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biology of the southern sea otter

Otter skeleton sketch based on 1947 illustration

 

Description

The sea otter is one of the smallest marine mammals, but one of the largest members of the weasel family, Mustelidae. Adult sea otters standard length is on average 4 ft. 3 in. (1.29m) for males and 3 ft. 8 in. (1.12m) for females. Adult males weigh on average 64 lbs. (29 kg), while adult females weigh 44 lbs. (19.8 kg). . Females otters nature at 3 years of age and males at age 5. Southern sea otters can live to approximately 15 years for males and 16 years for females.

Unlike other marine mammals, sea otters do not have a thick layer of blubber and rely upon their water-resistant fur for insulation. The sea otter is covered in dense fur that consists of two layers. The short, more dense, under layer of fur can have as many as 1 million hairs per square inch, which is the densest of any mammal. Humans only have about 100,000 hairs in total on our heads. A top layer of fur is long, waterproof hairs that keep the layer of underfur dry which helps with thermoregulation of the otter’s body temperature by keeping cold water away from their skin. Sea otters have a specialized gland that secrets oil to enhance the water-repellent quality of the fur.

Sea otter fur color varies in shades of brown turning silver-gray to white with age in males. Molting of their fur takes place gradually throughout the year and frequent grooming is essential to maintain its insulative properties. Their fur is sensitive to soiling from oil or other contaminants and soiling of the fur by oil generally results in hypothermia or pneumonia and eventually death.

 

Habitat and Range

Southern sea otters inhabit shallow nearshore coastal ecosystems within 1-2 km from shore. They are usually seen in rocky marine habitats where there is a high abundance of kelp canopy and typically in water depths about 20 m. Kelp canopy is an important habitat component used for resting and foraging. They are also seen in areas of expansive open soft sandy bottom habitat and coastal wetland waterways such as Elkhorn Slough. Occasionally, sea otters are seen further offshore in water depths of between 40 and 60 meters and on rare occasions in deeper offshore areas where there is an abundance of food.

From evidence found in the fossil record, sea otters and their ancestors have been a component of California’s coastal marine environment for the past five million years. Historically, southern sea otters have inhabited nearshore coastal ecosystem from northern California to Baja California in Mexico. The current sea otter range extends from north of Half Moon Bay in San Mateo county south along the California coast to Point Conception in Santa Barbara county. There is a small population that lives south of Point Conception in the “No-Otter” management zone and a small “experimental” population of translocated lives at San Nicolas Island. Males otters are often seen hundreds of miles north and south of the current home range, recently near San Diego, in search of new territory.

 

Diet and Foraging

Sea otters feed on a wide variety of marine invertebrates including clams, mussels, sea urchins, marine snails, crabs, and abalone. The sea otter’s teeth are adapted for crushing these prey items with their molars being broad and flat and canines rounded and blunt. Sea otters in general have a varied diet but research has shown that individuals tend to specialize on a few main prey species and this specialization in prey preference is passed on from mother to pup.

Sea otters have a very high metabolic rate and must consume the equivalent of 20 to 30 % of their body weight per day to maintain their body heat. Sea otters generally forage in nearshore waters shallower than 60 feet (18 m) but have been recorded diving to 300 feet (90 m) or more. Sea otter use a variety of strategies for finding their food. They have long whiskers and sensitive forepaws, with retractable claws, which help them to detect and capture prey underwater.

The sea otter is one of the few animals to use tools. Sea otters use rocks, other shellfish, or man-made objects to pry prey from rocks and use them as a hammer or anvil to break open the hard outer shell of some prey species. They also have pockets of loose skin under each foreleg for storing their tools and to hold prey it has found on the seafloor. Sea otters float on their back and break open their prey by pounding it on the rock or other tool on their chest.

 

Reproduction and pups

Male sea otters have multiple female partners and are territorial during the breeding season from September to November. Males are aggressive toward females and often having bloody noses or scaring from being bitten during copulation

Female otters give birth to a single pup nearly every year with peak pupping season from January through March. There is also a secondary pupping peak in late summer to early fall which may be associated with pup mortality during the winter months. The gestation period is about 6 months.

When pups are born they are about 2 feet (0.6 m) long and weigh 4-5 lbs. (2-2.3 kg). They have a thick coat of long black or brown fur and are so buoyant that they cannot dive underwater. Pups are constantly groomed while resting upon their mother’s chest for the first 2 months. Pups are often seen bobbing in the water or wrapped in kelp screeching for their mothers when they dive to forage for food. Pups begin diving for food at 2 months. They typically nurse for 6 to 12 months and are then on their own. Once a pup is weaned a female will soon mate again.

 

Ecosystem function

Sea otters are the textbook example of a “keystone” species and have a considerable impact on the structure and complexity of their nearshore ecological community. Sea otters increase biodiversity and primary productivity of the nearshore environment by controlling populations of grazers, sea urchins in particular, which feed on kelp.

California kelp forests have some of the highest primary productivity rates (conversion of sunlight to living material) of any naturally occurring ecosystem on earth and are the most diverse ecosystem of the temperate latitudes on earth. This predator-prey relationship enhances kelp production and it is a widely accepted ecological concept that the more diverse and complex an ecosystem, the more stable the ecosystem.

Sea otters eat between 20-35% of their own body weight daily. Because of their voracious appetite, they are occasionally accused of driving their prey species into decline by fishermen who compete for the same prey species. In fact, sea otters do not cause local extinction of species. They do however alter the distribution of prey species due to their predator-prey relationship.

Some researchers hypothesize that sea otters and their voracious appetites have played an important role in shaping the natural history of the California coastline and the Pacific Rim. Rather than driving their extinction, sea otters may be responsible for the relatively large size of abalones in California, in comparison with other parts of the world. Abalone numbers are often high within the sea otters range. However, abalones are restricted to deep crevices, out of reach of otter and human harvesters.

Sea otters are also known as an “indicator” species for the health of the nearshore marine ecosystem. An “indicator” species is an organism that is relatively sedentary and tends to bio-magnify and/or be sensitive to contaminants. Sea otter tends to be relatively sedentary in comparison to other marine mammals and eat a wide variety of prey species that are filter feeders. These prey species bioaccumulate toxins in their tissues and pass them onto sea otters.

Research has shown that up to 60 percent of southern sea otter mortality is due to infectious diseases which are attributed to known land-sea linkages. Incidence of disease in sea otters is also correlated with contaminants such as tributyltin (used as an anti-fouling agent in boat paint), DDT (a banned pesticide) and PCBs.

The sea otter is also considered an “umbrella” species. An umbrella species is a species whose conservation confers protection to a large number of naturally co-occurring species (Roberge and Angelstam, 2003). Protections given to sea otters tend to ‘shelter’ and offer benefits to other species, habitats, and ecosystems including human beings.