Birds of a feather flock together, and so do dolphins that use sponges as tools, biologists report, a sign of dolphin "culture".

For centuries, philosophers have debated whether other species possess culture, begins the Nature Communications journal study led by Janet Mann of Georgetown University. "Whether non-human animals have at least rudimentary culture is contested, partly because scholars disagree on the definition of culture," says the study.

Chimps and crows use tools and appear to pass the knowledge on to others, seen by some researchers as a sign of animal culture. In the new study, Mann and colleagues decided to look at tool use among dolphins.

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"In Shark Bay, Australia, a subset of the community of Indo-Pacific bottlenose dolphins procure and wear basket sponges on their beaks while lightly scouring the seafloor for prey in deep channels. Sponging is the best-documented case of tool use by wild cetaceans and is unique among wildlife in that only a small subset of the population uses tools," says the study.

Only 55 dolphins among the ones in Shark Bay use sponges. They cap their snouts with the sponges to scour the seafloor for fish. Only calves of sponge-using dolphins becomes spongers, mostly female dolphins which range less widely than males.

But whether there really is a "sub-culture" of sponging among some dolphins is tough to prove, the study notes, as sponging is a solitary activity and the dolphins swim around in ways hard to track.

So, Mann and colleagues created a social network of dolphins, measuring which ones flocked together in surveys, 14,651 collected from 1989 to 2010. Statistically, it appears they do:

"Spongers were more cliquish, had more sponger associates and stronger bonds with each other than with non-spongers. The significance of foraging homophily remained even after controlling for other factors that contribute to affiliation. These patterns are remarkable because spongers lead a relatively solitary lifestyle and have weaker ties with other dolphins than non-spongers do. Given that we included only non-spongers that are in the same habitat and had a very high chance of associating with spongers, our results are even more striking. The mutual interests of spongers seem to influence the nature of their social relationships. This is the first demonstration that a behaviour that is strictly vertically transmitted by a single parent serves an affiliative grouping function as well, thus meeting both criteria for culture. To date, no material subcultures have been identified outside of humans."

The study concludes that sponging appears to be a behavior transmitted mostly among three or four cliques of female dolphins in Shark Bay, more evidence for "culture" among dolphins.