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.
The otters seemed to prefer rocks closer to land that had points and ridges.
The otters would float upright in the water, hold up a mussel and strike it down against the stone. After enough strikes to weaken it, the otters would use their teeth to open the shell. Then the otter would float on its back and enjoy its hard-won food and perform a "cleaning roll" to slough off the shell pieces.
The researchers also studied the mussel shells that were left around the rocks, creating accumulations called shell middens. Some contained as many as 132,000 shells. The shells also had a distinctive damage pattern in which both sides of the shell were still attached but a diagonal crack ran through the right side of the shell.
In the video, the researchers saw that the animals held the shells evenly between their paws, but as the shell struck the rock, their right paw shifted slightly on top. This is similar to a human's tendency to favor one hand or the other, called handedness. In this case, the researchers call it "pawedness."
"The shell breakage patterns provide a novel way to distinguish mussels broken by sea otter pounding on emergent anvils from those broken by humans or other animals," study author Natalie Uomini of the Max Planck Institute for the Science of Human History said in a statement. "For archaeologists who excavate past human behavior, it is crucial to be able to distinguish the evidence of sea otter food consumption from that of humans."
Going forward, knowing what these distinctive markings look like can help archaeologists determine how humans and sea otters might have used rocks in coastal areas. It could also fill in the history of now-extinct sea otters by looking at how far back their use of anvils extends.
This could help extend the field of animal archaeology, which has previously focused on primates.
"Our study suggests that stationary anvil use can be detected in locations previously inhabited by sea otters. This information could help to document past sea otter presence and diet in locations where they are currently extirpated," study author Jessica Fujii of the Monterey Bay Aquarium said in a statement. "The recovery of past animal behavioral traces helps us to understand the evolution of behaviors like stone anvil use, which is rare in the animal kingdom and is extremely rare in marine animals. We hope that this study establishes a new path for the growing field of animal archaeology."
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