Sea otters use tools when feeding to survive a changing world

Sea otter using a rock
UC Santa Cruz researchers used biomechanical and functional morphology to measure how hard sea otters need to bite to break open prey of various sizes. (Photo by Jessica Fujii, Monterey Bay Aquarium)

Sea otters are one of the few animals that use rocks and other objects to access their food, and a new study has found that individual sea otters that use tools—most of whom are female—can eat larger prey and reduce tooth damage when their preferred prey becomes depleted.

The study’s researchers and their enlisted volunteer “otter spotters” followed 196 radio-tagged southern sea otters off the coast of California to better understand how the threatened species uses tools in a rapidly changing environment. The research team from UC Santa Cruz, the University of Texas at Austin, Monterey Bay Aquarium, and elsewhere monitored how the marine mammals used tools—such as rocks, shells and trash—to break open prey and identified links to the animals’ dietary patterns and dental health.

For the first time, researchers found that the use of tools among male and female otters led to a reduction in tooth injuries. The study is published in Science.

“Sea otters vary in how often they use tools,” said biologist Chris Law, who led the research while pursuing his Ph.D. at UC Santa Cruz. “The females are likely using tools to overcome their smaller body size and weaker biting ability in order to meet their calorie demands. Raising pups takes a lot of energy, and the females need to be efficient in their foraging. The study shows that, for some animals, tool use is an important behavior for survival.”

A dental dilemma

In the southern sea otter’s range of coastal Central California, preferred prey like large abalone and sea urchins are not difficult to break open or consume once acquired. However, these food resources dwindle or disappear over time as a result of sea otter predation, leading them to broaden their diets to include  crabs, clams, mussels, and small marine snails with hard shells that can damage their teeth as they pry them open.

Tooth condition is crucial for survival: When an otter’s teeth become too worn or damaged, they could starve. Using tools helped otters to meet their calorie needs by enabling individuals to specialize on  distinct types of prey that are difficult or dangerous to consume without using tools.

In a previous study estimating how hard otters can bite, the researchers realized certain prey were off the menu. That inspired them to bring in biomechanical and functional morphology to measure the forces needed to break open prey of various sizes, and determine how those forces compare to an otter’s bite strength. “We combined these extensive data sets with estimates of otter biting ability, their diet preferences, and frequency of tool use to show how that behavior is important for their survival,” said Professor Rita Mehta, chair of the Ecology and Evolutionary Biology Department at UC Santa Cruz, and Law’s Ph.D. advisor. 

Females use tools more

 Research shows that female otters are more likely to use tools, and in the study, those that did were able to access harder or larger prey than otters that did not use tools. In fact, females were able to consume prey that were up to 35% harder compared with that of males that used tools. The study also found female otters had less tooth damage than male otters did, and that females that used tools most frequently had less tooth damage than females that used tools rarely or never.

Female dolphins, chimps and bonobos are also known to use tools more than their male counterparts, probably for the same reasons. In these species, females tend to raise offspring, and they are often the ones that pass down tool-use behavior to offspring. 

Listed as a threatened species under the U.S. Endangered Species Act, southern sea otters number only about 3,000 in California, where they play a critical role in marine ecosystems preying on sea urchins that feed on kelp forests, and also have been shown to have positive effects on estuary habitats.

Law is now a postdoctoral researcher and an Early Career Provost Fellow at UT Austin. Other authors of the new study include Mehta, Tim Tinker, also on UC Santa Cruz’s ecology and evolutionary biology faculty, as well as Jessica Fujii, Teri Nicholson and Michelle Staedler with the Monterey Bay Aquarium. Other contributors include Joseph Tomoleoni of the U.S. Geological Survey, and Colleen Young of the California Department of Fish and Wildlife.

“We are fortunate to have these state and federal collaborative partnerships at our coastal campus location overlooking the vibrant Monterey Bay that so many marine species call home,” Mehta said. “These collaborations make UC Santa Cruz a leading location to study how these creatures adapt to a changing ecosystem and persevere.”

The research was funded by the U.S. National Science Foundation, Packard Foundation, Coastal Conservancy, U.S. Fish and Wildlife Service, Pacific Gas & Electric and Bureau of Ocean Energy Management.