Abstract
Wild sea otters (Enhydra lutris) are the only marine mammals that habitually use stones while foraging, using them to break open hard-shelled foods like marine snails and bivalves. However, the physical effects of this behavior on local environments are unknown. We show that sea otters pounding mussels on tidally emergent rocks leave distinct material traces, which can be recognized using methods from archaeology. We observed sea otters pounding mussels at the Bennett Slough Culverts site, California, USA, over a l0-year period. Sea otters repeatedly used the same rocks as anvils, which resulted in distinctive wear patterns on the rocks and accumulations of broken mussel shells, all fractured in a characteristic way, below them. Our results raise the potential for discovery of similar sea otter pounding sites in areas that no longer have resident sea otter populations.
Highlights
Sea otter (Enhydra lutris) populations currently survive only in remnants of their former habitat, which stretched from Baja California, Mexico, around the northern Pacific rim to Japan[1]
We report on two data sets, the first containing sporadic observations of sea otters foraging at the site from 2007 to 2017, and the second consisting of concentrated behavioral and archaeological observations at the Bennett Slough Culverts (BSC) site from July 7th to 27th, 2016
We opportunistically observed tagged and untagged individuals foraging at BSC, with observation effort varying over time, and records spanning from 2007 to 2017
Summary
Sea otter (Enhydra lutris) populations currently survive only in remnants of their former habitat, which stretched from Baja California, Mexico, around the northern Pacific rim to Japan[1]. Sea otter stone use while foraging takes three forms: (i) using a stone underwater to pry loose abalone from a substrate[11], (ii) pounding food using a stone as a hammer or anvil on the chest while floating at the surface[12] (Fig. 1A), and (iii) pounding food directly against a rocky substrate Both the underwater and chest anvil pounding behaviors are considered tool-use under current definitions[2], as they involve the controlled use of a detached object. In the third form of stone use, the sea otter repeatedly pounds a hard-shelled prey against a stationary, fixed stone anvil, typically a boulder at the water margin (Fig. 1B). We term this behavior emergent anvil use, to distinguish it from the use of chest anvils. Our study establishes a new path for the growing field of animal archaeology, which until now has focused on primates[15,16,17]
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