Wild sea otter mussel pounding leaves archaeological traces

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.

Figure 1

Wild sea otters at Bennett Slough Culverts opening mussels using stones. The otters are using (A) a chest anvil, and (B) an emergent anvil.

Figure 2

Map of the Bennett Slough Culverts (BSC) study site and Moss Landing, with foraging sea otter densities. Black triangles show the position of BSC North and South, and the insets show (A) BSC North facing northwest, and (B) BSC South facing southeast. Jetty Road is at the left of both inset photos. The map was created using ArcGIS 10.6.1 (ESRI 2018, Redlands, CA). The kernel density of foraging sea otters was created using the Spatial Analyst toolbox on sea otter location data from distribution surveys from January to December 2016. Kernel densities in raster format were calculated using a grid cell size of 400 m² and a kernel-smoothing window of 200 m. Kernel density is displayed with a transparency of 30% to see the features of Moss Landing on the ESRI World Imagery Basemap (Sources: Esri, DigitalGlobe, Earthstar Geographics, CNES/Airbus DS, GeoEye, USDA FSA, USGS, Aerogrid, IGN, IGP, and the GIS User Community, https://services.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer).

Figure 3

Plan of the Bennett Slough Culverts site. View from above, showing alternating pipes and piles of rocks (the width of Jetty Rd is reduced for conciseness). Darker shading on rocks indicates a higher use-intensity score.

Figure 4

Use-wear damage on rocks at the Bennett Slough Culverts (BSC) site. (A) Sea otter damage on the corner of a quartzite boulder at BSC South; the scale is 10 cm. (B) Emergent boulders damaged by sea otters on their upper surfaces (circled) at BSC North, with the rocks further from the water topographically higher; the water level is mid-height. (C) Emergent anvils at low tide at BSC North, with the boulders seen in (B) on the left. Mussel shell deposits are visible above and below water in the three views.

Figure 5

Mussel shell breakage patterns at the Bennett Slough Culverts site. (A) Outer and (B) inner faces of each valve; (C) schematic drawing of the exterior of a mussel shell showing the typical sea otter breakage pattern (illustration by Neil Smith); (D) broken mussel shells in situ.