Thinner Fur Keeps Numbats Warmer

close photograph of small brown and white striped marsupial.

Biological Strategy

Numbat

Lonny Lippsett

Image: dilettantiquity / Flickr / Some rights reserved

Capture, Absorb, or Filter Energy

Energy is naturally available in many forms, including kinetic, potential, thermal, elastic, radiant, chemical, and more. All living systems require energy to carry out their many activities, and have developed strategies appropriate to one or more of those forms. For example, some plants maximize their surface area available for capturing radiant energy from the sun while others have strategies to focus scattered light onto photosynthesizing areas.

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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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By making their hairs stand up, numbats expose more skin to the sun and create an insulating layer of air to reduce heat loss.

Introduction

In the animal kingdom, marsupials have several characteristics that set them apart. But even among marsupials, the numbat stands alone.

Marsupials are found primarily in and around Australia. They are perhaps best known as the mammals that are born prematurely and continue to develop in pouches. Numbats, however, are pouchless; babies simply cling to their mothers for six or seven months until they get too big to carry around.

And while most marsupials are nocturnal, numbats are active exclusively during the day. That’s because of their diet. Unlike most marsupials, which generally eat plants, fruits, and sometimes insects, numbats are entirely carnivorous. More particularly, they are termitivorous. They eat nothing but termites, which come out by day.

Given their singular schedule and diet, numbats have had to adapt to maintain ideal body temperatures. A major way they do that is with another feature that sets them apart. Unlike most marsupials, which have thick fur coats, numbats have sparse pelts—which counterintuitively help regulate their body temperatures and keep them warm.

The Strategy

Since numbats don’t get enough fuel from termites to maintain body heat, they rely on absorbing heat from the sun. Their pelts have shorter hairs and fewer hairs per square inch, compared with other marsupials and mammals. So their sparse coats are not made to insulate against the cold, but rather to expose more skin area to the sun, maximizing the amount of solar radiation they absorb.

They also make their pelt hairs stand up straight. The scientific term for this is piloerection. It happens to people when they get goosebumps.

Piloerection traps layers of motionless body-heated air close to the skin surface. These layers block heat loss from the body, creating a blanket that helps insulate numbats from cold. As temperatures drop or cold winds pick up, numbats effectively reduce heat loss by erecting their pelts. Altogether, numbat pelts strike a balance between regulating how much heat gets in and how much is allowed out.

The Potential

The insulating properties of piloerection offer ideas for designing movable hairlike systems, from microscopic to architectural, that can regulate heat in response to changing environmental conditions such as temperature, sunlight, winds, and humidity. These systems could be applied to fabrics, materials, and structures that efficiently capture heat and prevent heat loss and could lead to innovative fabrics or buildings that keep people safe and comfortable.

Image: Martin Pot / Wikimedia commons /

Numbat, Perth Zoo, Western Australia

Image: S.J. Bennet / Flickr /

Last Updated August 23, 2021

References

“The biophysical properties of numbat pelts presumably reflect adaptation to their diurnal lifestyle. Compared with other marsupials, their pelt is sparse and shallow and has poor insulative and low reflective properties, probably to maximise SHG (solar heat gain). … By erecting their fur when exposed to low TaS (ambient temperatures), numbats are potentially able to double the thermal resistance of their pelts and also reduce the effect of wind, therefore reducing the loss of metabolic heat.”

Journal article

Biophysical properties of the pelt of a diurnal marsupial, the numbat (Myrmecobius fasciatus), and its role in thermoregulation

Journal of Experimental Biology | August 7, 2003 | C. E. Cooper

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Reference

“Within mammals, fur plays a similar thermoregulatory role. …This means that the optimum density of fur for thermoregulation is a careful balance between regulating how much heat reaches the skin relative to how much may be allowed out. Similarly, in the diurnal numbat, sparse fur allows heating through solar absorption, whereas closely related nocturnal marsupials prioritize the reduced thermal resistance and heat retention of dense fur. … In terms of engineering, the creation of clothing (i.e. mimicking the presence of fur) is presumably one of the oldest examples of biomimetics. More recently, materials based on fur have been proposed and developed for building insulation and solar radiation harvest.”

Journal article

Design principles of hair-like structures as biological machines

Journal of the Royal Society Interface | March 30, 2018 | Madeleine Seale, Cathal Cummins, Ignazio Maria Viola, Enrico Mastropaolo, and Naomi Nakayama

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