Behavior Biology Conservation Ecology Genetics

A long history of tool use in marine mammals? You otter believe it!



Article: Ralls, Katherine, et al. “Mitogenomes and relatedness do not predict frequency of tool-use by sea otters.” Biology Letters 13.3 (2017): 20160880.


If you’re like me, you’ve gotten lost in YouTube “rabbit holes” of random animal videos. Some of the best videos are those of animals using tools to solve problems, usually related to getting food. From crows to octopi, from primates to bumblebees, animals using tools is not only fascinating to watch (Fig. 1) but might bruise our egos, as we tend to think of ourselves as the most intelligent species on the planet.

Fig. 1: Tool use can be seen in a wide variety on animals. Like this monkey, most animals use tools to access food (source:
Fig. 2: Dolphins have been observed using sponges as armor to root around the sea floor looking for food (Photo: Hugh Pearson).









Only about 20 years ago, a population of bottlenose dolphins in the Indian Ocean was observed using tools that had never before been documented. These dolphins would stick their noses into marine sponges and use the sponge as armor or protection while they sift around rocks for food (Fig. 2). As scientists began to investigate this behavior, they found that they were able to trace it to a single female about 180 years ago, and since then, this behavior has been passed down from mothers to daughters in this population. To do so, they looked at the mitochondrial DNA (or the strands of proteins that code for the mitochondria within cells – the places where food is turned into energy)  of dolphins and found similar markers in the dolphins that used this tool. It was thought that this behavior developed due in part to high population density of dolphins. With more individuals competing for the same food sources, individuals finding a new and unique way to get a meal helped the overall population survive. Similar to bottlenose dolphins, sea otters (another marine mammal) use tools to crack into hard shells and deal with high population densities. Not all individuals in sea otter populations use tools or even use them in the same way. Researchers decided to apply the same tests on sea otters as they did on bottlenose dolphins in an effort to find out when otters developed tool use and how that’s been passed down from generation to generation.




The Study:

Sea otters living off the coast of California eat a variety of hard shelled marine invertebrates such as snails, clams, abalone, and sea urchins. Otters use rocks or other hard objects to crack open the shells of their prey to get at the stuff they really want. While otters use tools for almost all their prey, tool use is most common in otters that eat snails.


Fig. 3: Sea otters will bring rocks to the surface, along with a hard-shelled treat, in order to crack open shells to get at their next meal (Photo: Hakai Magazine).


Over a 14-year period, otters were tagged and monitored. Observers watched as these otters would dive for food, recording whether anything was caught, what was caught, and if the otter used a tool or not to access the food. Data for otters was sorted by food type, and individuals were only considered frequent tool users if they used tools more than 40% of the time they were eating. In addition to observations of feeding and tool use, researchers also collected DNA samples of the otters. Looking at the mitochondrial DNA, researchers aimed to find haplotypes, or sets of DNA variations that are heritable, that might be associated with using certain tools. For example, if you are an otter who uses rocks to smash up snail shells, you might have a very specific variation in a section of your genetic code that you only share with other snail-smashing otters.


Fig. 4: This table shows the number of sea otters that use tools for a specific prey item (left hand column) and which haplotype (or genetic variation) they have. Overall, there is no real pattern, and variations of the genetic code are found in otters regardless of their preferred food.


Researchers found 3 haplotypes in otters that use tools, although one of the haplotypes was found in just one individual (Fig. 4). With the other haplotypes, researchers expected there to be an association with a certain type of prey and tool use, but ultimately found that the same haplotypes were found in otters eating a wide variety of food (Fig. 5). So what does that mean? Well, it means that tool use and diet are spread across otters in this population and otters don’t need to have a certain haplotype to exhibit a certain behavior. This also means that otters that use tools aren’t related more closely to each other than to the whole population. What’s interesting about these findings, in comparison to those from the dolphin study, is that because there is no correlation between haplotypes and tool use, otters likely started using tools a long time ago, perhaps even developing in ancient otter ancestors millions of years ago!


Fig. 5: A sea otter uses a rock to open a clam. Based on this research here, this otter might have the same genetic variation as an otter using a tool to feed on a snail (Photo: Smithsonian Insider).


The Significance:

Sea otters play an important role in coastal ecosystems of the eastern Pacific. They are part of the classic marine food web and their presence and abundance plays a major role in shaping kelp forest ecosystems as otters dine on sea urchins. Understanding more about sea otter behavior and its evolution will help us to better understand these valuable ecosystems.

It’s no question that conservation and biodiversity are critical for maintaining a functioning and healthy planet, but sometimes it’s hard to get people to care (Fig. 6). Studying animal behavior, like tool use, makes it easier for people to connect to wildlife and see that we share abilities and behaviors. Studying animal behavior offers insight into us as humans, understanding how and why animals use tools or help one another gives us perspective on how social groups evolve. Here, researchers have shown that otters are pretty intelligent, and that intelligence is nothing new. The more we discover about the world around us, the more we realize that we are all connected. We are not the only species capable of manipulating the environment around us and this gives us context and helps us understand our place in the natural world.


Fig. 6: Conservation of these creatures is otter-ly important (Photo:!


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