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.