Posted by: Rob Viens | September 20, 2012

Pondering How to Hold Water

On September 19th, Darwin’s never-ending curiosity was focused on how desert animals found water in the desert:

“Walked to the plains beyond the sand hillock & shot some small birds for specimens.— It is a complete puzzle to all of us, how the Ostriches, Deer, Cavies, &c which are so very numerous, contrive to get water. Not one of us has seen the smallest puddle (excepting the well which is 8 feet deep) & it is scarcely credible they can exist without drinking. I should think this sandy country in the summer time must be a complete desert; even now in spring & all the flowers in bud the sun is very powerful, there being no shelter & the heat being reflected from the sand hillocks.” (Sept 19)

A good question indeed, and one with several fascinating answers. As Darwin would later tell us, these animals adapted to this arid environment through natural selection. (One has to wonder every time he poses a question like this, if it was one of the many questions that pushed him further into formulating his theory.) Individuals with a slight advantage of surviving in drought conditions survive, passing that characteristic on to their offspring, and viola, overtime the population evolves to be able better adapted as a whole to the desert. (OK – big simplification, but that’s the 5 cent version.)

Dromedary (from Wikipedia Commons)

dromedary

So what are some adaptations to drought? They tend to fall in a few general categories:

Behavior modification:

Many desert animals are nocturnal to prevent the loss of water during sweating and panting (which is necessary to cool them down and prevent overheating during the day). Others, such as desert toads, tend to burrow in moist soils, or stay underground during the warmest weather. (Many toads can also survive at least partially drying out for a period of time.)

 Body structure:

Like with behavior, the key here is being able to cool off using means other than sweating (i.e., evaporation/loss of water). Light-colored skin/fur absorbs less heat, long legs keep animals further from heat radiating off the ground, and larger body surface-to-volume ratios help radiate heat more efficiently .  One of my favorites – large ears which act as organic radiators, such as the ears of elephants or jack rabbits.  The elephant ear is full of blood vessels and has a large surface area allowing that blood to lose heat as it passes through the ears.

African Elephant (from Elephant Pictures site):

Find water in unique places:

Many animals extract water from the food they eat, including plants and other animals. You may suggest that this is a bit of a weak answer since we still have to explain where the water in the food come from? A good questions, but remember that those animals and plants (which have their own adaptations) have been accumulating water in their system, so in a sense they are little concentrated morsels of water.

Be more efficient at storing water:

Camels and gila monsters store water in fatty tissue that can be turned back into water when it is dry.

Kangaroo Rat (from Wikipedia Commons)

kangaroo rat

Retain water more efficiently:

There are a lot of ways animals do this including:

  1. Excreting waste in the form of uric acid that can be expelled as a solid thereby allowing water retention (reptiles and birds can often do this).  Mammal’s urea is soluble though, so they need to pass more water with their urine.
  2. Camels have the ability to raise their body temperature, which reduces the need to lose water through sweating. This is called hyperthermia.
  3. Desert animals often have a lower metabolic rate, thereby producing less heat, and reducing the need to sweat.
  4. Kangaroo rats have several ways of retaining water, including specialized kidneys that are able to remove almost all water from urine, special organs in their nose that reduce the temperature of exhaled air and extract moisture from their breath before it exits the body, and the ability to extract water from the driest of seeds and nuts.

There are other animal adaptations and a lot of plant adaptations, too, but that is a story for another day (or feel free to share your favorite in the comments). (RJV)

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Responses

  1. I’ve always been fascinated with how desert animals survive without much or any water. One thing to add to sources of water is metabolic water, water produced from the metabolism of food and stored energy. Breaking down fat for energy produces quite a bit of water. Water is produced as the oxygen we breath in is combined with hydrogen ions released from fat molecules (and other energy sources). If an animal is really good at conserving water and can build up fat reserves during the wet season it can survive long dry periods. I wish we had this ability. Similar adaptations can be found in marine vertebrates, who are surrounded by water that they can’t drink.

    • Very cool – I was aware of this in concept but was not sure how it worked. Thanks for the addition, Jason!


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