Fur Keeps Heat in and Cold Water Out

Biological Strategy

Otters

Kathryn Krupin

Image: christels / Pixnio / Public Domain - No restrictions

Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Long scales on the underhair of otters and fur seals trap air to retain heat and prevent water penetration.

Introduction

Many aquatic mammals face a serious physical challenge: they must stay warm while spending significant periods of time in extremely cold waters. Sea lions, walruses, and true seals have thick layers of blubber to keep them warm, while fur seals have much thinner layers, and otters do not have any blubber at all. Instead, fur seals and otters rely on very specialized layers of hair to maintain their body temperature in and out of the water.

The Strategy

The pelts of all fur-bearing animals—whether they live on land or in the water—are actually made up of two different types of hair that grow in clusters together: guard hairs and underhairs. Underhairs are shorter and denser, with three or more underhairs growing in a follicle for every one guard hair. Guard hairs are longer, and usually extend out over the surrounding underhairs, creating a protective canopy for them.

illustration showing guard hairs and underhairs growing in clusters from a single pore.

Guard hairs and underhairs grow in clusters together from a single opening. The guard hairs extend out over the underhairs and create a protective canopy for them. For readability, three underhairs are shown for every guard hair in this illustration. Sea otters may have from 12 to as many 108 underhairs bundled with a single guard hair, depending on the location.

Studies have shown that aquatic mammals relying on fur for insulation have scales on the cuticle (outer layer) of their underhair that are more elongated compared to aquatic mammals that rely more on blubber. The protruding of the scales causes the hairs to interlock, preventing the penetration of water, and also trapping air bubbles. Air is a poor conductor of heat, so the bubbles provide an insulating layer, preventing heat loss from the animal’s body.

Scientists also noticed that fur-bearing marine carnivores have significantly flatter and denser hair than carnivores found on land, such as bobcats and ermines. They performed a test of the different pelt types, putting fur samples from each in a small pressure chamber, covering them with water, and then observing how far the water penetrated into the fur when subjected to increasing pressure. The marine carnivores’ pelts were far better at blocking out water and maintaining the layer of air within the fur, providing evidence that flatter, denser hair was an that helped mammals survive in aquatic environments.

While some aquatic mammals rely on blubber to stay warm, sea otters rely on their specialized fur. Their two-layer fur system keeps air trapped close to the skin even when submerged and allows them to keep warm in frigid waters. Sea otters have the densest fur of any mammal followed by fur seals, who use a similar system.

Scientists also noticed that fur-bearing marine carnivores have significantly flatter and denser hair than carnivores found on land, such as bobcats and ermines. They performed a test of the different pelt types, putting fur samples from each in a small pressure chamber, covering them with water, and then observing how far the water penetrated into the fur when subjected to increasing pressure. The marine carnivores’ pelts were far better at blocking out water and maintaining the layer of air within the fur, providing evidence that flatter, denser hair was an adaptation that helped mammals survive in cold aquatic environments. In fact, sea otters, followed by fur seals, have the densest fur of all mammals.

The Potential

Understanding how fur keeps aquatic mammals warm could inform the development of insulation and clothing for wet environments and, in particular, for situations where people or objects are moving frequently into and out of water.