Sea otters’ tool use leaves behind distinctive archaeological evidence

An international team of researchers has analyzed the use by sea otters of large, shoreline rocks as “anvils” to break open shells, as well as the resulting shell middens. The researchers used ecological and archaeological approaches to identify patterns that are characteristic of sea otter use of such locations. By looking at evidence of past anvil stone use, scientists could better understand sea otter habitat use.

Sea otters are an especially captivating marine mammal, well known for their use of rocks to break open shells. Sea otters are estimated to have once numbered between 150,000-300,000 individuals and their range stretched from Baja California, Mexico, around the northern Pacific Rim to Japan. Their numbers were dramatically reduced by the fur trade. In California, the southern sea otter population was reduced to around 50 individuals, but a massive conservation effort has resulted in increasing their numbers to around 3000 today. However, the southern sea otter is still considered threatened.

Sea otters are unique for being the only marine mammal to use stone tools. They often use rocks to crack open shells while floating on their back, and also sometimes use stationary rocks along the shoreline as “anvils” to crack open mollusks, particularly mussels. A joint project including the Max Planck Institute for the Science of Human History, the Monterey Bay Aquarium and the University of California, Santa Cruz, among others, has resulted in a first-of-its-kind interdisciplinary study published in Scientific Reports, combining ten years of observations of sea otters with archaeological methods to analyze sea otter use of such anvil stones, also known as emergent anvils.

The researchers hope that the study will be useful for archaeologists working with coastal populations, as a way to distinguish between human and sea otter use of rocks and consumption of marine resources. Additionally, the research could be helpful in future studies of the geographic spread of stationary anvil use throughout the former sea otter range, and how far into the past this behavior extends.

Research article: Wild sea otter mussel pounding leaves archaeological traces

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