Otters' Coats: Nature's Camouflage

Otters are adorable and playful semi-aquatic mammals found throughout North America. But why are they different colours?

Otters come in various shades of brown, from light tan to dark chocolate brown, with some even having a reddish tint to their fur. The most common otter fur colour is a rich, dark chestnut brown, which allows them to easily blend in with their riverbank environment as they hunt for food.

The different colours of otters can be attributed to a variety of factors, including species, age, location, and diet. For example, sea otters tend to be darker brown, while giant otters have reddish-brown fur. Otter pups are born with lighter-coloured fur, which darkens as they mature. Additionally, otters from different geographic regions can have slight variations in fur shade, with North American river otters tending to be lighter in colour than their European counterparts.

While the specific shade of an otter's fur can vary, the purpose it serves remains the same. Otters have two layers of fur – an outer layer of guard hairs and an inner dense underfur – which provide insulation and help them regulate their body temperature in cold environments.

Otters have two layers of fur

Underneath this protective barrier lies the second layer, a dense underfur that insulates otters against cold temperatures. This underfur is made up of extremely fine and short hairs that are packed tightly together, creating an effective barrier against water and cold air. The density of this underfur is remarkable, with around 150,000 hairs per square centimeter on otter pelts. This dense layer of fur traps air next to the otter's body, providing essential warmth.

The dual-layer structure of an otter's fur is well-adapted to their aquatic lifestyle. The outer layer's water-repellent properties allow otters to trap air next to their bodies, while the dense underfur provides insulation. These two layers work together to keep otters warm even in icy waters, demonstrating the functional benefits of their unique fur composition.

The outer layer protects against wind and water

Otters have two layers of fur. The outer layer, also known as the guard hairs, are longer and coarser than the inner layer. This outer layer forms the outer coat of the otter and serves to protect the otter against wind and water. These guard hairs are typically 1-2 inches in length.

The guard hairs are waterproof, thanks to secretions from the otter's sebaceous glands. This waterproofing allows otters to trap air next to their bodies when they are in the water. The trapped air is what keeps the otters warm.

The outer layer of guard hairs is so effective at repelling water that the fur closest to the otter's body is almost completely dry. This dryness is critical to keeping the animal warm.

Otters' fur is about 1,000 times denser than human hair. The density of their fur is one of the two key properties that make it so effective at creating an insulating layer of air. The other property is its spikiness. While otter pelts feel smooth to the touch, the individual hairs are covered in tiny, geometric barbs. These barbs help the hair to mat together, which is what prevents the trapped air from escaping.

The inner layer insulates otters against cold temperatures

Otters have two layers of fur: an outer layer of guard hairs and an inner layer of dense underfur. The outer layer is made up of longer, coarser hairs that form a protective coat to shield otters from wind and water. This outer layer is also waterproof, thanks to secretions from the otter's sebaceous glands, which allow otters to trap air next to their bodies when they are in the water.

The inner layer, on the other hand, is made up of extremely dense, short, and fine hairs that are packed tightly together, preventing water from penetrating this layer. This inner dense underfur is what insulates otters against cold air and water temperatures, allowing them to maintain their body temperature even in icy waters. This underfur is so effective that researchers have found around 150,000 underhairs per square centimetre on otter pelts, providing exceptional insulation.

The insulating power of the inner layer comes from its ability to trap a layer of air next to the otter's skin. The density and texture of the underfur are crucial to creating this insulating air layer. The hairs are packed so closely together that they form a nearly waterproof barrier, trapping air bubbles that provide insulation. This is further enhanced by the tiny, geometric barbs covering each hair. These barbs cause the hairs to mat together, ensuring that the fur closest to the otter's body remains dry.

The inner layer of dense underfur is essential for otters to survive in cold environments. It helps them retain heat and prevents heat loss, even when they are submerged in frigid waters. This adaptation is particularly crucial for otters as they lack the thick layer of blubber commonly found in other marine mammals, such as seals, whales, and walruses. Instead, otters rely on their specialised two-layer fur system to keep themselves warm.

Otters have sensitive whiskers to help them find prey

Otters have sensitive whiskers, known as vibrissae, which help them find prey in murky waters. They also have sharp tactile hairs on their nose pads that assist in feeling and capturing prey. Otters are semi-aquatic mammals that inhabit both freshwater and marine environments, and their whiskers are essential for their survival.

Sea otters, for example, use their whiskers to locate small prey inside crevices. They have robust and thick whiskers, similar to those of seals and sea lions. However, unlike other marine mammals, sea otters have retained their front paws instead of developing flippers. This makes their sense of touch particularly important for hunting.

To test how sea otters use their paws and whiskers, researchers conducted experiments with a sea otter named Selka. They presented Selka with textured plates and rewarded her for choosing a specific texture. As the experiment progressed, the unrewarded texture was gradually changed to be more similar to the rewarded one. This allowed researchers to estimate Selka's touch sensitivity.

The results showed that while Selka's paws were more sensitive than her whiskers, both were comparable to the touch sensitivity of primates, seals, and elephants in similar experiments. Additionally, Selka was much quicker at distinguishing textures than humans. She could make decisions with her paws in under 200 milliseconds and with her whiskers in under 400 milliseconds, while humans consistently took over a second.

The heightened sense of touch in their paws and whiskers enables sea otters to efficiently locate and capture prey in their aquatic environment. This is especially important as they need to consume a large amount of food to meet their energy requirements. Sea otters consume about 25% of their body weight in food each day to maintain their body temperature in cold waters.

Oil can be dangerous to otters

Otters have two layers of fur—an outer layer of guard hairs and an inner dense underfur. The guard hairs are longer, coarser hairs that form the outer coat of the otter and protect against wind and water. The inner underfur is extremely dense and helps insulate otters against cold air and water temperatures.

In the aftermath of the 1989 Exxon Valdez oil spill, an estimated one-fifth of Prince William Sound's 14,000 otters died from spill-related poisonings. Even years after the spill, residual oil was found on some beaches, and otters that returned to these areas experienced chronic poisoning from periodic ingestion of oil.

Oil spills can also affect otters in less direct ways. For example, oil can contaminate the clams that otters feed on, leading to indirect ingestion of oil. Additionally, oil can make it difficult for otters to dive and swim, as they rely on the air trapped in their fur to stay buoyant.

Therefore, it is crucial to address and prevent oil spills to protect otter populations and their sensitive ecosystems.

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