In cooler, more temperate lands many animals become inactive and hibernate
through the harsh season of winter. In deserts a similar “inaction strategy”—
known as estivation—allows creatures to survive the worst of the heat and
drought. Snails, for example, use their mucus to stick their shells to a hard
substrate such as the underside of a shady rock. The mucus dries, creating a
waterproof seal round the shell and keeping the snail moist inside for months.
Some desert amphibians, such as spadefoot toads (genus Scaphiopus) and waterholding
frogs (Cyclorana platycephalus; see pp. 102–103), burrow deep into soil
to estivate through the dry season.
Bats may not seem to be typical desert animals, but of the more than 900
species, some two dozen can survive in arid, treeless landscapes. The great
mouse-tailed bat (Rhinopoma microphyllum) of the Middle East and East Asia,
for example, roosts in
cracks and caves in rocky outcrops or in the eaves of ruined tombs and temples.
It feeds on all kinds of insects and, as the dry season approaches, lays down
extra layers of body fat before settling in its roost and entering an inactive or
torpid state that is a combination of hibernation and estivation. It subsists like
this for two to three months on its stored food, waking up at almost half as
heavy as before its quasi-sleep.
The sparse and unpredictable nature of food availability in deserts has led to
many animals evolving nomadic or migratory habits. In some less arid areas,
regular migrations are tied in with dependable seasonal rainfall, but in true
deserts, where such rains are lacking, migration is more haphazard. The zeren
gazelles (Prodorcas gutturosa) of the Gobi trek to new pastures mainly in spring
and fall and may cover hundreds of kilometers, but the direction they take
depends entirely on the weather and where the pastures of feather grass and
other plants are growing. In winter they may have to move on every two or three
days.
Many other desert creatures follow the same irregular nomadic lifestyle in
search of food and water—from birds such as emus and budgerigars in Australia
to large mammals such as the asses and gazelles of the Sahara. One of the most
notorious examples of these unpredictable migrants is the desert locust (mainly
Schistocerca gregaria and Locusta migratoria, but also many others), whose
arrival en masse from the deep desert inspires terror among the inhabitants of
the tropics. A major swarm can number 10 billion of these insects, turning the
sky black for hours and eating every shred of greenery, including precious crops.
These locust irruptions, or population outbursts, are less common today than 50
or 100 years ago, and early-warning systems, along with detailed weather
forecasts, spotter aircraft, and crop sprays, have helped to stem their devastation.
Now and then, however, unexpected swarms can still inflict damage, bringing
hunger to large areas. Worst affected are Africa and the Middle East, but many
other regions, including the Americas and Australia, also experience periodic
irruptions.
The young, or larval, forms of locusts, just hatched from eggs that their
mother laid buried in the sand, are called nymphs. These locust nymphs, whose
wings are present but too immature for flight, are known as “hoppers,” and this
is what they do. Hopping over the dry scrub and bush, like their close relatives
the grasshoppers and crickets, they use their hard mouthparts to cut and grind up
grasses and other plants. They are well camouflaged in blotchy greens and
browns but still form staple food items for many desert insect-eaters, such as
lizards, toads, and grasshopper-mice.
In most years, when the dry season persists as usual, vegetation is in short
supply and puts a
natural ceiling on hopper numbers. A spell of extra rainfall, perhaps just a week
longer than normal, means a boom in greenery. Along with many other
herbivores, the locusts thrive, growing faster and completing their life cycle
within two months. As the hoppers become more numerous, they also change
appearance, swapping their discreet camouflage for bright stripes of yellow,
black, and orange, entering what is known as their gregarious phase. Wearing
their new, easily spotted colors, they gather into bands of thousands that march
across the landscape, one or two kilometers a day, devouring every scrap of
vegetation. They feed at dusk and dawn and rest during the heat of midday.
After passing through the fifth and final skin-moult, their wings are fully
formed and, as adult locusts, they take to the skies in search of fresh pastures. As
soon as the dryness returns, the remaining locusts, no longer stimulated by
plentiful food and the presence of thousands of their kind, revert to their
restrained solitary phase once more.