Oxygen Regulation and Diving Abilities
Although sea otters are marine animals, they differ taxonomically from other marine mammals. Instead of a layer of blubber, they have pelage, or dense fur, that helps trap a layer of air next to their skin. They also have one of the highest mass-specific metabolic rates because of their small body size. They also have a high mass-specific lung capacity. The combination of the pelage, the small body size, and large lung capacity, means that sea otters are extremely buoyant.
Being buoyant is extremely helpful when it comes to resting, grooming, and taking care of the young on the surface. But when it comes to foraging, it can be difficult and it requires a lot of energy to be able to dive. Sea otters are similar to birds, they both dive with air trapped in their respiratory system and in their plumage. The aerobic dive limit (ADL) is a common way to measure how long an animal can dive before their blood lactate levels increase too much and they go into anaerobic metabolism. Animals can dive using anaerobic metabolism, but it's energetically very expensive and usually not feasible. The cADL (calculated aerobic dive limit) is 3.62 minutes for juvenile sea otters and 4.82 minutes for adult sea otters (Thometz et. al.).
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Thermoregulation
Sea otters are the smallest marine mammal, and being in a cold water environment, they have to maintain a high body temperature to keep warm. Because they are so small, they tend to lose heat quickly. Sea otters usually have a metabolic rate that is 2.5-3.0 times that of land mammals of similar size. Heat production is usually high during times of activity and body temperature rises. When there is high heat production, the heat can be stored and then lost when they are resting. During rest, sea otter body temperatures can drop as much as 1 degree celsius due to low heat production (Costa & Kooyman).
Another factor that is really important for maintaining their temperature is their fur. Sea otters have extremely dense fur, the most dense of all animals! The density of sea otter fur is about 130,000 hairs per cm squared or about 20,000 hairs per square inch. This is about 1000x more dense than human hair (Cassidy, 2021). They don't have any blubber like other sea mammals have so their fur is key in temperature regulation. Because their hair is so dense, it helps keep their skin dry and warm. The dense fur acts like a waterproof jacket and doesn't allow the frigid ocean water to penetrate (Kuhn et. al.).
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Feeding and foraging strategies
In a study done at the Center for Coastal Marine Studies in Santa Cruz, California, the diets of sea otters can be variable. Sea otters tend to forage for sea creatures that are easy to find and catch, but also provide adequate energy and nutrients. In this study, they analyzed different sea creatures and how often/when the sea otters would forage for them. In the beginning of their study, they noticed that sea otters were mainly foraging and eating sea urchins. After about a year, the sea urchin population was depleted due to foraging and the otters moved on to kelp crabs and clams. Poor foraging success rates cause sea otters to move onto different prey, like what happened with the sea urchins for example (Ostfeld, 1982). Small sea creatures that are easy to dive for are preferred because diving uses a lot of energy. Sea otters mainly eat small or soft prey but will occasionally use tools like rocks or stones to break open prey such as clams (Davis, 2021).
In the study, they used an electivity index to determine the sea otters' food preferences. They calculated the energy intake and foraging times for each major prey species.
Sensory Processes
Sea otters are marine aquatic mammals, however, they are adapted to live both on land and in the water. They spend most of their time in the water but they have evolved multiple sensory processes that are different to other marine mammals.
Sea otters have very small ears that stick upright when they are on the surface of the water or on land. However, when they dive, their ears fold over tightly towards their head so that water doesn't get in. In a study done by Asila Ghoul and Colleen Reichmuth, they examined the auditory processes of sea otters and aimed to figure out how well sea otters hear both on land and in the water, as well as what frequencies their ears can hear. In the experiment, they "trained an adult male sea otter to perform a psychophysical task in an acoustic chamber and at an underwater apparatus." They recorded both aerial and underwater hearing thresholds and found that aerial thresholds for sea otters were pretty similar to sea lions. They both have reduced low-frequency sensitivity. However, for the underwater threshold, they found that the hearing sensitivity was low compared to sea otters. This means that sea otter hearing capabilities are mostly adapted for land, even though they can hear underwater. Compared to other marine mammals, sea otters also have a harder time picking out noises if there is a lot of background noise. This is not a problem for sea otters because they don't need to rely on acoustic and auditory cues while foraging.
Sea otters also have visual capabilities that have adapted for use on land and underwater. Scientists have discovered that sea otters can focus their vision underwater as well as on land. The refractive power of a sea otter cornea is 59D, which helps them see underwater because it compensates for cornea refraction. The anterior epithelium of the cornea is also highly developed to help with the salinity of the sea water (Murphy et.al.).
Another important sensory tool for sea otters are their whiskers! Their whisker act like organs of touch, and they can use them to forage or identify different prey. Their whiskers have a forward orientation. meaning trey grow out towards the front of their face vs. the side of their face. Due to their small body size, tend to have less whiskers compared to seals and walruses (Marshall et. al.).
Communication and Navigation
When it comes to navigation, sea otters mainly use their whiskers and paws to navigate their surroundings when foraging. Sea otters usually forage at night when there is little light so their whiskers and paws are great navigational tools. Sea otter paws are similar to our human fingertips and they can discern different textures through touch. They have a quick response time when it comes to comparing different textures and this makes sense, because they need to be efficient in foraging. We previously talked about how sea otters can focus their vision clearly underwater. In addition to that, being to feel their environment through their whiskers and paws make navigating in the dark advantageous (Kobilinsky).
Sea otters communicate through visual, auditory, and tactile cues. They can communicate by licking body surfaces or whiskers. Licking is mostly observed in mothers grooming their young babies and communicates bonding and comfort to young sea otter pups. For example, licking the head doesn't necessarily help with grooming or cleanliness, but it helps calm down sea otter pups during stressful situations. Body contact also helps communicate a sense of comfort and it's been observed that sea otter pups stop crying when they can feel their mother.
Visual cues are also important for communication. When sea otters are alarmed, their posture and mannerisms change and they come up halfway out of the water. They hunch up and hiss to signal that they feel threatened and will also claw and bite (Winn & Schneider). Auditory cues include various calls and noises that sea otters make. They have the ability to recognize different voices and sounds and seem to understand what they mean. They have vocal patterns that allow them to communicate with familiar individuals (McShane et. al.).
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Reproduction
Unlike some other mammals, sea otters don't have a set breeding season so they breed all year round. Sea otter litter sizes are typically only one pup (Sinha). Although there are no set breeding seasons, scientists have noticed that there are more pups during the spring and the fall. Sea otter pups usually stay with their moms for anywhere between 80 and 120 days. Scientists have discovered that the inter-birth interval was approximately 389 days. This means that there is on average 389 days between when a female sea otter will give birth successfully (Siniff & Ralls). The average gestation time for female sea otters is about 6 months and otters will typically breed for the first time at around five years old (Jameson & Johnson).
While about 85-90% of female sea otters give birth every year, giving birth is extremely difficult on sea otters. Sea otters already have an extremely high metabolic rate and require a lot of energy for diving, foraging and staying warm. Add on the high amount of energy needed to carry, birth, and raise a pup until weaning can be exhausting. Sea otters are required to forage even when they are lactating because their own caloric needs need to be met as well as their pups. Otters don't have many energy reserves like other mammals do. They don't have much excess fat and they don't have any blubber that they can use for energy while caring for their pups. Because of this, scientists have discovered that fat reserves get depleted about only a week after giving birth. Unfortunately because of this, it leads to high mortality rates in female sea otters who give birth. This is called end-lactation syndrome and scientists estimate that it's responsible for over half of the deaths of female California sea otters (Thometz).
Muscles
Since sea otters are considered semi-aquatic animals, their muscular system has some distinct differences compared to other Mustelidae (i.e. badgers, weasels, river otters, minks, etc.) Some key differences include having larger gluteus, popliteus and peroneus muscles. They also have smaller adductor and ischiopubic muscles. The larger muscles help with propelling and swimming through the water. Sea otter tails are also extremely muscular and are responsible for acting like a propeller to help them steer and swim (Mori et. al.).
Skeletal muscle also plays a role in temperature regulation. Through a technique called energy leak respiration, they can used energy that's not necessarily used to perform tasks like foraging to keep warm. The skeletal muscle plays a big role in energy leak respiration and helps account for the high metabolic rates found in sea otters (Janak).
Respiratory System
As we mentioned early on, sea otters have large respiratory volumes/lung capacities. A large lung capacity helps to keep sea otters buoyant, since they spend most of their time on the surface of the water.
Sea otter lungs are large given the size of their body. The lungs are described as being triangular in outline. The ratio of total lung weight, which is measured as the weight of the lungs per 100g of body weight, is found to be 3.86 in. in the sea otter. Comparatively, in the harp seal, that number is about 1.31 in. Harp seals are much bigger in size than sea otters so we can see that sea otters have huge lungs even though they are so small! Because of this, sea otters can hold their breath for up to five minutes.
References
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Created with images by christels - "sea otter aquatic animal blue" • chetvergov - "Похлопаем" • Jonathan Chancasana - "Marine otter (Lontra felina), a beautiful sea otter underwater in flight, photography taken in captivity. Lima - Peru" • nvphoto - "Sea otter posing in the water" • nvphoto - "Wild Sea otter floating in the water" • anlo - "Seeotter im Hafen von Seward, Alaska" • Elisabeth - "Southern sea otters enjoying life on the Central California Coast" • manuel - "Mother sea otter kissing her young pup." • Patrick - "A wild sea otter in the waters of Seward, Alaska near Kenai Fjords National Park in the Kenai Peninsula." • Sam D'Cruz - "Sea Otter - Vancouver, Canada"