Sea otters leave archaeological evidence

Sea otters pounding mussels on rocks on the shoreline create distinctive traces which can be studied using methods of human archaeology, enabling scientists to determine the historic distribution and behaviour of sea otters in the distant past.

Until now, the emerging field of animal archaeology has focussed on primates, and examined stone tool use in chimpanzees, macaques, capuchins and of course humans.

For the latest contribution to the field, published in the journal Scientific Reports, a multidisciplinary team combined behavioural observations of a sea otter (Enhydra lutris) colony in California, US, with archaeological survey methods applied to the animals’ stone anvils and the large shell piles or middens created under the anvils.

Jessica Fuji from Monterey Bay Aquarium in California and colleagues observed the sea otters’ foraging behavior for a decade, and while the population size of the colony was still small, they were able to establish that the sea otters leave behind distinctive archaeological traces.

The sea otters grasped mussels in their paws and pounded them against a favoured rock, or emergent anvil, until the shell was sufficiently weakened to be levered open using teeth. The shellfish was consumed while the otter floated on its back, the shells sloughed off its chest via a few ‘cleaning rolls’.

The mussel-pounding always occurred on ridges and points of the anvils, and use-damage analysis of 421 rocks found that the otters were targeting landward rocks, because these were the most often emergent from the water. A method for assessing human use of stone anvils was used to analyse the sea otter ones.

The shell debris left behind by the foraging bouts were also examined by the researchers.  The team did not excavate the sea otter middens, because they did not wish to disturb the colony. However, they extrapolate that perhaps hundreds of thousands of shells were present.

What’s more, the middens could be distinguished from the detritus of sea bird foraging, or human archaeological remains, by a typical sea otter breakage pattern that occurs when the animals bite down on mussels.

Today, the species occupies a fraction of its once extensive range, which stretched from Mexico to the northern Pacific rim and Japan. The study site has only been occupied since 1994, recolonized after the original otter population was hunted out.

“Our study shows that a clear archaeological signature of sea otter emergent anvil use can result from the behavior of a small number (10-20) individuals over a few years,” the team notes.

The research can now be used to document sea otter presence in locations where they are now extinct, and distinguish otter derived shell middens from those of ancient humans.