The first thing to know is that dolphins can be divided into two groups, and those groups are "spongers" and "non-spongers." The non-spongers are the dolphins that are probably the ones you think about when you have occasion to think about dolphins: smooth, sleek, nimbly darting through the water.
But the spongers! The spongers are slightly less physically nimble, but possibly much more intellectually nimble, than their fellow cetaceans. And that's because, as they swim, they carry sea sponges in their beaks—an activity that may help to protect their sensitive snouts from sharp rocks, stingrays, urchins, and other things that might plague them, particularly as they forage for food along the seafloor. Dolphin sponging is a recent discovery: In 1997, scientists observed a group of Indo-Pacific bottlenose dolphins engaging in the practice in Shark Bay, off the coast of Australia.
The behavior, Justin Gregg notes in his book Are Dolphins Really Smart?, has since been traced back to approximately 180 years ago, to a single female who has been nicknamed "Sponging Eve." Scientists now believe that more than 60 percent of all female dolphins in the area practice sponging. And while the behavior seems to be transmitted for the most part along mother-daughter lines, as many as half of the males born to "spongers" in the area grow up to become spongers, too.
The discovery of sponging was the first published instance of tool use among dolphins. And on the one hand, it wasn't terribly surprising: We know dolphins rank among the smartest animals on Earth, and tool usage is an extension of intelligence. On the other hand, though, sponging suggests a level of cognitive sophistication that we'd previously reserved only for ourselves and our closest cousins.
And though scientists had observed the sponging behavior, they weren't exactly sure what the dolphins were really doing with the sponges—or what made them engage in the behavior in the first place. Was it more analogous, Gregg asks, to vultures' seemingly instinctual use stones as hammers to crack open ostrich eggs? Or was it learned behavior, the results of complex problem-solving capacities? Was sponging helping the dolphins exploit food sources that otherwise wouldn't be available to them?
To find out, a team of evolutionary biologists at the University of Zurich examined the tissues of the sponging dolphins, analyzing chemicals in tissue samples collected from 11 spongers and 27 non-spongers. They were looking, in particular, for the presence of fatty acids that come from prey, which would offer clues about the dolphins' diets.
Their finding? Spongers seem to have completely different diets from their non-sponging counterparts—which suggests, at least, a correlation between sponging and diet. The team believes that the sponges allow the dolphins that carry them to feed on fish that live on the seafloor: fish that lack the swim bladders that allow other fish to stay buoyant in the water. Fish that are basically sitting targets, but fish that are hard for most dolphins to find with their normal method, echolocation, since the rocks and other animals on the seafloor can impede their biological sonar. The Zurich researchers think the sponging behavior could be connected to the dolphins' ability to feed on those bottom-dwellers.
Though there's more research to be done, "we were blown away as to how strong the differences between tool users and non-tool users were, especially given that these animals live in the same habitat," study co-author Michael Krützen told Live Science.
The findings, published in the journal Proceedings of the Royal Society B, offer the first direct evidence that dolphins' use of tools can help them to exploit new food environments in their ecosystems. Which would be a form of environmental engineering. Which is something that very few species are capable of. And something that we've thought of as a significant element of human evolution. And something we might now need to rethink.
Via Live Science