| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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feature A Smorgasbord for Sea Otters The young California sea otter was sleeping soundly in her usual spot in Monterey Harbor. It was Female 515, also known as Josie, who had been raised here by her mother, amid the constant boat traffic and human activity. I watched with interest as a couple of kayakers glided by and stopped. I knew they might be in for a surprise, for Josie was a notorious harasser of kayaks and small rowboats, on which she liked to hitch rides. She seemed to find this particular type of kayak, which lies very low to the water, especially irresistible. By now the otter was staring intently at the kayakers. Josie stretched, twirled in the kelp and dove, disappearing beneath the glassy surface. Almost half a minute passed. Everything was quiet and the kayakers had started to paddle away. Suddenly, the otter exploded out of the water and landed on the back of one kayak, nearly tipping it over. I could see the shock and surprise on the kayaker's face. He'd probably been warned not to disturb or harass sea otters in any way, and he must have seen the posted signs cautioning people to stay at least 50 feet from the shy and easily disturbed otters. But Josie made this rule a little difficult to obey. As the man struggled to keep his balance, the young female slid off his boat and headed for the other kayak, but its occupant wasn't waiting around. He sprinted off as fast as he could, with Josie in hot pursuit. Since sea otters can swim up to six miles an hour in short bursts, it took a good two minutes of frantic paddling for him to outdistance her. I felt a little guilty for laughing, but it was hard to restrain myself when watching such otter antics. I'd been following Josie since she was a young pup,and now she was nearly three years old, still young for a sea otter. (Female sea otters live as long as 15-20 years, while male lifespan usually ranges from 10-15 years.) "Is that what otters usually do?" asked a surprised tourist who saw the incident. "No," I replied, "but that's what this otter usually does." "Why does she do that?" I could think of no reason other than it must be fun. Sea otters, especially pups and young juveniles, are marvelously curious and playful marine mammals. Of course, most wild otters are much more reserved around humans. But there were those fearlessly friendly individuals, like Josie, especially along the northern Monterey Peninsula where otters are constantly exposed to divers and boaters. Sea otters' curiosity and resourcefulness probably has survival value, for they must be intelligent and innovative learners to find, capture, and break open the prey on which they feed. And as I had come to learn, whether at work or at play there were many exceptions to "typical" otter behavior. Josie was one of over 60 tagged California sea otters (Enhydra lutris nereis) in Monterey Bay that I studied from 1985 to 1995, with the assistance of many observers. Ten years of intensive study have revealed that no two sea otters are alike. Each has a unique personality. In fact, a remarkable degree of individual variation attends almost everything about sea otters: their diet and foraging behavior, tool-use techniques, the tendency to haul out on land, reproductive behavior, vocalizations, and, as Josie illustrates, their response to humans. Sometimes I could even guess the identity of an otter based solely on its resting position, before I was able to see its hindflipper tags; Female 182, for instance, often rested with her forelegs stiffly outstretched at an angle, forming a characteristic and conspicuous V. The striking differences among sea otters underscore the importance of studying individual animals over long periods of time. The northern Monterey Peninsula offers an ideal study site. Here otters remain close to shore, and most stay in the same area for years. Sea otters in many parts of Alaska and Russia, representing two different subspecies from the California otter, can be much harder to track from shore. Fortunately for us, a Monterey Bay otter does nearly everything in plain view. Looking from shore with a Questar spotting scope, we can record the type of food an individual eats, how it uses tools, who it mates with, and when a female gives birth and weans her pup. Most carnivores, in contrast, are much more secretive and difficult to observe, and, unlike many other marine mammals, sea otters are only underwater for a minute or so during feeding dives. They rest, breed, and socialize on the surface. One of the more surprising findings to emerge from this long-term study is that each otter has its own distinctive diet, usually preferring to eat only one, two, or three kinds of food from 33 types of prey eaten by otters in Monterey Bay. For years, and probably throughout its lifetime, an otter continues to eat the same foods and use the same feeding strategies. A female known as the Ab Queen, for instance, specialized in abalone (Haliotis spp.), although she also ate some rock crab (Cancer spp.) and purple sea urchins (Strongylocentrotus purpuratus). Even though abalone are difficult for human divers to find here, the Ab Queen was amazingly adept at locating these scarce, choice delicacies. Each week she probably consumed two hundred or so. Female 508 favored another well-hidden species, octopus (Octopus spp.). Once she captured 20 small octopuses in a single feeding boutÑan astonishing feat to human divers, who hardly ever see this shy cephalopod underwater. Female 520 specialized in fat innkeeper worms (Urechis caupo). These huge, unappetizing-looking worms burrow deeply into the sandy bottom, and it's nearly impossible for a diver to dig one up. But during one foraging bout, this female collected 44 worms in 46 minutes and swallowed about one worm per minute. Rock oysters (Pododesmus cepio) and crabs rounded out her diet. A large territorial male named Nosebuster, due to the especially rough manner in which he grabbed females' noses while mating, ate practically nothing but small, brown turban snails (Tegula spp.). He seemed to be eating every time I saw him, perhaps because he had to capture so many of the tiny snails to obtain sufficient calories each day. Like other territorial males, he often stole food from adult females. Most of the thieves, both male and female, tended to steal the same type of prey they captured on their own. Unlike other marine mammals, sea otters have very little blubber. To fuel the high metabolism that helps them stay warm in the chilly 35 to 60 degrees F ocean waters, they have to eat a lot. Each day an otter must consume between 23 to 33 percent of its body weight in food. Nosebuster, who weighed a hefty 67 pounds, probably had to capture many hundreds of snails each day to ingest the 15 to 22 pounds of food needed just to sustain himself. The pronounced individual variation in foraging strategies and diet among sea otters of the same age and sex classes feeding in the same area has rarely been documented in other animals. Theoretically, there should be strong selection pressure towards an "optimal" diet for all individuals. An otter like Nosebuster would seem to be at a disadvantage, since he must presumably forage for many more hours to meet his daily food needs with tiny snails, compared to Ab Queen, who eats calorie-rich abolone. The fact that snails are much easier to find and capture than abalone somewhat balances out these inequities, yet the overall profitability per item of prey like abalone and rock crabs is much higher than for snails, mussels, or purple sea urchins. Although we don't fully understand why these strikingly specialized diets arose, we now have some idea how each otter acquires its specific diet. I suspected that feeding habits might be matrilineally inherited, with mothers passing on their dietary preferences to their offspring. After all, pups remain with their mothers constantly for about six months and have plenty of opportunity to learn how to forage by watching mother. Although they suckle until being weaned, pups over one month old usually receive or steal at least some of their mother's food each time she surfaces from a dive. On average, a mother shares just over a quarter of her prey with her young, though some are much more generous. We were able to collect sufficient information on three grown daughters and several dependent pups to show that the offspring did indeed tend to eat the same foods as their mothers. In fact, the adult daughters often also chose the same type of tool and method of tool use, fed in the same area, and employed other foraging tactics similar or identical to those of their mothers. The male pups we observed also tended to select the same foods as their mothers, but we were unable to follow juvenile male offspring because they so often left the area where they were weaned. Whitehead, a female that we studied until she was 15 years old, preferred to eat kelp crabs (Pugettia spp.), mussels (Mytilus spp.), and some purple urchins. When she was eight years old, Whitehead had a pup we named Notch because of a V-shaped notc Female 184 fed on mussels, turban snails, and kelp crabs, often in shallow water close to shore. Her daughter, Female 535, stayed by her side for over nine months--the longest pup dependency period ever recorded for a California sea otter. After she was weaned, Female 535 not only ate most of the same foods as her mother but even fed in the same area. From her mother, Female 535 had learned an unusual feeding strategy that we called the "surf grass salad bar." Both otters dove to the bottom and collected a large clump of surf grass (Phyllospadix spp.). On the surface, they rested this "salad" on their chests, while leisurely picking out tiny kelp crabs hidden among the bundle of long, green grasses. Sometimes, like a true crustacean connoisseur, Female 535 would pull strands through her mouth and nibble off the tiny crabs. Josie's mother, Female 190, liked to eat small, pearly shelled rock oysters plus the occasional fat innkeeper worm. Josie's younger sister, affectionately known as Tubehead after she slipped a clear plastic tube snugly over her entire head as she played with it, also ate rock oysters as a juvenile and adult and often fed in the harbor where she grew up. More intriguingly, Female 190 always used a very distinctive tool to dislodge the oysters: a glass bottle. After she was weaned, Tubehead, too, used a bottle to crack loose her oysters. On several occasions while she was a dependent pup, I also witnessed Josie using a bottle as a tool. Although she spent a lot of time in Monterey Harbor, we were not often able to observe Josie's foraging habits after she matured, so we cannot say whether this habit continued. The fact that two generations used such an unusual tool perhaps provides some of the most compelling evidence that offspring learn foraging tactics from their mothers. Even though dependent pups usually captured the same prey species as their mothers, they were much less successful at obtaining food. Apparently, it takes a long time to learn the difficult and complex skills of foraging in Monterey, where otters have lived for many years and populations of otter prey are somewhat reduced. Pups captured real food on only 13 percent of their dives, whereas mothers captured prey on 70 percent of their dives. Almost ten percent of a pup's dives resulted in the capture of a non-prey item. Such "prey" was often tasted repeatedly or pounded unsuccessfully. Josie provides an example here as well. One day several years ago, when Josie was a large pup, she foraged in the harbor with her mother. As she surfaced from one dive, I could see she was struggling to bring something to the surface. It was an old automobile tire, bigger than she was. I'd seen pups capture all sorts of "junk food," including aluminum cans, driftwood, pine cones, plastic bags, even golf balls, but this was a first. Josie chewed on the tire for a minute, as though it was the most natural food in the world, then let it sink to the bottom. Meanwhile, munching on a rock oyster, her mother watched, apparently unconcerned, as her pup retrieved the tire once more and wrestled it to the surface. While watching Josie and her mother, I often heard the pup squeal with the ear-splitting scream emitted by distressed or hungry pups, especially when they are separated or hungry. Sometimes Female 190 responded with her own piercing call. Analysis of the vocalizations of mothers and pups have shown that each one has a very distinctive "scream," which probably helps the mother and pup stay in contact. Even the human ear can detect differences. One female's call always sounded like a raspy, quacking duck to me. We identified at least ten types of sea otter vocalizations: screams, whines, whistles, squeals, whimpers, squeaks, hisses, growls, coos, and grunts. The sea otter's vocal patterns seem to be most suitable for short-range communication among familiar individuals, Many of their sounds are often quite soft and difficult to hear from shore. The soothing coo, for instance, is even made with the mouth closed. It occurs between mothers and pups and between pair-bonded adults during courtship, but otters may also coo while eating. Sea otters also appear to communicate through scent and with ritualized nuzzling and "head-jerk" interactions. Nuzzling involves a gentle rubbing with the nose, but for the exaggerated head-jerk, an otter buries its nose in another otter's fur and shakes its head from side-to-side in a series of rapid jerks. A sea otter that is entering, leaving, or simply swimming past a raft of otters will typically approach and make contact in this way with some or all of the animals in the group. I suspect that sea otters can easily recognize other individuals, and even preferentially interact with certain otters. Only a handful of animals, including some birds, insects, dolphins, primates, and, of course, humans, have the ability to use tools. Sea otters are members of this select group. While floating face up on the surface, an otter often places a flat rock on its chest as an anvil and pounds mollusks against it until the tough outer shell breaks open. From my observations, this technique accounts for over half of an otter's tool use. Sometimes a second rock serves as a hammer to crush the shell against the rock anvil. Otters also bring rocks underwater to pry loose stubborn abalone or urchins tightly wedged in crevices. Just as with their varied diets, sea otters have devised an astonishing array of innovative methods to capture, secure, and break open prey using many different kinds of tools. Adult otters are known to employ over 30 different tool-use techniques and use at least 14 different types of tools, including rocks, large cement slabs, glass bottles, crab claws, shells, kelp, wood, and bricks. Many otters always use the same kind of tool or have distinctive ways of using their tools. For instance, during all the years I studied her, when she was feeding, Female 182, or Flatstoner, carried a huge flat rock or a concrete slab found along Cannery Row. Underwater, she used this massive tool to bash off abalone stuck tightly to rocks. On the surface she placed the rock or slab across her chest like a "table" as she ate. Flatstoner didn't always rely on her rock, however. Sometimes she and her "neighbor" Whitehead fed on mussels in the same spot, but each female had her own way of opening them. Flatstoner grabbed clumps of mussels from shoreline rocks and ripped them open with her teeth. Whitehead dove underwater to capture her mussels, then dove again to find a rock, which she used to crack the mussel shells. Otters are able to vary their method of tool use in relation to specific types of prey, one of the only examples of such flexible tool-using behavior among non primates. One female who simultaneously captured both a purple urchin and turban snails first ate the purple urchin by cracking it apart with her teeth. Before consuming the snails, she made a dive, apparently to obtain a rock to break open the hard-shelled snails. Although there is considerable individual variation in the proportion of time a tool is used to capture a particular type of prey, tools tend to be used much more frequently for certain prey types than for others. For example, a tool is used about 95 percent of the time with turban snails, 47 percent of the time with rock oysters and rock scallops (Hinnites spp.), 15 percent of the time with purple urchins, and rarely, if ever, with fat innkeeper worms, squid (Loligo opalescens), octopus, and sea stars (Pisaster spp.). Besides using tools to capture prey, sea otters employ other tactics for preventing their escape. An otter fortunate enough to capture two or more large crabs at once has to subdue the struggling crustaceans before they get away. To keep the second crab from escaping or pinching while the first one is being eaten, the otter wraps kelp tightly around the crab until it looks like a mummy. We called this strategy "straitjacketing." Female 535 had learned to straitjacket large rock crabs by the time she was ten months old. Sometimes she used surf grass instead of kelp for a restraint. Another maneuver to prevent crabs from escaping is the "hindflipper crab block," in which the otter imprisons its catch by folding its hindflippers tightly over the crab. After capturing a sea urchin, an otter often employs a tactic we called "urchin rolling." To break off the urchin's spines before eating it, the otter, using the pads of both paws, rapidly spins the prickly sea urchin, like rolling balls of play dough. Sometimes the otter turns the urchin over and taps a spot on its bottom, causing the urchin to flatten its spines. The "sea star stick" is made when an otter folds a sea star's arms down and inward so the sticky tube feet adhere to each other. This otter can then concentrate on consuming the sea star without it constantly sticking to the otter's fur. This technique proves especially useful for eating a giant sunflower star (Pycnopodia helianthoides), which has up to 15 long, sticky arms. In "mollusk squishing," an otter that has captured a small clam or other mollusk rotates the shell and, at the same time, pushes it with its powerful forelegs. This push-and-twist motion usually pops open the clam. Another method of opening clams is called "siphon striping." Clams have two long tubes of flesh peeking out of the shell, called siphons, which suck in tiny food particles and push out waste. To open tightly clamped clam shells, some otters grab the siphon with their teeth and remove it with a quick jerk. One otter, Female 532, even came up with an unusual technique to eat abalone: she used a piece of abalone shell as a "spoon" to scoop out the abalone meat. A quick and easy way of collecting many turban snails at once, called the "snail shake," was a technique that Nosebuster and other snail-eaters use a lot. Twisting and wriggling just beneath the surface of the kelp canopy, the otter sends scores of snails falling off the kelp plant into its paws, like ripe fruit shaken from a tree. As the otter collects the snails, it stores them in a pouch of loose skin under its forelegs. Later the feeder will reach into its "armpit purse" and snack on the snails.
A few sea otters in Monterey Bay have actually learned to catch and eat seabirds--a very unusual behavior among all three subspecies of sea otters. We saw western grebes, sea gulls, surf scoters, common loons, and cormorants all fall victim to the stealthy bird-eaters. I couldn't help wondering if the bird-eating otters might have learned this practice by watching other otters, since most seabird attacks occurred in the same areas. We saw one male otter, who lived in Whaler's Cove at Point Lobos, eat at least six birds. Western grebes were his favorite, even though they are half as big as an otter and have long, dangerously sharp beaks. This resourceful male would sneak up on the grebe from underwater, grab the surprised bird's feet, and deliver a fatal bite to its neck or head. Although such behavior is anomalous, seabird foraging offers yet another example of how sea otters can learn new behaviors. The skills required to hunt, capture, subdue and consume a strong and sharp-billed grebe differ greatly from those needed to capture slowmoving or sessile invertebrates. Individual sea otters appear to forage more efficiently by specializing on prey. This fact may have contributed to the evolution of their distinctive diets, and the varied feeding patterns may also lessen competition for food in areas where sea otter populations are well established, such as around Monterey Bay. When California sea otters have lived in an area for many years, populations of preferred prey, such as large rock crabs, abalone, and red urchins, become less abundant and the diet of the otter population diversifies. Otters that are more efficient at processing specialized prey may have an adaptive advantage in finding enough food to meet their high energetic needs.
Marianne Riedman is a marine mammal biologist and writer who has recently published a children's book and audiocassette, The Adventures of Phokey the Sea Otter: Based on a True Story (Sequoyah Publishing, Capitola, California.) |
Fall 1996
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