Sea otters (Enhydra lutris) are the only marine mammals that use stone tools while foraging, using them to break open hard-shelled foods. In a new study, a team of researchers observed sea otters pounding mussels at the Bennett Slough Culverts site, California, over a ten-year period. The sea otters repeatedly used the same rocks as anvils, which resulted in distinctive wear patterns on the rocks and accumulations of broken mussel shells and created a distinct, recognizable archaeological record at the site.
A wild sea otter at Bennett Slough Culverts opening mussels using a stone. Image credit: Haslam et al, doi: 10.1038/s41598-019-39902-y.
The team, led by researchers from the Max Planck Institute for the Science of Human History and the Monterey Bay Aquarium spent ten years between 2007-2017 observing sea otters consuming mussels at the Bennett Slough Culverts site.
The analysis identified that mussels were the most common prey eaten at the site and were the only prey for which the sea otters used stationary anvil stones.
The sea otters used such stones for about 20% of the mussels they consumed.
Interestingly, careful analysis of the stationary anvil stones using archaeological methods showed that their use resulted in a recognizable damage pattern that was distinguishable from what would be caused by human use. For example, the sea otters preferentially struck the mussels against points and ridges on the rocks, and struck the rocks from a position in the water, rather than from the land or from on top of the rock.
In addition to the stones themselves, the team analyzed the mussel shells left around the stationary anvils.
The researchers took a random sample of the shell fragments from these shell middens, which likely contained as many as 132,000 individual mussel shells.
They found an extremely consistent damage pattern, with the two sides of the mussel shell still attached, but a diagonal fracture running through the right side of the shell.
“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,” said co-lead author Dr. Natalie Uomini, from the Max Planck Institute for the Science of Human History.
“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.”
In combination with analysis of videos they took of the otters using the anvils, the scientists could see that the otters held the shells evenly in both paws, but when striking the shell against the anvil tended to have their right paw slightly on top.
Though the total number of otters observed was small, these results suggest that otters may exhibit handedness, or ‘pawedness,’ as do humans and many other mammals.
“We hope that the study will be useful for archaeologists working with coastal populations, as a way to distinguish between human and sea otter use of rocks and consumption of marine resources,” the study authors said.
“Additionally, the research could be helpful in future studies of the geographic spread of stationary anvil use throughout the former sea otter range, and how far into the past this behavior extends.”
“Our study suggests that stationary anvil use can be detected in locations previously inhabited by sea otters,” said co-lead author Dr. Jessica Fujii, from the Monterey Bay Aquarium.
“This information could help to document past sea otter presence and diet in locations where they are currently extirpated.”
“More broadly, 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.”
The study was published in the journal Scientific Reports.
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Michael Haslam et al. 2019. Wild sea otter mussel pounding leaves archaeological traces. Scientific Reports 9, article number: 4417; doi: 10.1038/s41598-019-39902-y
