When you are looking to eat a walnut, most likely you will reach for a nutcracker instead of trying to break it with your teeth. The reward is a high-energy walnut and intact teeth (if you used the nutcracker!). Southern sea otters use stones to act like nutcrackers to crack hard-shelled prey, such as clams and mussels. Sea otters also evolved very strong bites and molars with fracture-resistant enamel to consume hard-shelled invertebrate prey. Yet, frequent clashes between teeth and tough shells lead to tooth decay, which can impact an animal's health and survival. As sea otters are known to use tools, such as stones, to smash open the shells of tasty treats – including clams, crabs, sea urchins and snails – Chris Law, from the University of Texas, USA, and colleagues set out to explore whether using stone tools often allows sea otters to consume tough prey more frequently, whether it reduces tooth decay and whether it enables the mammals to consume more energy.
The team monitored the food consumed by 196 sea otters from five populations spread along the California coast (including animals at Big Sur, Monterey, San Luis Obispo, Piedras Blancas and Elkhorn Slough) and how often they resorted to using stones to crack open their dinner. In addition, the team monitored the toughness (how much force it took to smash) of typical otter delicacies (including clams, crabs, sea urchins, mussels and snails), the size (length) of the delicacies, and how much energy the morsels typically provided. Lastly, the team recorded the condition of the otters’ teeth, ranging from excellent to extremely poor, as well as calculating the percentage of dentine – a major component of teeth that gets exposed as a result of enamel damage – on the premolars of 35 of the 196 otters, to determine whether the animals were suffering tooth decay.
Overall, the sea otters that used stones as tools had better teeth, regardless of age, with males experiencing 19% less tooth damage, whereas tool-wielding females had 16% less tooth wear than sea otters that always cracked food open with their teeth. Additionally, the frequent tool users suffered less tooth damage, with ∼20% less dentine exposure than the non-tool-using sea otters. The team found that females, which are typically smaller with a weaker bite, found tool use particularly helpful, allowing them to crush harder shells than they could crack with their teeth alone, allowing them to eat larger mollusks and crustaceans. The team points out that this can be particularly important for females, which have to consume even more food and are unable to range far and wide in search of food when caring for a pup.
Among the sea otters that used tools, the researchers also found two distinct dining strategies. The sea otters that used tools ∼24% of the time used ‘high risk, high gain’ feeding strategies, which allowed them to feed intermittently on scarce, energy-rich, well-armored foods that would otherwise pose a risk to their teeth if they couldn't open them by some other means, allowing them to consume sufficient amounts to meet their daily energy demands. In contrast, the sea otters that used stone tools most of the time mainly consumed snails, the lowest energy diet, which required less time to smash open and resulted in less tooth damage, but forced the animal to spend more time foraging.
Tool use enables sea otters to consume hard, energy-rich foods, such as clams and crabs, or switch to less energy-rich abundant foods with shells, such as snails, when softer foods, such as abalone, are scarce. This shows that tool use is a necessity that allows sea otters to munch on a wider variety of foods than they could consume otherwise, reducing competition for foods that are easier to consume. So environmental pressures, such as competition for soft, easy to eat foods, or even reductions in the availability of soft food, can lead animals to adopt alternative feeding strategies that can also benefit their health.
