1. How Sea Otters Use Rocks to Open Food
Researchers have identified three main ways sea otters use rocks. They can use a rock underwater to pry abalone off rocks, float on the surface with a rock on their chest to place food on it and strike it, or hold a shell and strike it directly against a rock [1]. Sea otters are considered the only marine mammals known to use stone tools regularly in the wild [1]. It is important to note that this uniqueness applies specifically within the group of marine mammals.
Observations show that otters grasp shells with both front paws. While floating on their backs, they strike the shells quickly and repeatedly against rocks, facing downward or sideways [1].
2. Why Sea Otters Use Rocks When Favorite Foods Become Scarce
In Southern California, sea otters can eat favorites like large abalone and sea urchins without tools. However, when these these foods become scarce, otters expand their diet to include crabs, clams, mussels, and small hard-shelled snails [3]. A 2024 study analyzing long-term data from 196 otters fitted with radio transmitters found that individuals who used tools, especially females, could eat larger or harder-shelled prey and had less tooth damage [2]. Females using tools were reported to eat prey up to 35% harder than males who used tools [3].
The study suggests that in environments where favorite foods are reduced, tool use might be necessary for some individuals to survive. It is not established that tool use is essential for every single otter [2].
3. How Tool Use Differs from One Sea Otter to Another
A 2017 study of 211 otters in California found significant differences in how often otters used tools. In areas with abundant prey, 4 to 38% of prey caught were taken using tools. In areas with limited prey, the range widened to 0 to 98% [4]. Otters that specialize in eating difficult-to-open prey tend to use tools more often. Some sea otters use tools often, while others use them less often.
4. What Shell Fragments Show About Sea Otters Using an Anvil Rock
At a location called Bennett Slough in California, researchers observed otters repeatedly using a large coastal rock as an anvil over ten years, from 2007 to 2017. Rocks used as anvils developed distinctive marks, and broken shell fragments accumulated beneath them [8].
There is a consistent pattern in how otters break mussel shells. The left shell often remains attached at the hinge, while the right shell breaks diagonally. In one sample of 29 fragments collected at a single site, 18 were left shells and 11 were broken right shells. This is not a number for all sea otters, but a specific local observation [1].
This pattern helps archaeologists distinguish otter-made breaks from those made by humans or other animals, aiding in the study of ancient shell middens. Because the breakage pattern tends to favor one side, it has been suggested that many otters there were right-handed. This is a suggestion, not a confirmed fact [5].
5. Do Sea Otters Keep a Favorite Rock in a Skin Pouch?
Japanese reference materials describe a loose fold of skin under the sea otter’s armpit, near the base of the front paw that acts like a pouch. Otters are said to store food collected in the sea in this pouch [7]. Some descriptions also suggest they store rocks there.
The idea that otters carry a "favorite rock" for months or years is widely circulated. However, sea otter researcher Jessica Fujii stated in a Hakai Magazine article that otters drop the rock after using it and return to one of the many rocks on the sea floor, rejecting the idea that they carry a specific rock around [6]. This article presents both the popular explanation and the researcher's counter-argument without declaring one as absolute fact.
6. Activities: Draw and Observe Sea Otters
If you want to explore this topic further, try drawing a sea otter floating on its back and cracking a shell. Note where the front paws are and what is resting on the chest. Using field guides or photos can help you understand how the body is used.
If there is an aquarium with sea otters nearby, ask an adult to check the feeding time, then watch from the visitor area and follow the aquarium’s rules. While captive behavior may not match wild behavior exactly, observing how they handle shells provides clues. For deeper reading, compare the abstract of the Science paper (Source 2) with the Hakai Magazine article (Source 6). They offer different perspectives on why otters use tools and the idea of carrying a rock, showing that the same animal can be viewed in different ways.