Zoology: Otters create their own archaeological records

Sea otters leave distinct wear patterns on the rocks that they use to break open hard-shelled food reports a paper in Scientific Reports. The patterns, which can be recognised using archaeological techniques, could help trace locations of previous sea otter populations that are now extinct.

Sea otters are the only marine mammal known to use stone tools to break open prey such as mussels. They have been observed striking prey against rocks under water, on their chests or by using a boulder at the water margin. However, the physical effect of this behaviour on local environments is not well known.

Jessica Fujii, Natalie Uomini and colleagues observed sea otters using rocks to break open prey over a 10 year period in Elkhorn Slough, a tidal estuary in central California. They surveyed 421 rocks in the area and found that 77 of the rocks had distinctive wear patterns of damage consistent with being used by otters to break open prey. In the same areas they also found a large number of discarded mussel shells with shell breakage patterns consistent with predator actions. These patterns enable the researchers to distinguish mussels that had been broken by otters using rocks from those broken by humans or other animals. The authors suggest that the distinct damage to rocks and shell presents a clear behavioural signature that can be used to identify previous foraging areas of sea otters. In addition, the findings may help us to understand the evolution of stone anvil use, which is rare in the animal kingdom and is extremely rare in marine animals.