litter the sea floor, making this behavior the first example of tool use in this species.
But why do dolphins go to all of this trouble when they could simply snag a fish from the open sea?
The answer, researchers hypothesize and report online today in PLoS ONE, is that the bottom-dwelling fish are a lot more nutritious. Some species also don't have swim bladders, gas chambers that help other fish control their buoyancy as they travel up and down the water column. In the Bahamas, where dolphins are also known to forage for bottom-dwelling fish, dolphins hunt partly by echolocating these bladders, which give off a strong acoustic signal. That helps the cetaceans find prey even when it's buried in sea sand.
But bottom-dwelling fish, such as barred sandperch, which are favored by some Shark Bay dolphins, don't have swim bladders and so are harder to find with echolocation. The sea floor is not nearly as soft here as it is in the Bahamas, so if dolphins want to probe for these fish, they risk injuring their rostra.
Enter the sea sponge. Some ingenious Shark Bay dolphin figured out that by prodding the sediments with a sponge attached to its beak, it could stir up these swim bladder-less fish without hurting itself.
To find out more about the types of fish that might respond to this "sponging" technique, Eric Patterson, a graduate student in behavioral ecology at Georgetown University in Washington, D.C., asked a colleague to try it in Shark Bay while he filmed the results. The pair fitted a marine basket sponge on the end of a pole and dove into the same channels where the dolphins hunt. "We had to switch sponges after every five to six dives because they were torn to bits by the sediments," Patterson says. Although not nearly as graceful as sponging dolphins, "which are really elegant in their moves," Patterson says, the human sponger nonetheless managed to scare up a hidden prey fish every 9 minutes. "The prey are numerous and reliable," Patterson says, "and their behavior is so predictable—they always dart out of the sands—that they make this hunting behavior worthwhile for the dolphins."
Patterson has also observed the sponging dolphins. After a fish emerges from its hiding spot, it swims a few meters away and doesn't immediately rebury itself. That interlude gives the dolphin just enough time to drop its sponge, surface to get a breath, and dive again to snag the fish, most of which are about 12 centimeters in length. "They are not a huge prey, but are very nutritious," Patterson says, because fish lacking swim bladders typically have a high lipid content.
Not every dolphin in Shark Bay hunts with sponges. "It's primarily done by females," says Janet Mann, a behavioral ecologist also at Georgetown University and Patterson's dissertation adviser. She believes the female dolphins invented the method because of the "selective pressures they face while raising a calf as long as they do," about 4 to 5 years. "These clever dolphins have figured out a way to target fish that other dolphins cannot," she says, adding that even the local fishermen do not catch, or even know about, this particular species. Mann's previous research has shown that dolphin mothers pass the sponging method to their daughters and some of their sons, rare evidence of a cultural tradition in an animal other than humans. The team has documented three generations of sponging dolphins.
The study provides a "better understanding of the why and how of sponging" by the Shark Bay dolphins, says Louis Herman, a cognitive psychologist and professor emeritus at the University of Hawaii, Manoa. The work "adds to previously documented" examples of "innovation by this highly intelligent species." Patterson's and Mann's results also "reinforce a pattern" often seen in other tool-using animals, says Simon Reader, a behavioral biologist at McGill University in Montreal, Canada. "Tool use appears to be almost a last option, taken when other options fail or are unavailable," he says, noting that woodpecker finches in the Galápagos Islands "turn to tool use only in arid areas," wielding cactus spines to extract grubs from tree branches. Using tools takes time and energy, Reader says, and animals tend to rely on them only when there's a guaranteed payoff, such as turning up a fatty fish that most other dolphins (and fishermen) know nothing about.
*This article has been corrected. Researchers first researchers spotted dolphins putting sponges over their rostra in Shark Bay in 1984, not 1991.