The Unique Characteristics of Sea Otters
Sea otters (Enhydra lutris) are extraordinary animals that play a vital role in the health and balance of their ocean habitats. These charismatic marine mammals are known for their unique physical, behavioral, and ecological characteristics — many of which are adaptations crucial to their survival — that set them apart from other otter species. For instance, unlike most mammals, sea otters are born with their eyes open. In this article, we’ll take a closer look at the various characteristics of sea otters and explore how these adaptations help them thrive in their aquatic environment.
Unique Dentition
Sea otter teeth are highly specialized for their diet, which consists primarily of invertebrates such as clams, urchins, and crabs. As Dr. Mike Murray from the Monterey Bay Aquarium explains, sea otter pups are born with 26 baby teeth, 10 of which are visible. By the time they reach juvenile age — around 2-3 months old — they have a set of 44 teeth, 26 of which are visible. When they reach adulthood, sea otters have thirty-two (32) rounded and flat teeth with two pairs of lower incisors. These teeth make them well-suited for crushing their prey rather than cutting into it. This unique dentition is a key adaptation that allows sea otters to effectively process their varied diet.
“As the adult teeth mature and start to erupt, while the skull and mandibles are simultaneously enlarging, the adult teeth either supplant the deciduous ones, or they erupt into a location where there is no deciduous tooth.” – Dr. Mike Murray
Fur and Buoyancy
One of the most well-known characteristics of sea otters is their dense, waterproof fur. Sea otters have the highest fur density of any mammal, with up to 1 million hairs per square inch. This remarkable adaptation helps them maintain their body temperature in cold ocean waters and provides buoyancy that allows them to float effortlessly on the surface. The dense fur also protects sea otters from the elements, keeping them warm and dry even when submerged in the water.
Whiskers and Olfaction
While sea otters may not have scent glands, they possess a keen sense of smell. They have sensitive whiskers (vibrissae) that can grow up to 8 inches (200 millimeters) in length in captive sea otters. In wild sea otters, the whiskers are shorter due to their foraging activities. The vibrissae are an important sensory organ that helps sea otters detect and locate their prey while navigating complex underwater habitats.
Behavioral Characteristics
Sea otters exhibit some intriguing behavioral characteristics that set them apart. For example, when they first awaken from sleep, they are often reluctant to get their flippers, tails, forepaws, and faces wet. Because of this, they can be seen elevating their appendages out of the water. Additionally, this behavior helps them maintain their body temperature.
Size and Subspecies Differences
The size of sea otters can vary considerably depending on the subspecies. The southern sea otter, found along the coast of California, is generally smaller than the northern sea otter found in Alaska and the Aleutian Islands. Southern sea otters typically measure up to 4 feet (1.2 meters) in length. Females in this subspecies can weigh up to 50 pounds (23 kilograms), while males can reach weights of up to 70 pounds (32 kilograms). In contrast, the northern sea otter subspecies is generally larger. Female northern sea otters can weigh up to 70 pounds (32 kilograms). Alternatively, male sea otters in this population can weigh of up to 100 pounds (45 kilograms).
Understanding these size differences is crucial for accurate prey measurements and population studies. Researchers measure the paw width of every captured and tagged sea otter to ensure that the conversion from size class to actual linear measurement (cm) takes into account these subspecies-specific differences.
Characteristics Used for Studies, Tagging, and Identification
Sea otter research often involves tagging and identifying individual animals to monitor their behavior, health, and movements. Michelle Staedler, former sea otter program manager at the Monterey Bay Aquarium and now assists various southern sea otter research projects, including one funded by SOFT, explains that placement of a tag on a southern sea otter’s right flipper reveals its gender. Tags positioned between the first and second digits indicate a female, while tags between the fourth and fifth digits indicate a male. These positions are referred to as 1/2 and 4/5, respectively. The color of the tag varies depending on the specific research project, making it easier for scientists to identify which study each otter is part of.
To ensure accurate identification, the tag color and placement must be correct. Researchers keep detailed records for each study and frequently consult the records during observations to confirm they have identified the correct animal.
Interestingly, the toes on a sea otter’s hind flipper are arranged differently than human fingers and toes – their longest digits are on the outside, and the shortest are on the inside.
Sea otter prey size
Another intriguing aspect of sea otter research is how scientists use otter paws to measure the size of their prey. Dr. M. Tim Tinker, a retired USGS Research Wildlife Biologist and co-founder of the Tinker Estes Laboratory at UC Santa Cruz, explains that “[sea otter] paw size has been used as the metric of prey size for virtually all published sea otter foraging studies.” Prey is categorized into size classes based on how many paw widths it spans, with sizes ranging from 1 (less than or equal to one paw width) to 4 (more than four paw widths). This system helps researchers estimate the biomass and energy content of the food sea otters consume.
In the late 1990s, scientists added “qualifier” letters (such as 1a, 1b, 1c) to further refine these size classes. For instance, a 1A item is very small (less than 1.67 cm), while a 1C item is larger (between 3.33 cm and 5 cm). Since different sea otter populations and age groups have varying paw sizes, researchers measure the paw width of every otter they capture and tag. This allows them to accurately convert the size class into an actual linear measurement in centimeters, taking into account the differences in paw size. This method provides valuable insights into the foraging habits and nutritional needs of sea otters.
Summary
The unique characteristics of sea otters are a testament to their remarkable adaptations and their crucial role within the ocean habitat. From their specialized dentition to their dense fur and distinctive behaviors, sea otters are truly one-of-a-kind creatures. By understanding and appreciating these fascinating features — and also using them to study and identify individual otters — we can better appreciate the importance of sea otter conservation and the vital role they play in maintaining the delicate balance of their ecosystems and our world as a whole.
At SOFT, we work to ensure the survival and recovery of sea otters in their habitats by building funds to support research, conservation, and education. You can learn more about the all-important efforts of our grant recipients by watching our interviews with them. These efforts are funded directly by our supporters, so consider advancing our crucial work by adopting an otter or making a donation today!
