Biology concepts – water conservation, kidney function, metabolic water, adaptation, water uptake
“Koala” in aborigine means “no drink.” The
moist eucalyptus leaves are poisonous
to most animals, but koalas have a special
bacteria that can break down the toxic
Koalas eat eucalyptus leaves, as well as mistletoe and a few other leaves. The leaves contain a good amount of water, and the koalas can survive on just this source of moisture. It also helps that they sleep about 18 hours each day, have a very slow metabolism, and feed about 80% of the time they are awake - it is apparent that they have evolved into teenagers. This doesn’t mean that koalas can’t or don’t drink, they just don’t require drinking to get their daily requirement of water unless a drought dries up the leaves.
However, there exist species that never drink. The kangaroo rat and the spinifex hopping mouse take temperance to the extreme. These rodents can live out their entire life (5-7 years) and never use the water fountain. They have chosen their lifestyles wisely, considering that the hopping mouse lives in the Australian outback and the kangaroo rat lives in Death Valley! We will use the kangaroo rat as our exemplar for this exception.
Unlike the koala that gets its water from its diet, the kangaroo rat eats seeds- not a great source of water. Therefore, it must have other strategies for survival. Foremost, it has developed ways to prevent water loss. Its kidneys super-distill its urine so it is up to 17 times more concentrated than its blood; the best we can do is 3-4 times concentration.
Please meet the nephron. The blood vessels form a
glomerulus, which is surrounded by the Bowman’s capsule.
Notice how the blood vessels surround the Loop of
Henle to take the retained water and salts back into
The kangaroo rat doesn’t look thirsty,
even though it doesn’t look like his
burrow has seen water for years.
I would imagine that despite the hot
weather and the fur coat, kangaroo
rats don’t sweat; they can’t afford the
The kangaroo rat doesn't stop there. He burrows deep and keeps his burrow small. This helps to trap and moisture that escapes via his exhalations. If you breathe on a mirror, it will show condensation; you invest a lot of water in keeping your lungs moist and functional. The rat can reabsorb some of the moisture present in its burrow via its skin, respiratory tract, and his seeds.
The dry seeds that the kangaroo rat finds are stored in a pouch in its mouth and taken back to the burrow. Here they are stored for several days in a corner, during which time they also absorb moisture from the burrow’s air. This is just another way the rat recycles some of its own moisture.
Finally, the kangaroo rat makes the most of the water it produces. Yes, it generates water – but so do you. Think of the production of ATP (aerobic respiration) as the opposite of photosynthesis. In the building of carbohydrates (during photosynthesis). In photosynthesis, water is split and the hydrogen is added to the growing carbohydrate. But in the electron transport chain for oxidative phosphorylation (making ATP) oxygen accepts an electron and then reacts with hydrogen to form water. Water made this way is called metabolic water. In humans, metabolic processes like generation of ATP produce about 2.5 liters of water each day. In the kangaroo rat, this process is more efficient and the water produced is kept in house.
Add all these measures up and the kangaroo rat changes its habitat from Death Valley to Life Valley. Unfortunately, not many other organisms can join it there.
Just because it doesn't drink or eat watery foods doesn’t necessarily mean that an organism doesn’t take in water. Amphibians absorb environmental (air or surface) water through their skin. Frogs are a group of amphibians that can be used as good examples. Frog skin is smooth, without hair or feathers, and is permeable to water. A ventral patch (sometimes called a seat patch) of skin is located on the underside of the frog between its two hind legs. This skin patch has a higher concentration of blood vessels just beneath the surface, ready to suck available water into the bloodstream.
To get to the blood vessels below the skin, the water passes through a series of aquaporin (aqua = water, pore = opening) protein channels in the skin cells. These proteins also control water entry into bacteria; they are evolutionarily very old and therefore must be important. The frog splays its legs and lays down on a surface that is moist from dew or rain, and the water flows through the ventral patch aquaporins and into the bloodstream. Interestingly, water doesn’t flow the other direction, although some water does evaporate through amphibian skin. That is why frogs must live close to water. Toad skin is much less likely to lose water, so they can live farther from water.
Some plants also garner water in unconventional ways. Non-vascular plants (mosses, lichens, liverworts, hornworts) as well as many epiphytes (bromeliads, orchids, some ferns and mosses, mistletoe) are plants without roots. However, a lack of roots or vessels doesn’t stop these plants, they have evolved marvelous adaptations to procure the water they must have.
Non vascular plants are just that – plants without vascular tissues (xylem and phloem). Plant vascular tissues are tubes inside the stem that transport water (phloem) and sugars (xylem) throughout the plant. Non-vascular plants don’t have roots and vessels to absorb and transport water and minerals, although mosses and ferns may have rhizoids that serve that purpose. In general, non-vascular plants grow close to water so that they can use all their structures to absorb water by capillary action as well as by absorbing water directly from the air.
Epiphytes are even better at pulling water from the air, although they still use pooled rainwater as well. This group of plants may have dense root systems, but some are not anchored in the ground to give support to the plant. Instead, many of them use other plants for support. Orchids are particularly good at storing water in their thick stems and absorbing water through their exposed roots. Velamen (latin for veil or cover) layer root cells of orchids are adapted to prevent water loss while a few cells in this layer and the layer below are hollow and allow water to pass through.
Bromeliad epiphytes are better at absorbing pooled water and humidity through their leaves than in taking water in through their roots. In tropical regions, they have two adaptations to aid this process. One, many bromeliads have near vertical leaves shaped to trap water at their bases (together called a tank) that may hold over a liter of water. Second, they have specialized cells at the base of the leaves to transfer this water (and minerals) to the interior of the plant. The most economically important of this Bromelioideae subfamily is the pineapple, which is a terrestrial bromeliad. It can absorb water through its roots in the ground, but if you are growing one, try to keep the tank from drying out as well.
The top picture is looking down on a bromeliad trichome.
The middle picture is looking from the side. See how they
curl up to allow water in. When they fill with water,
they fold down (lowest picture), to prevent water loss
from the cells underneath.
Curling forms a small cavity under the disc that draws water in to the protected foot cells under the disc by capillary action. These cells also have aquaporin proteins that draw the water into the interior tissues. When there is less water around, the disc cells flatten out and cover the stalk cells, preventing water loss. The whole structure acts like an anti-umbrella!
So organisms can get water from air, food, or metabolism - but we can go them one better. There is an animal that doesn’t eat or drink during its entire adult life, can you imagine? O.K. – so its life is only five minutes long, but it doesn’t eat or drink during that five minutes.
Adult female sand burrowing mayflies (Dolania Americana) emerge from their water-borne larval form and seek two things, a male for mating, and a place to deposit her eggs. Since all larvae are evolved to mature at once, males are around in large numbers; problem 1 solved. And since they live near water, place to lay eggs are also plentiful; problem 2 solved. Within five minutes, her work is done and she dies – not a glamorous life.
Different species of mayfly live varying amounts of time – some live as adults for up to 2 days - oldtimers! But even if the mayfly wanted to invest some of their precious time in eating and drinking, they couldn’t do it. Adult mayfly mouthparts are vestigial (having become nonfunctional through evolution) and their digestive systems disappear as they mature. So in this biological case, a lack of form follows a lack of function.
There is another crucial element of life that interacts with water, and ocean going organisms are intimately familiar with it. Salt is just as important for life as is water, but why? We will begin looking into the functions of salts and how they interact with water next time.
Banta MR (2003). Merriam's kangaroo rats (Dipodomys merriami) voluntarily select temperatures that conserve energy rather than water. Physiological and biochemical zoology : PBZ, 76 (4), 522-32 PMID: 13130431
King RF, Cooke C, Carroll S, & O'Hara J (2008). Estimating changes in hydration status from changes in body mass: considerations regarding metabolic water and glycogen storage. Journal of sports sciences, 26 (12), 1361-3 PMID: 18828029
For more information, classroom activities, and laboratories about water uptake, renal function, trichome, or mayflies:
Animals that don’t drink –