Tool use by sea otters

Sea otters are one of five species, and the only non-primate, known to regularly use percussive stone-tool technology in the wild. The others are humans, chimpanzees, the bearded capuchin, and the long-tailed macaque. Shellfish are found on rock structures at the bottom of the ocean, so collecting multiple rocks and shellfish from the bottom of the ocean in one diving episode and crushing the objects together on the surface may have led to the association of rocks with crushing shells of foods items. Skeletal changes Sea otters belong to the order Carnivora, whose members possess typical carnivoran teeth, or shearing carnassials. These are blade-like teeth formed by an upper premolar and lower molar. Sea otters have replaced their carnassial teeth with bunodont post-canines to improve their food crushing ability. These teeth favor a diet of aquatic invertebrates which is crucial to the otter's survival in the water. Sea otters also belong to the family Mustelidae. When compared with other mustelids, such as the river otter, weasels, and minks, the sea otter shows distinct hindlimb anatomy which could contribute to propulsion and stability at the surface of the water while the forelimbs manipulate tools and food. E. lutris has significantly larger gluteus muscles than other mustelids. Sea otter forelimbs are small and not used in swimming. Forelimb structure, particularly musculature and skeletal anatomy, are potentially adapted for, "tactile sensitivity and tool use associated with detection, handling, and consumption of prey". == Anatomy ==

hands which they use to grasp prey. During consumption of sea urchins, otters use a tool or their paws to crack open the sea urchins and scoop out the gonads and viscera with the lower incisors (12). The canines are blunt, and the post-canines are bunodont. Their flat and broad shape is useful for chewing a combination of soft invertebrates and the harder parts of tougher invertebrates, like shell fragments and sea urchin tests. Sea otters also have some of the largest lungs in the otter family, which may be helpful for buoyancy, especially because sea otters do not have blubber. Being buoyant along the length of the body allows otters to lie on their backs and manipulate food, tools, and young while on the surface of the water. Underneath the arm of each sea otter, at the axilla, is a flap of skin that can store stones and food. The hind legs are long and flattened like flippers, and the tail is also flattened. The tail moves in an undulating motion for propulsion. Paws and vibrissae work in tandem to find and grasp prey underwater. Sea otters also have retractile claws on their forelimbs. == Learning ==

Otters that use stones to open prey do not use stones every time they need to manipulate their prey. Crabs, for example, can be ripped apart by the forelimbs and then eaten. Otters will store a stone in the pouch of skin under the arm to eat prey with both forelimbs, such as crabs, and then retrieve the stone at a later instance. Otters have been observed to use a clam shell as a digging tool and as a pry tool as well. This seems to imply, "an anticipation of use that goes beyond the immediate situation". Until then, otter pups spend all of their time with their mothers. This is exacerbated by the fact that for the first three months of life, otter pups cannot swim or dive effectively. They are born with a natal pelage that differs in color and structure from adult pelage and is incredibly buoyant. This prevents them from submerging their bodies underwater and must be held or anchored to kelp. Sea otter pups display a propensity for manipulating objects between their paws and regularly pound rocks and little bits of coral against their bodies in a random and curious manner. According to some of the earlier otter behavior biologists, Hall and Schaller, this, "tendency to manipulate and pound is far from stereotyped in its application and seems to prove the basis for learning the use of tools in feeding behavior". Social transmission Otters forage independently, except for females with offspring, who feed with their single young. Twins are observed but very rare. This allows for the mother otters to focus their attention on the pup, especially because mating occurs without pair bonding, so the father of the pup is not present. Pups express the same preferences in tools, technique, and diet as their mothers, which is evidence of vertical social transmission. It has been observed that if a pup has not been taught by its mom to use tools by three years of age, it will never use tools as an adult. In addition, pups that are taught to use tools spend years refining their techniques. Mother otters appear to make sacrifices in order to help their pups. In Prince William Sound it was observed that, "the diet of females with pups was often of poor quality because the mothers frequently foraged on prey items that are easily captured by pups". Georg Steller wrote that "their love for their young is so intense that they expose themselves to the most manifest danger of death. When taken away from them, they cry bitterly, like a small child, and grieve so much that, as we have observed from rather authentic cases, after ten to fourteen days they grow as lean as a skeleton, become sick and feeble, and will not leave the shore." Sea otters show variation in their foraging techniques, with some examples of horizontal transmission within groups as well, such as the sudden spread of smooth lumpfish hunting among Amchitka Island sea otters. Other sea otters will dig trenches up to four feet long to hunt clams. == Current tool use behaviors ==

The foundation for observation studies of otters was conducted by scientists K. R. L. Hall and George B. Schaller in 1964. The scientists spent six days observing the Californian otters in Point Lobos State Park, California. Over the course of their time, they observed 30 separate instances of tool use behaviors, most of which involved otters using rocks to crack mussels. Otters of the Northern Pacific consume mostly sea urchins and fish, thereby exhibiting less tool use behavior. Otters of the southern Pacific Ocean feed on tougher macro invertebrates such as bivalves. In central California, sea otters feed mostly on sea urchins, abalones, and rock crabs. Rocks are used to knock abalones from the structure on which they are growing. Rocks can also be used to crack crab carapaces. Variation Intra-species variation Intra-species variation is observed with regards to the Northern Pacific and the Southern Pacific. In Point Lobos, the use of tools to open mussels is very common. This behavior is distinctly less common in northern areas like the Commander Islands and Kuril Islands. There, adult otters only use tools if they are unable to open food items after trying first with their teeth. A study that compiled seventeen years' worth of observational data demonstrated a significant difference between the occurrence of tool use in Amchitka Island, Alaska and Monterey, California. Alaskan otters used tools on 1% of dives, while Californian otters used tools on 16% of dives. Individual variation During long-term studies to record behavior, it is frequently recorded that some individual otters do not use tools at all. Instead, these otters target soft-bodied prey like fish, or urchins which can be ripped apart with the forelimbs. Among tool using otters, up to 21% of the day can be spent engaging in tool use. Daughter Californian otters display the same tool techniques as their mothers, expressing explicit preference for certain methods when eating bivalves or eating crabs. In an aforementioned study, which compiled 17 years of observational data on otters from southern California to the Aleutian Islands, it was discovered that anywhere from 10% to 93% of individuals in a population use tools. Some otters have developed very specific, individual behaviors that do not necessarily demonstrate tool use, but do demonstrate dexterity. Individuals in California have learned how to tear open aluminum cans that float in the water from incidents of pollution. Small octopuses commonly reside in the cans and the sea otters attempt to eat the small octopuses. Others have learned to reach on the stern of small boats to obtain bait fish or squid. == Paralytic shellfish poisoning ==

Otters are not immune to paralytic shellfish poisoning, despite some popular belief that they are. However, they have the ability to manipulate their prey enough to avoid the paralytic shellfish poisoning toxins so that they do not consume lethal amounts. Alaskan sea otters prey heavily on the butter clam, which has the ability to retain toxins obtained from dinoflagellate blooms. == See also ==