As far as we know, water is fundamental to all life on this planet. But there are places on Earth that receive as little as 0.76 mm of rainfall a year and have no open freshwater sources. To live in these extremely arid environments, evolution has favoured those able to adjust to life with little water, which has resulted in a huge array of interesting and unique adaptations.
While some animals have learned neat behavioural tricks to deal with the pressure of hydration, others physically alter their bodies in some pretty amazing ways. From the poles to the plains of Africa, plenty of critters have figured out how to become the ultimate water-savers, allowing them to live in some of the harshest habitats in the world.
First off, a bit about the driest place on Earth
Sandy deserts mostly come to mind when thinking of arid climates, but it’s Antarctica’s McMurdo Dry Valleys that holds the title of driest place on earth, technically receiving zero rainfall on the average year. The valleys are the largest ice-free stretch of the continent, at 15,000sq.km, with very low humidity levels and usually almost no snow cover. Tucked between snow-laden mountains, the coastal wind that would normally generate rainfall in the region condenses in the high ranges, falling before it can reach the valleys.
Researchers trying to figure out when the last time significant precipitation visited the valley have looked for the presence of beryllium-10, an atmospheric radioactive isotope that falls to the Earth’s surface in rain. So far there’s been no hint of the isotope as deep down as 60cm in the soil, indicating it may have been 14 million years since the Dry Valleys saw rain.
And, the next driest place on Earth
The location of the world’s driest sandy desert may surprise you too, found on the east coast of South America. Chile’s Atacama Desert sits 4km above sea level, making it also the highest-altitude desert on the planet. Being so high up in the atmosphere, there’s far less room in the thin air for moisture, or oxygen. With an estimated 1mm or less of rainfall a year, it’s no wonder NASA uses the Atacama as a rover testing ground, mimicking the surface of Mars. Though 1mm is the average rainfall for the desert, most regions of Atacama only receive rainfall a few times a century, some going without a drop for more than half a millennia.
We’d be amiss here to not mention the planet’s largest hot desert, North Africa’s 9.1 million sq. km Sahara Desert. Smack dab in the middle of a climatic divide, winds arriving from the south are halted before reaching the Sahara and the winter rainfalls of North Africa don’t reach far enough inland. Rainfall is pretty sporadic in the desert on a whole, dependent on the perfect mixture of atmospheric conditions. The whole desert on average is thought to get just 25mm of precipitation per year, with easterly regions seeing 5mm or less on a yearly basis.
Life without water
Evolution truly is a many splendored force. For those species willing to stick it out, or take a risk and try to make a go of it in these incredibly harsh conditions, the rewards can be great. First off, the competition is usually far less intense in such difficult environments, meaning potentially easier to access and more abundant resources to spread around. Despite the Sahara’s massive size, there’s thought to only be some 500-plant species that call the desert home, 70 species of mammals, 90 species of birds and about 100 species of reptile.
When it comes to the Dry Valleys, mostly only extremophiles inhabit the region’s ancient cold soils like mosses and lichen, plus microscopic critters like cyanobacteria, yeasts and nematodes. In the most intense regions of the Atacama Desert microbes also represent a bulk of the holdout populations. Just last year scientists cultured micro-communities from the driest spot in the Atacama, and there’s currently a ‘Dry Limit of Life‘ project running working in the desert to establish precisely what this earthly maximum may be. Birds are the largest group of animals to inhabit the desert, with only a few mammal species and even fewer reptiles found in the region. Some parts of the desert don’t even support insect life.
But keep in mind that there’s generally a lot less resources in arid environments to begin with, imposing limits on the breadth of life that can even contend in these habitats. And acclimatising to such intense conditions comes with consequences.
Gazelles shrink their hearts and livers
How do you decrease water evaporation (aka sweat or breath-droplets) in the world’s hottest deserts? Well one surefire way is to program the body to use less oxygen, ultimately, breathing less and exposing less internal moisture to the outside environment. Gazelles in the Saudi Arabian desert are a prime subscriber to this method, capable of physically altering the mass of their most oxygen-demanding organs.
‘Sand gazelles reduce the size of their heart, liver and stomach under periods of drought,’ says Joe Williams with Ohio State University, one of the researchers who discovered the phenomena. ‘To an extent this reduces the animals’ metabolic rate and the amount of oxygen-demanding tissue.’
Unexpectedly, the same study also concluded that sand gazelles seem to store fat in their brains, helping them through periods of starvation.
To add to their organ-shrinking trick, sand gazelles also employ behavioural habits to hold onto precious water reserves. ‘The gazelles spend a bulk of their time during the day under the shade of the acai tree, whose leaves provide them with water even in the driest times of the year,’ says Williams. ‘They save most activity for night, under cooler conditions.’
Sand grouse parents hold water in their feathers
Found throughout parts of Asia and South Africa, the male sand grouse has adapted special feathers on their bellies and breasts, capable of soaking up water to deliver to their mate and young. Unlike many desert animals, which have adapted to go without water for substantial periods of time, the sand grouse requires the necessary life-liquid daily, meaning watering hole visits are a crucial part of the birds’ lives. Males will often fly in large flocks to locate waterholes, sometimes up to 50 miles, a tiring day-to-day journey for the bird. Sometimes males land as far as five miles from the water source and walk the rest of the way—the breadwinners themselves usually only hanging around long enough to imbibe ten big gulps before heading back to the nest.
The greater roadrunner cries salt
The roadrunner’s no chump when it comes to survival; no wonder Wyllie Coyote had such a hard time catching it—the bird can outrun a human easily and reign victor in a combat battle with a rattlesnake (however the speed record of land mammals like the coyote should far outpace the 2ft bird). While you may not consider crying a water-saving act, in the case of the North American greater roadrunner, tears are a lifesaver. Just like seabirds, roadrunners excrete a salt-liquid concoction through a special gland just in front of the eye, greatly decreasing the need for the water-demanding work of the kidney.
Darkling beetles fog bask
That’s right, it’s not just humans and seals that like to take advantage of basking behaviour. The darkling beetle, a well-adjusted desert insect, is represented by some 25 different species throughout the Namib Desert. Thomas Norgaard spent numerous years studying the water-saving traits of various darkling species, in particular, a skill called fog basking. He explains that the Benguela cold current which passes through the region wards of precipitation, but does lend itself to the creation of rather hardcore fog events.
‘The rainfall in the region is only 10-15mm a year, about 80-90 per cent of which is lost in evaporation,’ says Norgaard. ‘Yet intense fog events can occur up to 60 times per year, moving up to 100 km inland. The beetles have come to utilise this predictable occurrence, as one of the few and only sources yearly of fresh water.’
Norgaard confirmed that two species of darkling use their bodies to trap fog, climbing to the top of sand dunes and positioning themselves head-down. As the fog collides with their bodies, droplets condense, thanks to gravity, pooling together and flowing downwards into the beetle’s mouth.
Other species of darkling have come up with other methods to collect water during these fog events, explains Norgaard. ‘Some create small trenches in the sand, which collect fog,’ he says, ‘then the beetles drink from the sand.’
Thorny devils drink through their skin
At some point during this exploration of deserts we had to come to the Australian Outback. Being so varied, the down-under’s epically arid environment doesn’t get top nods for being so incredibly dry, but once you factor in the heat, cold, bareness and sheer scale of the outback, its likely one of the cruelest environments on this planet next to the poles.
One of the most famous water-scavengers in this environment is the Thorny Devil, aka the thorny dragon or mountain devil. The outback lizard has narrow channels between their scales that draw on capillary action, directing water towards their mouth and eyes. Amazingly these desert reptiles are obligate myrmecophages, their diet more or less restricted to ants. And as far as us humans know, the dragonish devils can live between six and 20 years.
Sand larks let themselves get super hot
Before Williams studied sand gazelles, he worked with desert larks, showing they had substantially lower water-loss levels than larks in other parts of the world, like the Netherlands and arid regions of Spain. ‘Sand larks have the ability to let their body temperatures got up to 45ºC during the day in the sun,’ he says. ‘They also go into burrows in the sand and press their bodies against the cooler sand, which can be around 37ºC, and drop five, six or seven degrees.’
Larks in the desert also tend to have lower metabolic rates and smaller clutch sizes, to accommodate the scarcity of resources, explains Williams. Even the one or so eggs females lay per year often succumb to predation, he adds, via crows, lizards, foxes, small passerines and snakes. Williams found sand gazelles allow themselves to get pretty hot too, but not nearly as warm as the larks, only reaching 42ºC or so during the day.
It’s hard to say how climate changes will impact arid ecosystems and their inhabitants
By now we’re all well aware the planet’s climate is undergoing changes at an unprecedented rate. Even parts of the famously wet Amazon are suffering from the climatic shifts taking place.
But it’s too tricky just yet to quantify what these changes will mean for arid environments and the flora and fauna adapted to them. For starters, these types of changes have happened before. Based on ancient dust blown offshore into the Atlantic Ocean the Sahara Desert was a fairly lush stretch of land just 5 to 6,000 years ago, more like the savannah woodlands of the east and south of Africa. Then the earth went through a period of global cooling, lowering the ability of the air to hold precious moisture from the oceans and carry it inland, and the desert of today was born.
Now some researchers believe this process could be happening in reverse, actually shrinking the desert’s extent. As the global temperature rises, more ocean water droplets make it further inland. Satellite images confirm the southern edges of the Sahara are greening. But could this effect outweigh the multitude of predicted negative climate change impacts? That’s something we probably won’t know for a long, long, time.
The documented climate change impacts on dry-adjusted animals have so far been mostly negative. The famous Quiver tree of western South Africa and Namibia is unable to develop and spread out quick enough to persist under current rates of climate change, and the range of the American Southwest’s iconic Joshua tree is decreasing with less dependable yearly soaking events.
But on an optimistic-concluding note, as with all cases in the natural world, these scary instances could be the result of simply not looking in the right places, or asking the right questions. Just as desert species could be more vulnerable to climate changes being so specified to their environment, it would also make sense that life well suited to extreme weather events, especially dry spells, could have a big leg up in the future. We’ll have to keep our fingers, toes, hooves, and claws crossed that these extreme survivors can rise to the challenge—once again.
More from the desert on Love Nature: