Ins & Outs of Fluid balance in Desert Animals (Part 1)

On the edge of the vast & usually dry Etosha pan waterholes like this one at Salvadora are an essential water supply to large herds of hoofed animals such as  wildebeest, zebra, & springbok

On the edge of the vast & usually dry Etosha pan waterholes like this one at Salvadora are an essential water supply to large herds of hoofed animals such as wildebeest, zebra, & springbok. (click to enlarge)

Maintaining their hydration is a major problem for desert animals. Large mammals e.g. Gemsbok & small mammals e.g. rodents use different strategies to cope with arid conditions. We will discuss the special behavioral or physiological adaptations that have evolved in various animals, concentrating on those found in Africa.

We will in part 1 first explain the model we will use. We will try to simplify the adaptations used to maintain water balance, once again, using our simple analogy of a wash basin. The level of water in a washbasin depends on how much is flowing in versus how much is flowing out. Similarly the level of hydration in all animals will depend on the balance between the water input & output. (Figure 1)

Wash basin model of fluyid baslance

Figure 1:this simple input output model analogous toa wash basin demonstrates that fluid balance will depend on the input & output of various fluids. Copyright of Old Age Adventurers (Click to enlarge)

Water Input:- Most of the water input usually depends on the intake from exogenous sources. This is the amount we drink or the amount ingested with food e.g. fluid present in cells or surrounding tissue of animals & plants. However water is also produced endogenously when nutrients are catabolized e.g. when  glucose is converted to energy ( ATP), CO2 & water are released.

Water Output:- Although water can shift compartments e.g. move from extracellular fluid to intracellular fluid & vice versa this will not change total body water. This occurs in disease states & will not be discussed. Thus for water balance we will only discuss water lost from the body i.e. exogenously, resulting in dehydration. This can occur from body sites which are permeable to water and have a large surface area. The water output thus occurs in the kidneys, skin, lungs, & gastrointestinal tract.

Renal output: Large volumes of blood pass through the kidneys for filtration. The small molecules in blood plasma e.g. water, glucose, salts, & break down products of metabolism such as urea & uric acid pass through the small pores of the glomeruli into the renal tubules where they can be reabsorbed or pass onto the bladder for excretion. In the distal part of the tubules water can be reabsorbed depending on hormonal signals & the urine concentrated. Thus the water output via the kidneys can be regulated to maintain water balance.

Pulmonary Fluid loss: Water molecules pass back & forth over the large surface area in the lungs. The alveolar/capillary interface is very thin & permeable to water. The water forms a thin alveolar/bronchiolar interface with the inhaled air &will evaporate & be carried out on exhalation. The condensation of exhaled water into steam when we breathe out on cold winter days clearly illustrates this water loss.

Skin losses: Here as in the lung the large surface area allows a large amount of water to be lost. Since surface area varies inversely with the size of the animal small animals have relatively larger surface areas & lose water more easily. This point is illustrated by the fact that premature babies dehydrate very easily & are nursed in humidified incubators. This water loss is further aggravated if the skin is very thin e.g. preterm babies. This loss is known as insensible loss. The other water loss from the skin is via active sweating. Newborn babies and many animals are unable to sweat. This decreases their water losses but hinders temperature regulation in hot conditions.

Gastrointestinal losses: A large amount of fluid is released into the gastrointestinal tract to aid digestion. Saliva, gastric, fluid, pancreatic juice & bile, as well as secretions along the small intestine contribute to this potentially large output of water but most is normally reabsorbed in the large intestine & thus feces has a relatively firm consistency. When this mechanism is disturbed e.g. vomiting or diarrhea dehydration can rapidly follow.

We will use this simple input output model in following posts on how animals adapt to desert conditions & maintain their hydration.


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2 Responses to Ins & Outs of Fluid balance in Desert Animals (Part 1)

  1. Patrycja Chyla-Malima says:

    Dear H&M Berger,

    I am a publisher working at Namibia Publishing House (Pty) Ltd.
    We are publishing textbooks for government schools from pre-primary to high school level.
    Our aim is to provide quality affordable educational materials for underprivileged kids, mainly in rural areas of Namibia.
    We strive to use the most up-to-date information and attractive relevant illustrations and photographs in all our publications.
    At the moment, I am working on Social Studies Grade 4 Learner’s Book and I would like to use a photograph I found on your blog: ,
    while searching for images of animals at a waterhole in Etosha National Park.
    I would like to use the first image appearing on your post from 25 March 2013, that has the following caption from your blog: On the edge of the vast & usually dry Etosha pan waterholes like this one at Salvadora are an essential water supply to large herds of hoofed animals such as wildebeest, zebra, & springbok.
    I would like to use it inside the text as an example of Etosha National Park and the type of animals that live there.

    I kindly ask for your permission to use the photograph in question inside the text of this publication. We oblige ourselves to attribute the photograph in a manner specified by yourselves.

    Thank you in advance for positive consideration of my request.

    I look forward to hearing from you.

    Warm regards,

    Patrycja Chyla-Malima

    Namibia Publishing House
    19 Faraday Street
    P.O. Box 22830
    Windhoek, Namibia
    Tel: +264 61 232165
    Fax: +264 61 233538

    • old age adventurers says:

      Thanks for the request. It would be a pleasure to help young Namibian students. Just acknowledge photo is from

      I am particularly pleased about your request because my wife & I are helping a security guard at Namutoni to pass his previously failed maths & physics Senior secondary certificate. We are supporting him via NAMCOL. So great to also help in a very small way with the younger children in Namibia.

      Best wishes
      Howard & Marianne Berger

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