The Great Sand Sea
Although it's not easy living in the extreme landscape of Africa's Kalahari, some fascinating wildlife thrives there
by Miles Roberts
At almost a million square miles, the Kalahari sprawls over the heartland of southern Africa like a great sandy sea. It is a vast, arid plateau ringed by ancient mountains and blanketed by the Kalahari sand sheet, probably the largest continuous mass of sand in the world.
From the air, the Kalahari presents the quintessential desert landscape. Ochre-red dunes ripple endlessly in neat parallel lines. Dry, fossilized riverbeds snake among them, vestiges of a time when surface water flowed freely. And countless pans—shallow, circular depressions a few meters to several kilometers in diameter—dot the landscape, revealing where lifegiving water once was and will be again after rain.
|The extraordinary color of the Kalahari sands come from the kaleidoscopic mixture of minerals that blankets the bedrock. (Miles Roberts/NZP)|
On the ground, it is all about sand—every conceivable manifestation of the stuff. Nine-tenths of the Kalahari’s sand is fine-grained, gritty white, pink, or red quartz. The rest is a kaleidoscopic mix of zircon, garnet, feldspar, and tourmaline. It covers ancient bedrock in a deep, structureless infertile sheet. This layer is topped by a thin veneer of organic matter that is distributed in tiny islands of fertility known as “biological soil crusts.” Drought-resistant microorganisms reside here, binding soil particles together, fixing atmospheric carbon dioxide, and adding significant nutrients to the soil.
The Kalahari environment is famously harsh. Summers are brutally hot, winters bone-chillingly cold. Incessant winds sandblast and disfigure the landscape, and the vegetation is eternally coated with dust and grit. While there is an annual “wet season,” months, sometimes years, pass in some places with little or no rain. With the exceptions of the Orange River on its extreme southern edge and the Kwando and Okavango rivers in its center, the Kalahari has almost no year-round surface water. Yet it is alive with plants and animals remarkably adapted to surviving in this extreme environment.
In late spring and summer, after months of winter drought, clouds gather in the vast Kalahari sky and rain falls on the parched landscape. The rains awaken dormant plants and their pollinators and stimulate great movements of animals seeking water and food. Lions, hyenas, jackals, and cheetahs shadow herds of hoofed mammals. Water birds—flamingos, pelicans, ducks, and geese—flock to the pans in surprising numbers to feed and reproduce. But the water’s presence is ephemeral. In the pans, it remains only a few days or weeks before evaporating. Elsewhere it is quickly used by vegetation or absorbed by the sandy soils.
Though sometimes called a desert, the Kalahari is almost entirely covered by a mosaic of grass and woodland. But for long stretches of the year, the only water available for plants and animals lies underground, mostly sealed away in deep aquifers. Kalahari plants are supremely adapted to these arid conditions and are adept at finding, sequestering, and conserving scarce water. Many, like grasses and vlei lilies, sit out long dry periods on or below the soil surface in the form of dormant seeds, bulbs, and corms that develop rapidly when exposed to water. Many shrubs, trees, and creepers resist dehydration with physical adaptations: small, tough, waxy leaves; thick bark; a small size; spongy, water-storing wood; and sprawling, water-seeking root systems. A few, like the tsamma melon and gemsbok cucumber, store water in prominent fruits and are an important source of moisture for many animals.
|Despite the harsh climate, vegetation thrives, and gives much-needed shade to the other desert residents. (Miles Roberts/NZP)|
The Kalahari’s animals are as adept as its plants at conserving water and contending with extreme heat and cold. Ectotherms, the so-called “cold-blooded” animals (such as reptiles, amphibians, and arthropods) whose body temperatures vary with that of the surrounding environment, have a fairly simple strategy. To warm up, they are active during the day and hang out in warmer, sunnier spots as needed. To cool down, they seek shade, burrows, or crevices away from the sun and heat. Because they are unable to adjust their body temperatures, many ectotherms find themselves at the mercy of cooler temperatures at night and retreat to safe hiding places.
Some ectotherms take matters into their own hands and construct climate-controlled habitats that would make environmental engineers proud. The most prominent of these are desert termites, whose homes are massive constructions made of bits of soil “glued” together with pasty spit. Each termite species has a distinct architectural style: some construct amorphous subterranean bunkers, while others create great obelisks and fluted spires that may rise three or more meters into the air. The interiors of these structures are lined with honeycombed passageways that serve as safe transit routes for their soft-bodied inhabitants, and as a ventilation system that helps maintain a constant interior temperature and humidity.
The termites’ communal industriousness inadvertently benefits other wildlife as well. Many Kalahari animals—aardvarks, aardwolves, dwarf mongooses, some lizards, birds, and even ants—are specialized termite predators. Some catch their prey by stealthily entering mounds, others sit and wait outside for prey to emerge, or simply bulldoze their way through the thick walls.
A blue agama lizard basks under the Kalahari sun. (Miles Roberts/NZP)
Among the larger ectotherms, two groups stand out. Most Kalahari snakes are what herpetologists call “hot”—they are highly venomous. One notable is the puff adder, a common, highly venomous, but sluggish serpent whose hunting strategy is to sit motionless in its camouflaged skin and wait for unsuspecting prey to amble within striking range. This habit is certainly an energy-saver, but a particularly dangerous one for non-prey–especially bare-legged tourists in flip-flops, who occasionally venture too close to a hiding puff adder. But the reigning monarch of Kalahari snakes is the cape cobra—a large, startlingly yellow snake with cold, jet black eyes, lightening speed, and a decidedly cranky disposition. And, if that’s not scary enough, one of its favorite meals is the puff adder.
Scorpions, which are ubiquitous in the Kalahari, come mainly in two forms: large and black with slender tails and huge pinchers used for capturing and crushing small prey, or small and tan ones with small pinchers and bulbous tails loaded with deadly venom. Both types are everywhere— on the ground, in houses, in shoes, in shower drains—but surprisingly the small, tan scorpions are more dangerous.
The larger Kalahari animals—mammals and birds in particular—are mostly endotherms, or animals that control their own body temperature more or less independently of the environment. Many are familiar to visitors of the Smithsonian’s National Zoo and other zoos: lions, leopards, cheetahs, wild dogs, spotted hyenas, zebras, giraffes, ostriches, and meerkats. Others may be less well known, but are icons of the Kalahari landscape: gemsbok, springbok, wildebeest, kori bustards, palechanting goshawks, ground squirrels, and the ever-present jackals. Nighttime in the Kalahari brings out aardvarks, aardwolves, bat-eared foxes, and springhaas.
Zebras have adapted to the unforgiving Kalahari climate. (Miles Roberts/NZP)
The endotherms have a wide range of physiological and behavioral adaptations that enable them to live and even thrive in this hostile, near waterless environment. A gemsbok—a large antelope—can live for months without drinking and has a remarkable blood-vessel, heat-exchange system in its sinuses that helps it get rid of heat. Bat-eared foxes have giant ears for detecting subterranean animal prey like termites and scorpions. Ground squirrels shade themselves against the hot sun with their huge, umbrella-like tails. Antelope (springbok, wildebeest, hartebeest, dikdik, eland, kudu, and others) forage at night, dawn, and dusk, and take refuge in the shade of trees and shrubs during the heat of the day.
Meerkats live in cooperative groups that forage collectively during the day for protection and huddle together in burrows at night for warmth. Sand grouse carry water back to their nestlings in spongy spaces between feather barbules. Rodents of all sizes (striped mice, springhaas, porcupines, and others) spend hot days in insulated burrows or arboreal nests and forage at night when it’s cool. Social weaver birds build massive communal nests, some weighing over a ton, which provide protection from predators and insulation against temperature extremes. Many birds simply migrate to more suitable climates and return when rain brings a flush of vegetation and fresh food.
The severe Kalahari environment has been the crucible of evolution for a remarkable group of animals and plants that are superbly adapted to life within the thinnest of biological margins. But the Kalahari is a fragile environment, and we must wonder how it will respond to change, already evident in fence lines, pipelines, roads, and tracks that crisscross the landscape. How will the lives of its creatures, already in a delicate balance, be affected? Only the passing of the sands of time will tell.
—Miles Roberts is a wildlife biologist with the National Zoo’s Center for Conservation and Sustainability. His interest in ancient deserts began when he was growing up in Australia.
Pans—small, closed basins with hard, alkaline floors—are common in low-lying areas. They capture and temporarily
Cooperation is a strategy that pays off big time in extreme conditions like the Kalahari’s. Meerkats inhabit the most arid environments of any mongoose, and, not coincidentally, are legendary practitioners of social cooperation. They typically live in bands of four to ten individuals that forage together and vigorously defend their territory against other bands. They also sleep communally in burrow systems that are often shared with ground squirrels. Meerkats are cooperative breeders and while usually only a single dominant pair in each band reproduces, all adult group members help with the guarding and feeding of young. In the Kalahari, meerkat survival and breeding rates are closely linked to rainfall. In very dry years, overall mortality increases and reproduction may cease altogether, leading to a marked decline in the number and size
Lions face extraordinary challenges in the harsh Kalahari environment. They must roam great distances in search of prey, but must also remain close to water. In the dry season, lions frequent waterholes where they can wait and ambush thirsty prey, from porcupines to giraffes. In wetter summer months, lions often travel more than 12 miles per day in open habitat in search of dispersed prey. It is under these difficult foraging conditions, which can last eight months or more each year, that hunting in coordinated, cooperative groups becomes particularly critical to survival. But even under the best of conditions, the mortality of Kalahari lions is among the highest in Africa.
If you have a comment about Smithsonian Zoogoer magazine, please email it to us.ZooGoer 38(4) 2009. Copyright 2009 Friends of the National Zoo.