Bottlenose dolphins that carry sea sponges on their beaks probably learned the trick from their moms rather than inheriting a sponge-shuttling gene, researchers say.
The sponges appear to protect the dolphins’ beaks during foraging along rugged ocean bottoms, explains Michael Krützen of the University of Zurich in Switzerland. In search of the behavior’s origin, he and an international team of collaborators studied the genetics of the sponging dolphins, known only in Australia’s Shark Bay.
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The researchers say that the spongers belong almost exclusively to a single maternal lineage, although sponging doesn’t follow any of the patterns that would be expected if it were genetically based. Therefore, Krützen and his colleagues argue in the June 21 Proceedings of the National Academy of Sciences, dolphins’ sponge use is a case of cultural transmission—the passing along of a learned behavior.
“This is an exciting addition to the catalog of what we can be increasingly confident are culturally transmitted forms of tool use in nonhuman populations,” comments Andrew Whiten of the University of St. Andrews in Scotland, who studies tool-using chimpanzees.
Sponge carrying came to the attention of scientists 20 years ago when a boater reported seeing a dolphin in Shark Bay with a “tumor” on its beak. The tumor turned out to be a sponge, and in 1997 researchers proposed sponge carrying as the first known example of tool use in dolphins.
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The practice has been difficult to study. “Shark Bay has its name for a reason,” says Krützen. Observations, primarily from the surface, suggest that some dolphins “wear sponges like a glove” as they search for food, he says. Working alone, the spongers poke around with their sensitive snouts, particularly in deep waters in rough terrain where they risk stings from bottom dwellers.
Working with DNA from dolphins in Shark Bay—1 male sponger, 12 female spongers, and 172 nonspongers—the researchers found that all but 1 of the spongers shared markers in the DNA of their mitochondria, cellular organelles inherited exclusively from mothers. Despite examining 10 scenarios of inheritance, both for mitochondrial DNA and DNA from cell nuclei, the researchers were unable to explain genetically the observed female-biased pattern of sponge carrying.
Spongers and nonspongers live side by side, so Krützen dismisses the possibility that ecological factors drive sponge carrying.
Whiten says that the “painstaking genetic analysis” and the improbability of an ecological cause suggest that cultural transmission is “the most compelling interpretation” for the bottlenose’s penchant for carrying sponges on their beaks.