The adaptable sea otter is one of the only known marine mammals to use stone tools while foraging, most notably to crack open hard-shelled delicacies like mussels, crabs, and clams. The otters use stones in multiple ways, but the technique of smashing mussels against large ‘anvil’ stones has been of particular interest to a group of Canadian and international researchers.
The team were curious about what we could learn about the distinct markings that this activity leaves on the larger rocks, and as it turns out, they are akin to an ‘archaeological signature’. By better understanding these damage patterns, researchers believe they could track sea otter populations in areas they are now extinct. It may also help archaeologists on the trail of ancient humans to distinguish between the activity of man and animal. The team’s research was published in Nature.
Identifiable patterns in anvil stones and shells indicate history of otter activity
The inter-disciplinary team involved in this study spent a decade (2007-2017) examining the nifty tool work of sea otters at the Bennetts Slough Culverts site using archaeological methods. Their analysis showed that mussels were the favourite dish of the otters and the anvil technique was used for about 20% of their mussel consumption.
A total of 421 local rocks were studied for damage patterns indicating repeated shell strikes and 77% were found to have features consistent with the sea otter technique. Some examples of this include the preference for jagged edges and ridges instead of the flat side of the rock, and angles indicative of a strike instigated from a water-bound position as opposed to a land one. Shells could be struck up to 30 times against the anvil rock before being finished off with teeth.
Randomized samples of approximately 132,000 individual shell fragments were extracted from the shell middens and analyzed. On the shells themselves, there was a consistent pattern: the two sides remained intact but a diagonal fracture ran through the right side.
The authors suggest this pattern may be a result of “pawedness” – a preference for one paw over the other – and with otters, it tends to be the right paw. Most of the otters under observation turned their right wrist before impact with the anvil rock so the right side of the shell smacked off the target.
“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,” says co-author Natalie Uomini of the Max Planck Institute. “For archaeologists who excavate past human behaviour, it is crucial to be able to distinguish the evidence of sea otter food consumption from that of humans.”
Co-author Jessica Fuji from the Monterey Bay Aquarium commented that uncovering past animal behavioural traces helps us to understand the evolution of rare activity like stone tool use in marine animals. “We hope that this study establishes a new path for the growing field of animal archaeology,” she adds.
Sea otter numbers used to range somewhere between 150,000-300,000 and their range spread from Baja California to Japan, but they suffered devastation on account of the fur trade. Canadian populations have recovered well since a reintroduction program coordinated between 1969-1972, and some 5,000 were estimated to exist in 2008. Despite this, they remain listed as a creature of “special concern” by the government.