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Sea Otter - Enhydra lutris
Sea Otter - Enhydra lutris

[Image: photo.jpg]

Scientific classification 
Kingdom: Animalia 
Phylum: Chordata 
Subphylum: Vertebrata 
Class: Mammalia 
Order: Carnivora 
Family: Mustelidae 
Subfamily: Lutrinae 
Genus: Enhydra 
Species: Enhydra lutris

The sea otter (Enhydra lutris) is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg (30 to 100 lb), making them the heaviest members of the weasel family, but among the smallest marine mammals. Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom. Although it can walk on land, the sea otter lives mostly in the ocean.

The first scientific description of the sea otter is contained in the field notes of Georg Steller from 1751, and the species was described by Linnaeus in his Systema Naturae of 1758. Originally named Lutra marina, it underwent numerous name changes before being accepted as Enhydra lutris in 1922. The generic name Enhydra, derives from the Ancient Greek en/εν "in" and hydra/ύδρα "water", meaning "in the water", and the Latin word lutris, meaning "otter". It was formerly sometimes referred to as the "sea beaver", although it is only distantly related to beavers. It is not to be confused with the marine otter, a rare otter species native to the southern west coast of South America. A number of other otter species, while predominantly living in fresh water, are commonly found in marine coastal habitats. The extinct sea mink of northeast North America is another mustelid that adapted to a marine environment.

Although it is a relatively new marine mammal lineage, the sea otter can live in the ocean at all stages of life.The sea otter is the heaviest member of the family Mustelidae, a diverse group that includes the thirteen otter species and terrestrial animals such as weasels, badgers, and minks. It is unique among the mustelids in not making dens or burrows, in having no functional anal scent glands, and in being able to live its entire life without leaving the water. The only member of the genus Enhydra, the sea otter is so different from other mustelid species that as recently as 1982, some scientists believed it was more closely related to the earless seals. Genetic analysis indicates that the sea otter and its closest extant relatives, which include the African speckle-throated otter, Eurasian otter, African clawless otter and oriental small-clawed otter, shared an ancestor approximately 5 million years ago (mya).
Fossil evidence indicates that the Enhydra lineage became isolated in the North Pacific approximately 2 mya, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter, Enhydra lutris. The sea otter evolved initially in northern Hokkaidō and Russia, then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast. In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50 mya, 40 mya, and 20 mya, respectively, the sea otter is a relative newcomer to a marine existence. In some respects, however, the sea otter is more fully aquatically adapted than pinnipeds, which must haul out on land or ice to give birth.

Population and distribution
The sea otter inhabits nearshore environments where it dives to the sea floor to forage. It preys mostly upon marine invertebrates such as sea urchins, various molluscs and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects. First, its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems. Its diet includes prey species that are also valued by humans as food, leading to conflicts between sea otters and fisheries.
Sea otters, whose numbers were once estimated at 150,000–300,000, were hunted extensively for their fur between 1741 and 1911, and the world population fell to 1,000–2,000 individuals in a fraction of their historic range. A subsequent international ban on hunting, conservation efforts, and reintroduction programs into previously populated areas have contributed to numbers rebounding, and the species now occupies about two-thirds of its former range. The recovery of the sea otter is considered an important success in marine conservation, although populations in the Aleutian Islands and California have recently declined or have plateaued at depressed levels. For these reasons (as well as its particular vulnerability to oil spills) the sea otter remains classified as an endangered species.

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There are three recognized subspecies, which vary in body size and in some skull and dental characteristics:
[blockquote]1. Common (Asian) Sea OtterEnhydra lutris lutris (Linnaeus, 1758), ranges from the Kuril Islands to the Commander Islands in the western Pacific Ocean. Also known as the Asian sea otter, it is the largest subspecies with a wide skull and short nasal bones. 
2. Southern (Californian) Sea OtterEnhydra lutris nereis (Merriam, 1904), is found off the coast of central California. Also known as the Californian sea otter, it has a narrower skull with a long rostrum and small teeth. 
3. Northern (Alaskan) Sea OtterEnhydra lutris kenyoni (Wilson, 1991), is native to Alaska and the Pacific west coast from the Aleutian islands to British Columbia, Washington, and northern Oregon . After being extirpated from southern British Columbia due to overhunting, it has since been re-introduced off Vancouver Island and the Olympic Peninsula. [/blockquote]
The reintroduction effort off the Oregon coast was not successful. However, reintroductions in 1969 and 1970 off the Washington coast were very successful and sea otters have been expanding their range since. They have now entered the Strait of Juan de Fuca and can be found almost as far east as Pillar Point. Individuals have even been seen in the San Juan Islands and northern Puget Sound.

Physical characteristics
A sea otter's thick fur makes its body appear much plumper on land than in the water.The sea otter is one of the smallest marine mammal species. Male sea otters weigh 22 to 45 kg (49 to 99 lb) and are 1.2 to 1.5 m (4 to 5 ft) in length. Females are smaller, weighing 14 to 33 kg (30 to 73 lb) and measuring 1.0 to 1.4 m (3 ft 3 in to 4 ft 7 in) in length.
Unlike other marine mammals, the sea otter has no blubber and relies on its exceptionally thick fur to keep warm. With up to 150,000 strands of hair per square centimeter (nearly one million per sq in), its fur is the most dense of any animal. The fur consists of long waterproof guard hairs and short underfur; the guard hairs keep the dense underfur layer dry. Cold water is thus kept completely away from the skin and heat loss is limited. The fur is thick year-round, as it is shed and replaced gradually rather than in a distinct molting season. As the ability of the guard hairs to repel water depends on utmost cleanliness, the sea otter has the ability to reach and groom the fur on any part of its body, taking advantage of its loose skin and an unusually supple skeleton. The coloration of the pelage is usually deep brown with silver-gray speckles, however it can range from yellowish or grayish brown to almost black. In adults, the head, throat, and chest are lighter in color than the rest of the body.
The sea otter displays numerous adaptations to its marine environment. The nostrils and small ears can close. The hind feet, which provide most of its propulsion in swimming, are long, broadly flattened, and fully webbed. The fifth digit on each hind foot is longest, facilitating swimming while on its back, but making walking difficult. The tail is fairly short, thick, slightly flattened, and muscular. The front paws are short with retractable claws, with tough pads on the palms that enable gripping slippery prey.
The hind flippers are larger than the mitten-like front paws.The sea otter propels itself underwater by moving the rear end of its body, including its tail and hind feet, up and down, and is capable of speeds of up to 9 km/h (5.6 mph). When underwater, its body is long and streamlined, with the short forelimbs pressed closely against the chest. When at the surface, it usually floats on its back and moves by sculling its feet and tail from side to side. At rest, all four limbs can be folded onto the torso to conserve heat, whereas on particularly hot days the hind feet may be held underwater for cooling. The sea otter's body is highly buoyant because of its large lung capacity – about 2.5 times greater than that of similar-sized land mammals – and the air trapped in its fur. The sea otter walks with a clumsy rolling gait on land, and can run in a bounding motion.
Long, highly sensitive whiskers and front paws help the sea otter find prey by touch when waters are dark or murky. Researchers have noted that when they approach in plain view, sea otters react more rapidly when the wind is blowing towards the animals, indicating that the sense of smell is more important than sight as a warning sense. Other observations indicate that the sea otter's sense of sight is useful above and below the water, although not as good as that of seals. Its hearing is neither particularly acute nor poor.
An adult's 32 teeth, particularly the molars, are flattened and rounded, designed to crush rather than cut food. Seals and sea otters are the only carnivores with two pairs of lower incisor teeth rather than three; the adult dental formula is:


The sea otter has a metabolic rate two or three times that of comparatively sized terrestrial mammals. It must eat an estimated 25 to 38% of its own body weight in food each day in order to burn the calories necessary to counteract the loss of heat due to the cold water environment. Its digestive efficiency is estimated at 80 to 85%, and food is digested and passed in as little as three hours. Most of its need for water is met through food, although, in contrast to most other marine mammals, it also drinks seawater. Its relatively large kidneys enable it to derive fresh water from sea water and excrete concentrated urine.

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Sensitive whiskers and forepaws enable sea otters to find prey using their sense of touch.The sea otter is diurnal. It has a period of foraging and eating in the morning, starting about an hour before sunrise, then rests or sleeps in mid-day. Foraging resumes for a few hours in the afternoon and subsides before sunset, and there may be a third foraging period around midnight. Females with pups appear to be more inclined to feed at night. Observations of the amount of time a sea otter must spend each day foraging range from 24 to 60%, apparently depending on the availability of food in the area.
The sea otter spends much of its time grooming, which consists of cleaning the fur, untangling knots, removing loose fur, rubbing the fur to squeeze out water and introduce air, and blowing air into the fur. To an observer it appears as if the animal is scratching, however sea otters are not known to have lice or other parasites in the fur. When eating, the sea otter rolls in the water frequently, apparently to wash food scraps from its fur.

The sea otter hunts in short dives, often to the sea floor. Although it can hold its breath for up to five minutes, its dives typically last about one minute and no more than four. It is the only marine animal capable of lifting and turning over boulders, which it often does with its front paws when searching for prey. The sea otter may also pluck snails and other organisms from kelp and dig deep into underwater mud for clams. It is the only marine mammal that catches fish with its forepaws rather than with its teeth.
Under each foreleg, the sea otter has a loose pouch of skin that extends across the chest. In this pouch (preferentially the left one), the animal stores collected food to bring to the surface. There, the sea otter eats while floating on its back, using its forepaws to tear food apart and bring it to its mouth. It can chew and swallow small mussels with their shells, whereas large mussel shells may be twisted apart. It uses its lower incisor teeth to access the meat in shellfish. To eat large sea urchins, which are mostly covered with spines, the sea otter bites through the underside where the spines are shortest, and licks the soft contents out of the urchin's shell.
The sea otter's use of rocks when hunting and feeding makes it one of the few mammal species to use tools. To open hard shells, it may pound its prey with both paws against a rock on its chest. To pry an abalone off its rock, it hammers the abalone shell using a large stone, with observed rates of 45 blows in 15 seconds. Releasing an abalone, which can cling to rock with a force equal to 4,000 times its own body weight, requires multiple dives.

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Sea otters consume over 100 different prey species. In most of its range, the sea otter's diet consists almost exclusively of marine invertebrates, including sea urchins, a variety of bivalves such as clams and mussels, abalone, other mollusks, crustaceans, and snails. Its prey ranges in size from tiny limpets crabs to giant octopuses. Where prey such as sea urchins, clams, and abalone are present in a range of sizes, sea otters tend to select larger items over smaller ones of similar type. In California, it has been noted that sea otters ignore Pismo clams smaller than 3 inches (7 cm) across.
In a few northern areas, fish are also eaten. In studies performed at Amchitka Island in the 1960s, where the sea otter population was at carrying capacity, 50% of food found in sea otter stomachs was fish. The fish species were usually bottom-dwelling and sedentary or sluggish forms, such as Hemilepidotus hemilepidotus and family Tetraodontidae. However, south of Alaska on the North American coast, fish are a negligible or extremely minor part of the sea otter's diet. Contrary to popular depictions, sea otters rarely eat starfish, and any kelp that is consumed apparently passes through the sea otter's system undigested.
The individuals within a particular area often differ in their foraging methods and their prey types, and tend to follow the same patterns as their mothers. The diet of local populations also changes over time, as sea otters can significantly deplete populations of highly preferred prey such as large sea urchins, and prey availability is also affected by other factors such as fishing by humans. Sea otters can thoroughly remove abalone from an area except for specimens in deep rock crevices, however, they never completely wipe out a prey species from an area. A 2007 California study demonstrated that in areas where food was relatively scarce, a wider variety of prey was consumed. However, surprisingly, the diets of individuals were more specialized in these areas than in areas where food was plentiful.

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Social structure
Although each adult and independent juvenile forages alone, sea otters tend to rest together in single-sex groups called rafts. A raft typically contains 10 to 100 animals, with male rafts being larger than female ones. The largest raft ever seen contained over 2000 sea otters. To keep from drifting out to sea when resting and eating, sea otters may wrap themselves in kelp.
A male sea otter is most likely to mate if he maintains a breeding territory in an area that is also favored by females. As autumn is the peak breeding season in most areas, males typically defend their territory only from spring to autumn. During this time, males patrol the boundaries of their territories to exclude other males, although actual fighting is rare. Adult females move freely between male territories, where they outnumber adult males by an average of five to one. Males who do not have territories tend to congregate in large male-only groups, and swim through female areas when searching for a mate.
The species exhibits a variety of vocal behaviors. The cry of a pup is often compared to that of a seagull. Females coo when they are apparently content; males may grunt instead. Distressed or frightened adults may whistle, hiss, or in extreme circumstances, scream.
Although sea otters can be playful and sociable, they are not considered to be truly social animals. They spend much time alone, and each adult can meet its own needs in terms of hunting, grooming, and defense.

Reproduction and lifecycle
Sea otters are polygynous: males have multiple female partners. However, temporary pair-bonding occurs for a few days between a female in estrus and her mate. Mating takes place in the water and can be rough, the male biting the female on the muzzle – which often leaves scars on the nose – and sometimes holding her head under water.
Births occur year-round, with peaks between May and June in northern populations and between January and March in southern populations. Gestation appears to vary from four to twelve months, as the species is capable of delayed implantation followed by four months of pregnancy. In California, sea otters usually breed every year, about twice as often as sea otters in Alaska.
Birth usually takes place in the water and typically produces a single pup weighing 1.4 to 2.3 kg (3 to 5 lb). Twins occur in 2% of births; however, usually only one pup survives. At birth, the eyes are open, ten teeth are visible, and the pup has a thick coat of baby fur. Mothers have been observed to lick and fluff a newborn for hours; after grooming, the pup's fur retains so much air that the pup floats like a cork and cannot dive. The fluffy baby fur is replaced by adult fur after about thirteen weeks.
Nursing lasts six to eight months in California populations and four to twelve months in Alaska, with the mother beginning to offer bits of prey at one to two months. The milk from a sea otter's two abdominal nipples is rich in fat and more similar to the milk of other marine mammals than to that of other mustelids. A pup, with guidance from its mother, practices swimming and diving for several weeks before it is able to reach the sea floor. Initially the objects it retrieves are of little food value, such as brightly colored starfish and pebbles. Juveniles are typically independent at six to eight months, however a mother may be forced to abandon a pup if she cannot find enough food for it and at the other extreme, a pup may nurse until it is almost adult size. Pup mortality is high, particularly during an individual's first winter – by one estimate, only 25% of pups survive their first year. Pups born to experienced mothers have the highest survival rates.
Females perform all tasks of feeding and raising offspring, and have occasionally been observed caring for orphaned pups. Much has been written about the level of devotion of sea otter mothers for their pups – a mother gives her infant almost constant attention, cradling it on her chest away from the cold water and attentively grooming its fur. When foraging, she leaves her pup floating on the water, sometimes wrapped in kelp to keep it from floating away; if the pup is not sleeping, it cries loudly until she returns. Mothers have been known to carry their pup for days after the pup's death.

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Females become sexually mature at around three or four years of age and males at around five; however, males often do not successfully breed until a few years later. A captive male sired offspring at age 19.] In the wild, sea otters live to a maximum age of 23 years, with average lifespans of 10–15 years for males and 15–20 years for females. Several captive individuals have lived past 20 years, and a female at the Seattle Aquarium died at the age of 28 years. Sea otters in the wild often develop worn teeth, which may account for their apparently shorter lifespans.

As a keystone species
Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. Sea otters keep the population of certain benthic (sea floor) herbivores, particularly sea urchins, in check. Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. North Pacific areas that do not have sea otters often turn into urchin barrens, with abundant sea urchins and no kelp forest. 
Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems, and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the Big Sur coast of California. However, some kelp forest ecosystems in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors. The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and estuaries.
In addition to promoting growth of kelp forests, sea otters can also have a profound effect in rocky areas that tend to be dominated by mussel beds. They remove mussels from rocks, liberating space for competitive species and thereby increasing the diversity of species in the area.

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Predators of sea otters include orcas and sea lions; bald eagles also prey on pups by snatching them from the water surface. In California, bites from sharks, particularly great white sharks, have been estimated to cause 10% of sea otter deaths and are one of the reasons the population has not expanded further north. Dead sea otters have been found with injuries from shark bites, although there is no evidence that sharks actually eat them.
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  • Claudiu Constantin Nicolaescu
Otters fight climate change by keeping kelp forests lush

Santa Cruz Sentinel
Posted: 09/17/2012 08:23:36 PM PDT
Updated: 09/18/2012 09:31:22 AM PDT

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A raft of otters floats off of Moss Landing. Two UC Santa Cruz researchers have published a paper showing that the otter's positive impact on the environment extends beyond the ocean. (DAVID ROYAL/Herald file)

Monterey Bay's main mascot may turn out to be a secret agent in the fight against climate change.

It has long been known that sea otters, nursed back from brink of extinction in the past several decades, provide huge benefits for the vitality of undersea kelp forests. But a pair of University of California-Santa Cruz scientists recently found that those benefits extend into the atmosphere, finding a strong connection between otters, kelp and global warming.

"We just looked at the question, 'Does it matter?'" said James Estes, a UCSC biologist. "And the answer was yes."

In a paper published in the October issue of Frontiers in Ecology and the Environment, Estes and UCSC environmental studies professor Chris Wilmers found that if otters covered the globe, the resulting growth in kelp forests would strip 10 percent of the carbon dioxide from the atmosphere.

"We discovered that in a world with otters versus a world without otters, the effect was significant," Estes said.

To put that in context, carbon dioxide levels have gone up 40percent since the Industrial Revolution. Otters would solve a quarter of that problem.

The actual impact is significantly lower, isolated to regions where otters are found. Sea otters range along the West Coast of North America, with the number off the coast of California estimated at just more than 2,700.

The broader lesson of the paper is how the species can impact an ecosystem, potentially playing an important role mitigating climate change.

"You don't think of predators walking around influencing the carbon cycle," Wilmers said.

The authors went so far as to estimate sea otters' value in term of carbon credits, noting their impact would be worth between $205 million and $408 million on the European Carbon Exchange.

The process begins with one of otters' favorite snacks: sea urchins.

The word "urchin" has a negative connotation for a reason. They can be ravenous creatures, moving in packs across the ocean floor and destroying as much as 30 feet of kelp forest a month.

A key limitation on their population is otters, with the relationship between otters and healthy kelp forests having long been recognized — more otters mean more kelp.

Similar to terrestrial forests, kelp removes carbon from the environment, and is being studied for its environment impact. Scientists are building an underwater platform called the Kelp Forest Array near Hopkins Marine Station in Pacific Grove, partly to study climate change and human impacts on marine ecosystems.

Studies also have shown that domesticated animals can hurt the environment. Methane from cows, for example, has been shown to be similar to human-linked carbon emissions as a threat to the ozone layer. 
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  • Claudiu Constantin Nicolaescu
Sea otter return boosts ailing seagrass in California

By Suzi Gage
BBC News
26 August 2013 Last updated at 18:40 GMT

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A sea otter enjoys a crab in California, and helps seagrass in the process

The return of sea otters to an estuary on the central Californian coast has significantly improved the health of seagrass, new research has found.

Seagrass was deemed to be heading for extinction in this region before the otters returned.

But scientists found that the animals triggered a chain reaction of events that boosted the water-dwelling plants.

The research is published in the journal, PNAS.

The urbanisation of California has led to a huge increase in nutrient pollution in coastal waters, from increasing use of nitrogen-rich fertilizers.

This is said to be the reason for the dieback of seagrass, which has also been declining worldwide.

This research suggests that the hunting to near-extinction of sea otters in the late 19th and early 20th Century may have exacerbated the problem, and conversely that their reintroduction is helping revive ailing seagrass populations, even in the face of hugely nutrient-rich water.

Links in the chain

The researchers assessed seagrass levels over the past 50 years in the Elkhorn Slough in Monterey Bay, and mapped their increases and declines.

They looked at a variety of changes that may have affected the grass, but the only factor that really matched the changes in seagrass was sea otter numbers.

They theorised that sea otters were eating the crabs which prey upon small invertebrates in the water.

These invertebrates eat a type of algae which blooms when there are more nutrients in the soil. It grows on the leaves of the seagrass, shading them from sunlight and causing them to die back.

This is quite a complex cascade of effects, so the researchers tested out their theory by comparing similar estuaries with and without sea otters, and by doing experiments in the lab, and in the field.

These experiments, which included putting cages that sea otters either could or couldn't access, down on the seagrass, confirmed their hypothesis.

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Sea otters have been responsible for improving the health of the seagrass in these estuaries.

Brent Hughes, lead author of the study, said: "This estuary is part of one of the most polluted systems in the entire world, but you can still get this healthy thriving habitat, and it's all because of the sea otters.

"So it's almost like these sea otters are fighting the effects of poor water quality."

Hughes described seagrass as "the canary in the coalmine" in terms of predicting levels of nutrient pollution in the water.

Foundation species

It also acts as a nursery habitat for many species of fish and it uses CO2 from sea water and the atmosphere, thus potentially helping with climate change.

Not only that, but it acts as protection to the stability of the shoreline.

Hughes said: "It's what we call a foundation species, like kelp forest, salt marsh or coral reef. The major problem from a global perspective is that seagrass is declining worldwide. And one of the major drivers of this decline has been nutrient inputs from anthropogenic sources, via agriculture or urban runoff."

These findings are of particular interest at the moment, as a ban on sea otters moving along the coast to southern California was lifted last year. The ban was in place as there was a fear the sea otters would impinge on fisheries in the area.

Hughes told BBC news: "That's important because there's a lot of these kind of degraded estuaries in southern California because of all the urban runoff from places like Los Angeles and San Diego.

"Coastal managers will now have a better sense of what's going to happen when sea otters move in to their systems.

"There's a huge potential benefit to sea otters returning to these estuaries, and in to these seagrass beds that might be threatened."
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  • Claudiu Constantin Nicolaescu
Vodmeister Wrote:Sea Otters are renowned predators of the Giant Pacific Octopus.

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Octopus: The Ocean's Intelligent Invertebrate - Roland C. Anderson, Jennifer A. Mather, James B. Wood
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For Otter Moms, Nursing Burnout Is Sometimes Deadly

By Mindy Weisberger, Senior Writer | August 4, 2016 12:45pm ET

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Cute baby otters can suck the life right out of their mothers.
Credit: Joe Tomoleoni

What scene could be more tranquil than that of a sea otter mother cradling her nursing pup? But there's a darker side to this heartwarming tableau. Suckling a baby comes with a high metabolic cost, one that some female otters' bodies just can't meet — and the experience can be fatal.

Scientists knew that mortality rates are unusually high in female southern sea otters that have just finished lactating, but researchers had yet to pinpoint the cause.

However, a new study offers the first evidence of what's happening in the bodies of female otters as they nurse their young, helping scientists to understand how the animals' metabolisms kick into overdrive to handle the demands of lactation — and why they sometimes burn out. 

"This had been a big question within the marine mammal scientific community for many years," said study co-author Nicole Thometz, a postdoctoral researcher with the Department of Ecology and Evolutionary Biology at the University of California, Santa Cruz.

Thometz told Live Science in an email that in an earlier study, she and her colleagues had evaluated the energy demands of sea otter pups at the Sea Otter Research and Conservation (SORAC) program at the Monterey Bay Aquarium in California. As the otters grew from pups to juveniles, the scientists calculated how many calories a female otter would require to keep up with the youngsters' needs, estimating that she would need about twice as many as a nonlactating female.

But Thometz said a crucial piece of the puzzle was missing: metabolic analysis of a lactating female. Without that critical data, scientists had no way of knowing how a female otter's body responds to lactation, and whether that challenge might be even greater than they suspected.

Meanwhile, the scientists faced a challenge of their own; breeding otters in captivity is illegal, so the researchers had no way of collecting the missing data. The unexpected arrival of a pair of young female sea otters, one of whom was pregnant, at the Monterey Bay Aquarium handed the researchers exactly the opportunity they needed.

The two otters were moved to a lab at Santa Cruz, where the researchers used a special chamber to measure the animals' oxygen intake.

The scientists monitored the pregnant otter, nicknamed Clara, before the pup's birth, while she nursed it, and after the pup was weaned. They found that over four months of producing milk for the pup, Clara's energy requirements doubled, with her resting metabolic rate increasing by 51 percent.

"Due to high lactation costs, we found the cost of pup rearing to be significantly higher than previously estimated — approximately twice nonreproductive levels for the last three months of lactation," Thometz told Live Science. 

For a species whose normal energy demands are already high and whose energy reserves are typically low, this represents a significant energy burden, she explained.

"This is likely one of the underlying reasons why we are seeing high mortality rates for prime-age females at the end of lactation," Thometz said, particularly in areas where sea otter populations are more numerous and there is limited access to prey.

The findings were published online Aug. 3 in the Journal of Experimental Biology.

Journal Reference:
Nicole M. Thometz, Traci L. Kendall, Beau P. Richter, Terrie M. Williams The high cost of reproduction in sea otters necessitates unique physiological adaptations Journal of Experimental Biology 2016 219: 2260-2264; doi: 10.1242/jeb.138891

Superimposed on inherently high basal metabolic demands, the additional energetic requirements of reproduction can push female sea otters beyond physiological limits. Indeed, the resulting energy imbalance contributes to disproportionately high rates of mortality at the end of lactation in this species. To examine and quantify metabolic changes associated with reproduction, we measured the resting metabolic rate (RMR) of a female sea otter across gestation, lactation and non-reproductive periods. Concurrently, measurements were made on a non-breeding control female. Our results suggest that RMR declines during gestation. Conversely, RMR increases during lactation, reaches a peak at 3–4 months postpartum, and remains elevated until weaning. Combining these direct measurements with published data, we found the cost of pup rearing to be significantly higher than previously estimated. High baseline energy demands and limited energy reserves, combined with significant lactation and pup rearing costs, appear to necessitate metabolic and thermal lability during key reproductive stages. 
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  • Claudiu Constantin Nicolaescu
Robust jaws and crushing bites allow sea otters to specialize their diets

October 25, 2017 by Kristin Campbell

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Credit: Robert Shea

A sea otter's survival depends on their ability to catch and eat prey. Unlike most marine mammals, sea otters lack a thick layer of blubber to insulate them from the frigid waters of the Pacific Ocean. Instead, they rely on dense fur and a very active metabolism to keep warm.
Sea otters have a diverse diet, eating anything from clams to sea urchins to crabs. However, while sea otter diets are diverse at a population level, individual sea otters can have highly specialized diets.
This is especially true for the subspecies of sea otter living in the waters of southern California. Known as the southern sea otter, this subspecies lives in rocky environments where marine invertebrates are plentiful.
As the number of sea otters in a particular area grows, each sea otter must compete for food with its neighbor. To overcome this, individual southern sea otters start to specialize by eating specific prey species, alleviating the competition. In this way, southern sea otters act as diet specialists.
However, this behavior is not observed in northern sea otters, the subspecies that lives in Washington, British Columbia, and Alaska.
In these mixed-sediment marine environments, sea otters quickly deplete their favorite prey, like sea urchins, and are left to forage mostly on bivalves, such as clams and mussels. Therefore, in contrast to southern sea otters, northern sea otters act as generalists.

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Articulated sea otter skeleton in the Burke Museum mammalogy collection. Credit: Burke Museum

Why are northern and southern sea otters behaving differently?
While previous research has found that habitat type and the diversity and abundance of prey influence this behavior, little is known about whether a sea otter's anatomy and feeding performance plays a role in enhancing or restricting their ability to specialize on certain prey types.
Specialized behaviors, like feeding on specific food items, are often associated with specialized morphologies, like the size and shape of an animal's body or its skull.
Working with Dr. Sharlene Santana, curator of mammals at the Burke Museum, I set out to investigate whether differences in food-resource use in sea otters can be explained by differences in the size and shape of their skull and their ability to efficiently bite into prey.
I used more than 100 sea otter skulls from the Burke Museum's Mammalogy collection to create mathematical models that allowed us to compare bite force, jaw strength, and skull size and shape differences between northern and southern sea otters.

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Kristin Campbell holds a sea otter skull in the Burke Museum mammalogy collection. Credit: Burke Museum

I photographed each skull (both the cranium and mandible) in three different views. Then, using these photographs, I estimated the size of the muscles used during biting and chewing to estimate the bite force of each individual sea otter.
To further investigate biting ability, I used digital calipers to measure specific dimensions of the lower jaw, which allowed me to estimate how strong and robust this bone is. Lastly, I used the skull photographs to compare the size and shape of sea otter skulls using a method known as geometric morphometrics.
Sea otters have highly specialized skulls and teeth that allow them to pierce through the tough exoskeletons of sea urchins, and pry open clams. We found that their short, blunt skulls and flat, fracture-resistant teeth allow them to generate high bite forces for their body size (nearly 80lbs of force!). These high bite forces allow them to eat otherwise hard-to-crack marine invertebrates.
Comparing the skulls of the two subspecies, we found subtle differences in skull size and shape between the northern and southern sea otters. However, these differences do not translate to significant differences in bite force.
This means that each subspecies has the potential to become a specialist or remain a generalist depending on prey availability, habitat characteristics, and competition. Southern sea otters specialize their diets, however, when conditions require.

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Using computer software, digital landmarks are placed on each sea otter skull at distinct locations. These landmarks are used to compare skull shape and size in a process known as geometric morphometrics. Credit: Kristin Campbell

With these results in mind, we decided to look even deeper at potential differences within populations; that is, between male and female sea otters.
Male sea otters are larger than female sea otters; males on average weigh 90lbs while females weigh 40-60lbs. Therefore, we expected to also see this trend in skull size—and we did!
However male sea otters not only have larger skulls than females, but also have skull features that would allow them to have more forceful bites for their size, including larger crests and ridges that provide greater surface area for muscle attachment (more muscle equals stronger bites!). In addition, male sea otters have stronger, more robust lower jaws that allow them to crush the toughest of prey.
Our results suggest that male sea otters have the potential to consume harder, tougher prey than females just by virtue of their skull size and shape.
But do these differences between males and females give males the upper hand when feeding? Not necessarily. What females lack in size, they make up for with brains!

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The landmarks are turned into x-y coordinates and analyzed to quantify and visualize shape differences between sea otter skulls. Credit: Kristin Campbell

Female sea otters are more prone than males to use tools, like rocks and stones, to crack open tougher-than-usual prey, like clams. Some males also do this behavior, but their mothers likely taught them how to do it.
It is possible that males are larger and generate more powerful bites due to sexual selection, and not to have an advantage over females during feeding. Males fight vigorously with each other for territory and reproductive opportunities with females, and the biggest and strongest sea otter generally wins.
Sea otters are keystone species in coastal ecosystems, but sadly they are also endangered. A better understanding of their adaptations is critical for their conservation.
By using museum specimens, we were able to identify anatomical performance differences between and within sea otter populations. We have gained valuable insight into how their ability to produce forceful bites might allow them to act both as generalists or specialists depending on environmental conditions.
The study, "Do differences in skull morphology and bite performance explain dietary specialization in sea otters?" is published in the Journal of Mammalogy.

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Sea otters have flat, fracture-resistant teeth and strong canines to crush and pry open hard-bodied marine invertebrates like crabs, urchins, and clams. Credit: Burke Museum

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Male sea otter skulls are overall larger and more robust than females. Notice the large crests and ridges of the male skull. These features provide more area for muscle attachment and, in-turn, greater biting ability. Credit: Burke Museum

Journal Reference:
Kristin M Campbell et al. Do differences in skull morphology and bite performance explain dietary specialization in sea otters?, Journal of Mammalogy (2017). DOI: 10.1093/jmammal/gyx091

Intraspecific studies of morphology and performance are essential for understanding the factors that enable resource partitioning within ecological communities. The sea otter (Enhydra lutris) is one of the few mammal species in which individual-level dietary specialization has been documented, making them an ideal system to investigate the morphological basis of food resource partitioning. Here, we test if differences in food resource use within and between sea otter subspecies can be explained by differences in ecologically relevant metrics of bite performance that are mainly the product of variation in size, cranial morphology, or a combination of these traits. We use geometric morphometrics to evaluate variation in cranium size and morphology, and 2-dimensional models to estimate bite performance differences between 2 sea otter subspecies that differ in dietary ecology: the northern sea otter (Enhydra lutris kenyoni, a facultative generalist) and the southern sea otter (E. l. nereis, a specialist). We found significant differences in cranium shape and size between subspecies and between male and female sea otters. These differences were subtle yet consistent with most subspecies classifications and known sexually dimorphic traits. Cranial morphological differences did not translate into differences in estimated bite force between subspecies or sexes, but dentary strength differed significantly between male and female sea otters. Sea otters have short, blunt crania with pronounced sagittal and lambdoidal crests, and strong mandibles. These traits combine to produce high bite forces for their size. We propose that high bite performance capacity in sea otters enables resource-use variation by widening the diversity of available food resources they can procure from their environment; this allows them to behave as either generalists or specialists within different habitats.
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  • Claudiu Constantin Nicolaescu
Vivyx Wrote:Mentions of predation on sea otters by salmon sharks. Always thought they were solely fish and squid eaters:

At least two accounts of salmon sharks taking sea otters were reported in Prince William Sound, Alaska. In one account the 300–400 pound (136–181 kg) shark attacked a female otter as her pup swam nearby. The shark grabbed the sea otter and shook, tearing the otter to pieces only ten feet (3 m) from a fishing boat. Otter blood was splattered across the boat’s stern and onto the fishermen. The pup sea otter escaped, but likely died of starvation or was preyed upon by bald eagles, which are on the lookout for unattended young otters. I'm still not convinced these were not great white sharks taking the sea otters.

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Sea otters' tool use leaves behind distinctive archaeological evidence

March 14, 2019, Max Planck Society

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Wild sea otter at Bennett Slough Culverts opening mussels using emergent anvil stone. Credit: Monterey Bay Aquarium, Jessica Fujii. Haslam et al. 2019. Wild sea otter mussel pounding leaves archaeological traces. Scientific Reports.

An international team of researchers has analyzed the use by sea otters of large, shoreline rocks as "anvils" to break open shells, as well as the resulting shell middens. The researchers used ecological and archaeological approaches to identify patterns that are characteristic of sea otter use of such locations. By looking at evidence of past anvil stone use, scientists could better understand sea otter habitat use.

Sea otters are an especially captivating marine mammal, well known for their use of rocks to break open shells. Sea otters are estimated to have once numbered between 150,000-300,000 individuals and their range stretched from Baja California, Mexico, around the northern Pacific Rim to Japan. Their numbers were dramatically reduced by the fur trade. In California, the southern sea otter population was reduced to around 50 individuals, but a massive conservation effort has resulted in increasing their numbers to around 3000 today. However, the southern sea otter is still considered threatened.

Sea otters are unique for being the only marine mammal to use stone tools. They often use rocks to crack open shells while floating on their back, and also sometimes use stationary rocks along the shoreline as "anvils" to crack open mollusks, particularly mussels. A joint project including the Max Planck Institute for the Science of Human History, the Monterey Bay Aquarium and the University of California, Santa Cruz, among others, has resulted in a first-of-its-kind interdisciplinary study published in Scientific Reports, combining ten years of observations of sea otters with archaeological methods to analyze sea otter use of such anvil stones, also known as emergent anvils.

Researchers spent ten years between 2007-2017 observing sea otters consuming mussels at the Bennett Slough Culverts site in California. Their analysis identified that mussels were the most common prey eaten at the site and were the only prey for which the sea otters used stationary anvil stones. The sea otters used such stones for about 20% of the mussels they consumed.

Interestingly, careful analysis of the stationary anvil stones using archaeological methods showed that their use resulted in a recognizable damage pattern that was distinguishable from what would be caused by human use. For example, the sea otters preferentially struck the mussels against points and ridges on the rocks, and struck the rocks from a position in the water, rather than from the land or from on top of the rock.

In addition to the stones themselves, the researchers also carefully analyzed the mussel shells left around the stationary anvils. The researchers took a random sample of the shell fragments from these shell middens, which likely contained as many as 132,000 individual mussel shells. They found an extremely consistent damage pattern, with the two sides of the mussel shell still attached, but a diagonal fracture running through the right side of the shell.

"The shell breakage patterns provide a novel way to distinguish mussels broken by sea otter pounding on emergent anvils from those broken by humans or other animals," explains Natalie Uomini of the Max Planck Institute for the Science of Human History. "For archaeologists who excavate past human behavior, it is crucial to be able to distinguish the evidence of sea otter food consumption from that of humans."

In combination with analysis of videos they took of the otters using the anvils, researchers could see that the otters held the shells evenly in both paws, but when striking the shell against the anvil tended to have their right paw slightly on top. Though the total number of otters observed was small, these results suggest that otters may exhibit handedness, or "pawedness," as do humans and many other mammals.

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Mussel shell breakage patterns at the Bennett Slough Culverts site. (A) Outer and (B) inner faces of each valve; © drawing of the exterior of a mussel shell showing the typical sea otter breakage pattern (illustration by Neil Smith); (D) broken mussel shells in situ. Credit: Image ©: Neil Smith; Images (A), (B), & (D): Michael Haslam. Haslam et al. 2019. Wild sea otter mussel pounding leaves archaeological traces. Scientific Reports.

The researchers hope that the study will be useful for archaeologists working with coastal populations, as a way to distinguish between human and sea otter use of rocks and consumption of marine resources. Additionally, the research could be helpful in future studies of the geographic spread of stationary anvil use throughout the former sea otter range, and how far into the past this behavior extends.

"Our study suggests that stationary anvil use can be detected in locations previously inhabited by sea otters. This information could help to document past sea otter presence and diet in locations where they are currently extirpated," explains Jessica Fujii of the Monterey Bay Aquarium.

"More broadly," she adds, "the recovery of past animal behavioral traces helps us to understand the evolution of behaviors like stone anvil use, which is rare in the animal kingdom and is extremely rare in marine animals. We hope that this study establishes a new path for the growing field of animal archaeology."

Journal Reference:
Michael Haslam, Jessica Fujii, Sarah Espinosa, Karl Mayer, Katherine Ralls, M. TimTinker, & Natalie Uomini. Wild sea otter mussel pounding leaves archaeological traces, Scientific Reports (2019). DOI: 10.1038/s41598-019-39902-y

Wild sea otters (Enhydra lutris) are the only marine mammals that habitually use stones while foraging, using them to break open hard-shelled foods like marine snails and bivalves. However, the physical effects of this behavior on local environments are unknown. We show that sea otters pounding mussels on tidally emergent rocks leave distinct material traces, which can be recognized using methods from archaeology. We observed sea otters pounding mussels at the Bennett Slough Culverts site, California, USA, over a l0-year period. Sea otters repeatedly used the same rocks as anvils, which resulted in distinctive wear patterns on the rocks and accumulations of broken mussel shells, all fractured in a characteristic way, below them. Our results raise the potential for discovery of similar sea otter pounding sites in areas that no longer have resident sea otter populations.
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