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TROPICAL GRASSLANDS
In general, grassland forms a transitional belt between areas of desert and forest. They are regions of intermediate and highly seasonal rainfall where there is sufficient moisture to support a drought-resistant vegetation of grasses, shrubs and in some cases trees.
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Between the fierce aridity of the desert belt and the constant humidity
of the tropical forest regions lies an area where the rainfall is intermittent
and erratic. The dominant plants are grasses and the habitat is one of
open
plains with scattered scraps of brush and woodland. As the region lies
wholly within the tropics, the sun is therefore directly overhead at any
one place
on two occasions each year. Most of the rainfall comes at these times because
the tropical convergence of global winds and the wet conditions associated
with them move north and south with the sun twice every year. The intervening
dry seasons are due to the dry high-pressure belt associated with the deserts
moving over the grassland area.
The dominance of grasses over trees has more to do with the level of soil
moisture than it has to do with the total amount of rainfall. Typically only
the upper soil layers contain water, whereas the lower strata, where tree
roots would be found, remain dry all year. Some areas of grassland receive
substantial amounts of rain, but because it occurs only at certain times
of the year trees are unable to establish themselves.
Because of the general dryness of the region it is highly susceptible to
fire. Indeed it is repeated destruction by frequent grassland fires that
has produced the plants that are characteristic of the habitat. The trees
are particularly hard and fire-resistant and the grasses grow from their
bases rather than from the tips of their leaves and stalks. They also spread
by means of underground runners, which allows their instant recovery after
fire has swept the area and destroyed the exposed parts.
The rapid recovery of trees and grasses from damage permits the grasslands
to support large numbers of grass-eating animals despite the frequent fires.
Only the upper parts of the leaves and stems are eaten, leaving the growing
bases and the underground runners intact. Another feature of the grasslands
that has an important influence on the fauna is the sparseness of cover.
A grazing animal can therefore be seen by a predator from a great distance,
and conversely the grazing animal can see danger coming. Hence both grazers
and predators in these regions are highly adapted for speed and pursuit,
and have long legs and quick reactions. Some birds, too, have found that
they can survive on the grasslands using only their legs to take them out
of trouble without recourse to flight.
Another feature of tropical grassland life is migration. Because of the seasonality
of rainfall, different areas of the grasslands provide food at different
times of the year and hence great migrations of grazing herds occur throughout
the year. Migration also takes place to the grasslands from other parts of
the world. Many birds summer in the temperate woodlands, far to the north,
and fly south to the grasslands to escape the winters.
As with the desert belts, the total land area of the globe lying within the
tropical grassland climatic belt has diminished since the Age of Man due
to the constant northward movement of the Australian continent. Although
the largest extent of tropical grassland is on the African sub-continent,
considerable expanses also exist on the South American island continent south
of the equatorial rain forests.
Ground-dwelling birds and herbivorous herds
The picktooth.
Only two of the picktooth's toes are functional. The fourth toe on each of its front feet has developed into a spur.
The picktooth's scull is similar to a rabbit's. Its tusks are developed from the front incisors.
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The strank's stripes produce a confused impression from a distance. The strank.
The watoo. Blotched markings give the watoo camouflage in areas of scrubby woodland.
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The grasslands, both tropical
and temperate, are the home of the running animals. The long vistas and
the general lack of cover make concealment
difficult and speed is the most practical means of defence.
The grasslands first appeared on a large scale about 80 million years
ago, when a general reduction of global temperatures, causing a drop
in average
rainfall, produced a reduction in the area of forest found on the earth.
At this time the mammals that had been in existence for about 20 million
years developed running forms in large numbers for the first time.
The grasses, representing a vast untapped food source, had however a
high
silica content,
which made them much tougher than the leaves of trees to which the
browsing mammals were accustomed. To deal with this more fibrous material
new
tooth structures appeared that had hard enamel ridges and cusps to
grind down the
grass before it was swallowed. New, elaborate digestive systems were
also evolved to deal effectively with it,
By the Age of Man, the long-legged grazers, the ungulates such as the
zebra, Equus, and the gazelle, Gazella, were the most successful animals
of the
tropical grasslands. However the rabbucks, which originated in the
temperate woodlands, after man's extinction, spread southwards, round
the mountain
barriers, into the African and Indian sub-continents, where they flourished
and competed so effectively with the ungulates that in time they largely
replaced them.
Although many forms of rabbuck inhabit the same region, because of
their different feeding habits they do not compete directly with one
another.
The little picktooth, Dolabrodon fossor, feeds on low-growing herbs
and roots,
which it digs up with its tusks and spurs. Its second incisor teeth
are developed into strong laterally directed tusks and it has long
spur-like
claws on the
fourth digit of each forefoot. As it runs only on the second and third
toes of each foot, the spurs do not hinder it.
The taller grasses are grazed by vast herds of larger rabbucks, Ungulagus spp. They tend to be similar to their temperate cousins, but are on
the whole lighter in build and have longer legs and ears. Their coloration
is very
different, consisting mostly of pale brown and white arranged in stripes
or spots depending on the species. The strank, U. virgatus, has a dazzling
pattern of stripes like the extinct zebra, while the larger watoo,
U.
cento, carries large angular blotches similar to those once possessed
by the giraffe.
Such patterns make individuals merge into one another so that a distant
predator gets only a confused impression of the herd as a whole. It
is particularly
effective in thorn thickets and areas of scrubby woodland. All rabbucks,
whether temperate or tropical, retain the dazzling white tail of their
rabbit ancestors. It is used as a warning signal when the herd is attacked.
The tropical grasslands are the home of a species of large flightless
guinea fowl, Pseudostruthio gularis. Standing about 1.7 metres high,
it sports a
startling selection of erectile wattles and inflatable throat pouches,
which are used in threat displays when dominance or pecking order is
threatened.
It is an omnivorous bird and feeds on seeds, grasses, insects and small
reptiles. Although it can deal a lethal blow with its broad feet in
common with most
plains-dwelling animals it runs off when real danger threatens.
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The place of large herbivores in a tropical environment
The long-necked gigantelope is a browser and eats the leaves and shoots of trees.
It has two vestigial horns, no more than bony pads on the top of the head.
Unlike most members of the gigantelope family, long-necked gigantelopes are not herd animals. They are typically found in ones and twos in lightly wooded areas around the margins of the tropical forests.
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The gigantelopes' immediate ancestors had long, double-pointed antler-like horns.
In some, the rear portions disappeared, leaving long forward-pointed prongs. Until recently the shovel-horned gigantelope was found on the grasslands. It is thought to have lived near rivers and lakes, and to have fed mainly on water plants.
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The elephants flourished
throughout the first half of the Age of Mammals, but with man's appearance
their numbers fell until they had almost
become extinct. Two genera only, Elephas and Loxodonta,
were latterly contemporaries
of man and both of these died out shortly before man's disappearance,
leaving no descendants. The ecological niche which they vacated
was eventually filled
by the descendants of a surviving group of antelopes, the gigantelopes.
These enormous creatures with tree-trunk legs and weighing up to
ten tonnes became
the giant herbivores of the tropical plains, a group of animals
feeding on trees, grasses or roots depending on the species. They had long
since abandoned
the antelope's running gait and had instead taken up a plodding
existence
- the two-toed feet of their ancestors having become broad-hooved
pads.
The typical grassland-dwelling type, Megalodorcas giganteus, has
four horns - one pair curving down behind its ears and another
pair pointing
out in
front of its snout. Each horn has a pick-like point, enabling the
animal to scrape soil away from the plant roots and bulbs on which
it feeds.
The animal's basic shape was highly successful and in the course
of time the gigantelopes spread northwards from tropical Africa,
crossing
the
Himalayan Uplands in two separate waves of migration; one spreading
into the coniferous
forests and giving rise to the hornheads, Cornudens spp., and the
other, much later, reaching the tundra and providing the ancestors
of the
woolly gigantelope, Megalodorcas borealis.
Once the massive body of the gigantelope had been established a
number of variations appeared. The earliest was the long-necked
gigantelope,
Grandidorcas
roeselmivi, a gigantelope able to browse on twigs and branches
7 metres above the ground, well out of reach of the smaller herbivores
and even
of its own
massive cousins. As well as a long neck this animal also has a
long,
narrow head, enabling it to push its thick muscular lips between
the branches of
the trees to reach the tastiest morsels. The horns of its ancestors
are reduced to long, low, bony pads at the top of the skull. Anything
more
elaborate
would become entangled in the branches.
At first glance these massive beasts seem to contradict the general
rule that animals of hotter climates tend to be smaller than their
equivalents
in cooler areas. The larger an animal is, the smaller its surface
area is in relation to its body mass, and the more difficult it
is for it
to lose
excess heat. In the case of gigantelopes, however, this problem
is overcome by the possession of a large dewlap beneath the neck,
which
is well served
with blood vessels and effectively increases the creature's body
area by about a fifth, thus providing an efficient heat radiator.
The rhinoceros, another of the massive tropical grassland animals
that became extinct during the Age of Man, has an almost direct
equivalent in the gigantelopes
- the rundihorn, Tetraceras africanus. It has adopted a body size
and
a horn arrangement not unlike its predecessor's and is a grazing
animal, a fact
that is reflected by its broad snout and muzzle. Its alarming horn
array is used for defence, although the animal has few enemies
likely to risk
a frontal attack. For the males, however, its secondary function
- for sexual
display - is now more important.
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Predators and scavengers of the plains
The raboon is directly descended from baboon. It has evolved a bipedal stance and much heavier hind quarters.
Male raboons are larger than females, and only males have manes. Their teeth follow the general carnivore pattern. Female
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The ghole's head and neck are totally hairless.
Its massive canine teeth and molars are designed for breaking and crushing bones.
Gholes frequently devour their food beneath the shelter of overhanging termite mound, where they find protection from the sun. The termites in return feed on the remaining scraps.
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Although the two principal predators of the tropical grasslands
of the African sub-continent are both primates, they have
evolved along
very
different lines
and hunt different prey.
The horrane, Phobocebus hamungulus, is descended from the
tree-dwelling apes of the tropical forests, a fact indicated
by the way that
the animal walks
on the knuckles of its forefeet. It leads a totally ground-dwelling
carnivorous mode of life. Lying in the long grass, where
it is camouflaged by its
stripes and mane, it waits for its chief prey, the gigantelopes.
As they pass by,
the horrane leaps out on to the back or neck of its quarry,
using its sickle-like claws to rip deep wounds around the
neck and
throat. Severely
wounded, the
gigantelope soon dies, providing a meal for the whole horrane
family group.
The other main predator is the raboon, Carnopapio spp. Descended
from the baboons that flourished on the grasslands during
the Age of Man,
their diet
changed from omnivorous to carnivorous during the period
that the big cats, of the grasslands, died out. At the same
time
they increased
their speed
by taking to their hind limbs and adopting a totally bipedal
locomotion. The forelimbs became reduced and the head was
carried further forward,
balanced by a thick, heavy tail. In physical form the raboon
bears a distinct resemblance
to the carnivorous dinosaurs that died out more than a hundred
million years ago.
A number of species of raboon, each living on a different
species of prey, exist in family-based tribes, like the ancestral
baboons.
Carnopapio
longipes is a very small, lightly built species about 1.8 metres high
that hunts smaller animals. C. vulgaris is the most widely
ranging species
and
preys on the
rabbuck herds. C. grandis is the most massive member of the
genus. It stands about 2.3 metres high at the hip and lives
purely as
a scavenger. As predators
such as the horrane eat only the softer tissues and muscles
of the gigantelope's
belly and anal regions there is always plenty of meat left
for the scavengers. The giant raboon concentrates on the
meat of
the limbs
and neck, leaving
the rest to smaller, less powerful carrion feeders.
The most efficient scavenger of the African grasslands is
the ghole, Pallidogale nudicollum, a creature that
resembles a
large mongoose.
Its head and neck
are almost totally devoid of hair, allowing it to reach inside
the body cavities of carcases without its coat becoming fouled.
Its canine
teeth
are particularly
huge and are capable of crushing most bones to get at the
marrow. Gholes live in packs of about a dozen and have developed
an
almost symbiotic
relationship with a species of termite. This termite builds
its mound with a horizontal
shelf projecting out all round, a metre or so above the ground.
The shelf provides shelter from the fierce midday sun where
the ghole
can bring
bones and other tough parts of its meal to chew at leisure,
The termites feed on
the scraps of carrion that the ghole invariably leaves scattered
around the mound, thus benefiting from the relationship.
It usually takes
about three
days for the predators and scavengers of the grasslands to
reduce a gigantelope to no more than a few pieces of bone
and hide and
a patch
of stained,
trampled ground. The final remnants are consumed by insects
and micro-organisms.
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INTRODUCTION BY DESMOND MORRIS 9
Cell Genetics : Natural Selection : Animal Behaviour
: Form and Development :
Food Chains
The Origins of Life : Early Living Forms : The Age of
Reptiles :
The Age of Mammals : The Age of Man
The World after Man
TEMPERATE WOODLANDS AND GRASSLANDS 36
The Rabbucks : The Predators : Creatures of the Undergrowth
:
The Tree Dwellers : Nocturnal Animals : The Wetlands
The Browsing Mammals : The Hunters and the Hunted : Tree Life
TUNDRA AND THE POLAR REGIONS 58
The Migrants : The Meaching and its Enemies : The Polar
Ocean :
The Southern Ocean : The Mountains
The Sand Dwellers : Large Desert Animals : The North American Deserts
TROPICAL GRASSLANDS 78
The Grass-eaters : Giants of the Plains : The Meat-eaters
The Tree-top Canopy : Living in the Trees : The Forest
Floor :
Living with Water : Australian Forests : The Australian Forest Undergrowth
ISLANDS AND ISLAND CONTINENTS 100
South American Forests : South American Grasslands :
The Island of Lemuria :
The Islands of Batavia : The Islands of Pacaus
The Destiny of Life
Glossary : The Tree of Life : Index : Acknowledgements
