Clams, abalone and sea urchins can all make a delicious meal—if you know how to get them open. Fortunately, sea otters have figured out this trick. Three different subspecies of otters are known to use tools to open hard-shelled prey, typically finding a rock and then floating adorably on their backs while banging the rock and the clam together over their chests.
Sometimes, they get even more creative. “There was one that used an old Coke bottle that was a really heavy glass, that worked well,” says Katherine Ralls, a senior research biologist with the Smithsonian Conservation Biology Institute (SCBI). “They'll use crab claws and use them to open the crab with their own claws. It's just this arms race between the invertebrates and the sea otters.”
But is this remarkable trait genetic or learned? That was the question behind a recent study led by Ralls, which was published last week in the journal Biology Letters. To find out, researchers at SCBI and its partners compared patterns of tool use in sea otters to that of dolphins and found some key differences—suggesting that rock-bashing among otters may have been going on for millions of years.
Orphaned sea otters that are raised in captivity will experiment with tool-use without being taught, which points towards the behavior being genetic, says Ralls. But not all wild adult sea otters use tools. So Ralls decided to compare the DNA of many otters to see whether the craftier individuals have something special in common. Then she compared those results with data on tool-using dolphins to see whether these different groups of marine mammals might share a similar story.
Among the 42 known species of dolphins, only the bottlenose dolphin is known to use tools. Even within the species, only a relatively small group of bottlenose dolphins living in two gulfs of Shark Bay, Australia, have learned to slide conical sponges over their noses as protection while rooting around on the sea floor for food.
But while tool-using dolphins are all closely linked genetically, Ralls found that otters who prefer to use tools to open their food are actually no more closely related to one another than they are to otters that don't use tools. For her, this was a surprise. “In the beginning we thought they would be more like the dolphins,” Ralls says. “We began to think of the reasons that might explain the difference and we came up with the age of the behavior.”
Dolphins are thought to have started using tools only very recently. A 2012 study observed the rate at which the tool-use behavior is transmitted from one dolphin to another (usually from mother to offspring). For example, in one of the two gulfs, 91 percent of the female offspring of 'spongers' become spongers themselves, while only 25 percent of the male offspring do.
This allowed scientists to construct a mathematical model that works backwards to see how old the behavior is. They found that the dolphins probably started using sponges as tools no more than 200 years ago.
Sea otters, by contrast, have probably been using tools for hundreds of thousands, if not millions, of years. Scientists can infer this in part because three different subspecies in different parts of the world all use tools, and comparing their mitrochondrial DNA allows them to calculate roughly when they diverged from a common ancestor. According to the study the genes involved in their use of tools have probably had enough time to become widespread among all sea otters. Bottlenose dolphins haven't been using sponges as tools for long enough for that to happen yet.
One of the next steps will be to confirm the history of sea otters using tools, says Ralls. She hopes that archaeologists will be able to find ancient sea otter archaeological sites and date the tools in order to find out for certain how long otters have been using them. Fortunately, otters have made the job of identifying otter-utilized stones and shells easier: “A shell, like a mussel shell, looks different if a sea otter has been using it,” Ralls says.