Of fog-basking, metabolic water and milli-osmoles: Animal adaptation

Text and Photos by Hu Berry

How are some animals able to exist without drinking water? The answer lies in the fact that drinking is not the only way to ingest this magical fluid.

H umans are totally dependent on a regular intake of water, but there are some animals in Namibia have evolved mechanisms to survive without drinking water for lengthy periods.

Biologists refer to water as ‘The Universal Biological Solvent’. This is arguably the best way to describe the liquid that possesses some of the most unique properties imaginable. Without the simple combination of two atoms of hydrogen and one of oxygen (H₂0), no plant or animal life would be possible. Considering that all warm-blooded animals, namely mammals and birds, consist of no less than two-thirds water, it is clear that without it, life would cease to exist.

Take humans for example: if a person has a body mass of 75 kilograms, then he or she will be carrying no less than 50 kilograms or 50 litres of water. Only 25 kilograms of this person will be dry, organic matter. It is an absolute verification of the biblical reminder that “Dust thou art and to dust thou shall return”. Moreover, physiologists calculate that, exposed to the extreme conditions existing in a desert, the human body will be unable to meet the demands made on it for very long. Relatively inefficient at saving water, the average person will lose as much as one litre of liquid per hour by perspiring, breathing and excreting. If there is no water intake, after five hours of this draining depletion the frail human physique will reach a state of severe dehydration.

Consequently, how are some animals able to exist without drinking water? The answer lies in the fact that drinking is not the only way to ingest of this magical fluid. There are various avenues of acquiring moisture and drinking is only one of them. The other, well–known method is obtaining water via food. This is best illustrated by desert dwellers like the gemsbok. When inhabiting a true desert realm, this prince of antelopes will eat tsamma melons in the Kalahari’s waterless dunes. In the Namib’s merciless ‘sand sea’, dune grass captures precipitation from the frequent fogs, thereby providing essential moisture for survival.

Skulking along the Skeleton Coast’s salty shoreline, the strandwolf or brown hyaena gleans its moisture from the rich array of marine offerings. Carcasses of fur seals, beached whales and birds provide this ultimate scavenger with all the water it needs. In the semi–arid Etosha National Park, lions demonstrate their ability to go without drinking for at least ten days. When their hunts take them into waterless areas of trackless bush, lions rely solely on the liquid content of their prey. Research shows that the fresh carcass of an equally water–independent gemsbok provides 60% of these lions’ water requirements. The remaining 40% they obtain by drinking at isolated waterholes.

If these abilities sound impressive, there are animals that surpass gemsbok, brown hyaena and lion by several orders of magnitude. Take the ground squirrel for example, a small rodent that survives with ease on the grasses, herbs and shrubs that dot the sparsely vegetated gravel plains of the Namib Desert. In this unforgiving environment these terrestrial squirrels have no access to open water for most of the year. They compensate for this by greatly increasing their urine concentration whilst retaining a high level of water in their blood plasma. They also employ methods of temperature control that further reduce the loss of liquid. The flared tail of a ground squirrel foraging under the merciless desert sun provides a unique umbrella that shades their body from incoming solar radiation. When the heat becomes unbearable, their underground burrows remain at a constant coolness of less than 30⁰C, providing instant refuge. Indeed, the ground squirrel possesses one of the most efficient kidney functions known in mammals.

The greatest achievement of water independence is found in certain desert rodents. They eat only bone-dry food and theirs is the ultimate ingenuity because, without taking in any moisture, they are nevertheless able to produce some. Their secret lies in the driest seeds that these small mammals eat. Within the laboratory of their digestive systems they separate sugars into the basic elements of hydrogen and oxygen, recombining these vital ingredients into life-sustaining water. This incredible feat of fundamental chemistry provides these never-drinking rodents with the water that does not exist in their surroundings. Science only recently unravelled this mystery of how some animals can produce their own water internally, terming it ‘metabolic water’, which means that it is a product of food oxidation. This illustrates how resourceful animal life can be when survival is at stake.

Lesser animals possess alternative strategies to deal with a waterless environment. Some insects are capable of absorbing water vapour directly from the atmosphere via their porous skin. Others, like the Namib black beetle or toktokkie, dehydrate severely in their sandy world where rain seldom falls. Their cue for survival comes in the form of drenching fogs, which blow in from the chilly Benguela Current and cover the desert dunes in nightly blankets of moisture. When this happens, the tiny beetle specialises in ‘fog–basking’. It emerges slowly from the dune’s soft slipface and climbs to the crest. There it pauses, facing the incoming fog, to perform a headstand. This unusual posture allows the finely–dispersed fog droplets to condense and trickle down the tiny furrows on its back to its mouth. A few drops of water captured in this way are sufficient for survival. Measurements prove that this exclusive manner of drinking allows the dehydrated beetle to gain up to 34% of its body water, the equivalent of a 50–kilogram human drinking 17 litres of water at one time!

This superbly adapted array of defences by animals dealing with dehydration, all rely on maintaining a near perfect balance of body fluid concentration, measured in miniscule milli-osmoles. A mere 10% decrease in this equilibrium can result in death by dehydration. Thus, life on our Earth evolved in and from water. When it is scarce or virtually absent, as in the great deserts of this planet, plants, animals and humans are constantly reminded of the reality that “Water is the currency of life”.

This article was first published in the Flamingo August 2004 issue.