Lessons from a Kangaroo Rat and Other Desert Tails

In 1946 Knut Schmidt-Neilson visited the Arizona Deserts. He had studied in Norway and Denmark but it was Arizona’s desert creatures that inspired much of his career’s work at Duke University where he remained for the rest of his career.

Among his works on comparative physiology, is the little book, “How Animals Work,” which became a classic in comparative physiology and popular textbook which I studied long before moving to the desert.

I thought it would be fun in this blog to explore some of these amazing adaptations and challenge readers to think about how design and innovation in sustainable desert living might follow the same or similar principles.

So put on your thinking caps!

The Kangaroo Rat – A Cold Nose

Schmidt-Nielsen was surprised by the numerous small mammals thriving in Sonoran Desert habitats. How could they survive without drinking a drop of water?

Facts. Mammals lose water from evaporation, urine and feces. The professor observed that Kangaroo Rats have highly concentrated urine and dry droppings, but how did they solve the evaporation problem?

Principles. For all animals, water gain and water loss must be equal to survive. Many animals get water from what they eat (free water) and from metabolic water (when food stuff is broken down.)

Kangaroo Rat

The Kangaroo Rat eats dry seeds. For all practical purposes there is very little free water gained. Therefore this small mammal must conserve almost all its metabolic water. But, doesn’t it lose water when it breathes through that long nose?

Schmidt-Nielsen measured the temperature of its expelled air and found it was at ambient air temperature – not body temperature. Besides being a nocturnal animal and having no sweat glands (two other adaptations that reduce water loss) the nose of the Kangaroo Rat is just long enough to create what amounts to a rain cloud in its nose!

Here’s how it works: ambient air is warmed and moisture added as it enters the narrow nasal passages of the rat and proceeds into its lungs when it is saturated and at body temperature. When the animal exhales, the cooled mucosa lining in the nose cools the warm air, condensing the moisture into fine droplets (like a rain cloud) before it leaves the nose at ambient temperature again. The Kangaroo Rat loses no water through breathing.

In comparison the human nasal passage is wide and short and significant water is lost through breathing. Humans sweat to transfer heat out of the body in the form of water. Under extreme heat conditions, a human being can lose about a half gallon of moisture in an hour and up to three gallons in a day. This water must be replaced or extreme dehydration occurs and perhaps death.