(CNN)When sea otters hammer mussel shells against rocks to help open them, they're leaving behind their own archaeological record that can't be compared to any other creature, according to a new study.
Otters use tools to eat, and it's recording their history
Sea otters were once a typical sight between Baja California and Japan, but the fur trade nearly wiped them out. Once numbering between 150,000 and 300,000, California's southern sea otter population dropped to 50 at its lowest. Now, there are 3,000 thanks to conservation efforts, but their status is still threatened.
They're the only marine mammal that uses stone tools and rocks to break open shells. Sea otters have been known to crack open shells on their chests using rocks as they float on their backs, but they've also been observed using rocks along the shoreline as "anvils" to crack open mussels, clams and crabs.
A study published Thursday in the journal Scientific Reports shows how the markings left behind on these rocks are recording the otters' history.
A research team observed sea otters for 10 years between 2007 and 2017 and took video of them at the Bennett Slough Culverts site, a tidal estuary in central California. The site includes six large metal drainage pipes that are surrounded by boulders. The pipes connect two wetland areas, and mussels attach themselves to the sides of the pipes.
The otters would dive for mussels, return to the surface hugging a clump of them to their chests and open each one with their teeth or the stones. They could go through 25 to 75 mussels per hour, depending on how many otters were present. The otters would spend about an hour feeding at a time, and they spend about 35% to 55% of their time eating.
Mussels were the food of choice for the otters in this area, and they used stationary stone anvils to open 20% of them. Of 421 rocks studied in the area, 77 bore distinctive patterns of wear where the otters used them to open mussels.