Kangaroo rats

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Heteromyidae
Genus: Dipodomys
22 species
Dipodomys - the kangaroo rat
Ord's kangaroo rat Dipodomys ordii
Note the large hind legs and long tail, and also the large eyes.

These rodents are fairly widespread in dry areas of North and central America. They are examples of mammals which are specialised to live in a rather inhospitable environment.
It is worth pointing out certain facts about their lifestyle and biology.

The animals' diet is principally seeds, gathered at night and stored in underground burrows. They have large cheek pouches like hamsters for temporarily storing this material during their brief forays to the surface at night time when the air is cooler and more humid than in the heat of the day.

Their food is fairly dry and they do not take in liquid water by drinking.

Like most animals, they produce water due to the oxidation of foods used in respiration. This "metabolic water" is the product of aerobic respiration - the final stage of oxidative phosphorylation being the reduction of oxygen using hydrogen ions and electrons.

Lipids are especially important in this respect because on oxidation they produce more water than carbohydrates and proteins.

There is a great difference between the environment beneath ground and above ground.

The nasal passages within the kangaroo rat's skull have a large surface area and the turbinate bone is well developed.

It is said that whilst underground in their cool humid burrows (where they spend the majority of their time), kangaroo rats retain as much water as possible by effectively recycling water which would otherwise be lost in their breath.

Air with a high water vapour content leaves the lungs - the unavoidable consequence of exposing a relatively large surface area for the absorption of oxygen. Therefore air which is exhaled has a very high relative humidity - close to 100 per cent saturation, and it is also at core body temperature - 38C.

If the temperature of the kangaroo rat's nasal passage is any cooler than 38C due to the cooler external environment, then water will condense before leaving the animal. The liquid water can then be reabsorbed, possibly by osmosis into the blood, or by being simply swallowed. It is also said that the stored seeds absorb water from the air - possibly originating from the kangaroo rats breath - or from the soil of the burrow. This results in greater water conservation (less water loss) than other mammals of a similar size.

Conversely it is said that the nasal passages may have a role in controlling the kangaroo rat's body temperature. In the brief periods when the animal is active above ground and breathing in air with a lower humidity, water will evaporate from the nasal passages, causing cooling of this area which is not far from the brain. This could cause a cooling effect of up to 5C. This allows the kangaroo rat to be active, for example foraging for food and moving to avoid predators so they are not restricted to staying in the burrow.

The kangaroo rat's kidneys are especially efficient and produce only small quantities of urine, which is highly concentrated. They are said to have very long loops of Henle.

These long loops of Henle give a greater opportunity for water to be reabsorbed. This is because the increase in concentration of ions deep in the medulla is effectively proportional to the length of the loops of Henle.

Further info: Wikipedia http: en.wikipedia.org wiki Kangaroo_rat

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