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TROPICAL FORESTS
Tropical forest is found in equatorial latitudes, where converging air currents bring large quantities of rain to the region at all seasons. This, combined with the constant high temperature, produces the forest's characteristic luxuriant vegetation.
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The tropical forests are found in a broad belt encircling the world at the
equator, broken only by oceans and mountains. Their distribution coincides
with the band of low-pressure areas that occurs where rising tropical air
is replaced by moist air flowing in from the north and south to form a system
of converging winds.
The rain forest is the floral product of great heat and copious moisture. At
all times the average temperature must be between about 21°C and 32°C and the
annual rainfall in excess of 150 centimetres. As the sun is roughly overhead
throughout the year, the climatic conditions have a constancy found in no other
habitat.
Tropical forests are often associated with great rivers, which carry away the
copious rainfall. Such rivers are found in the South American island continent,
the African sub-continent and the sub-continent of Australia.
Despite the constant fall of discarded leaves the soils of the rain forests are
very thin. The conditions are so favourable for decomposition that humus does
not have a chance to form. The tropical rain washes the clay minerals out of
the soil, preventing important nutrients such as nitrates, phosphates, potassium,
sodium and calcium from being retained as they are in temperate soils. The only
nutrients found in tropical soils are contained in the decomposing plants themselves.
There are many variations on the basic form of tropical forest resulting both
from climatic and local environmental differences. Gallery forest is found where
the forest comes to an abrupt halt, as at the edge of a broad river. Here the
branches and leaves form a
dense wall of vegetation reaching to the ground, to take advantage of light coming
in from the side. Less luxuriant monsoon forests exist in regions where there
is a distinct dry season. They are found at the edge of continental areas, where
the prevailing winds blow from the dry interior at one particular time of year,
and are typical of the Indian peninsula and parts of the Australian sub-continent.
Mangrove forest is found in saline swamp areas along muddy shorelines and the
mouths of rivers.
There are no dominant species of trees in the tropical forest as there are in
other forest habitats. This is because there are no seasons and therefore the
insect population does not fluctuate; the insects that feed on a particular species
of tree are always present and will destroy the seeds and seedlings of that tree
if they are sown nearby. Therefore the only seeds that flourish are those that
are transported some distance away from their parent and its permanent insect
population. In this way stands of particular tree species are prevented from
forming.
The area of tropical forest has increased considerably since the Age of Man.
In the past a great deal of damage was done to the habitat by man's agricultural
practices. Primitive societies cut down areas of trees and farmed the clearings
for a few years until the thin soil became exhausted, compelling them to move
on to another area. In the cleared areas the original forest did not immediately
re-establish itself and it was many thousands of years after man's extinction
before the tropical forest belt returned to anything like its natural condition.
A world of gliders, climbers and perchers
The long-armed ziddah. The ziddah curls itself into a ball to sleep. First it wraps its arms across its body, and then brings its legs close in against its chest.
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Coiled against a tree, anchorwhip lies in wait.
With jaws gaping it shoots out to seize a passing bird in mid-flight. |
The tropical forest is one of the most luxuriant habitats on earth. The
high rainfall and stable climate mean that there is a perpetual growing season
and there are therefore no periods in which there is nothing to eat. The
copious vegetation, thrusting upwards to reach the light, although continuous,
is arranged very roughly in horizontal layers. Most photosynthesis takes
place at the very top, in the canopy layer, where the tops of the trees branch
out to form an almost continuous blanket of greenery and flowers. Beneath
this the sunlight is more diffused and the habitat consists of the trunks
of the taller trees and the crowns of those that do not quite reach the canopy.
The forest floor is the gloomy domain of shrubs and herbs, which sprawl out
to make the best use of the little light that filters down.
Although the tremendous variety of plant species supports an equal diversity
of animal species, the number of individuals in each is comparatively small.
This situation is exactly the opposite of that found in harsh environments
such as the tundra, where, because few life forms can adapt to the conditions
of the region, there are many fewer species of either plants or animals but
correspondingly more individuals in each. As a result the animal population
of the tropical forests remains stable and there are no cyclical plagues
of either predator or prey species.
Birds of prey such as eagles and hawks are the important predators of the
tree tops, as they are in any other habitat. The tree-living animals of these
regions must be swift enough to elude them and also to escape from tree-climbing
predators coming up from below. The mammals that accomplish this best are
the primates - the monkeys, apes and lemurs. The long-armed ziddah, Araneapithecus
manucaudata, of the African sub-continent has taken these specializations
to the extreme, and has developed long arms and legs, fingers and toes, so
that it can brachiate, or swing, its tiny globular body through the branches
of the trees at high speed. It has also evolved a prehensile tail, just as
its South American cousins did in the first half of the Age of Mammals. Its
tail, however, is not used for locomotion but only for hanging from when
resting or asleep.
The flunkey, Alesimia lapsus, a very small marmoset-like monkey, has become
adapted to a gliding mode of locomotion. In this development it parallels
the evolution of many other mammals that have evolved gliding wings, or patagia,
from folds of skin between the limbs and tail. To support the patagia and
deal with the stresses involved in flight the backbone and the limb bones
have become remarkably strong for an animal of this size. Steered by its
rudder-like tail the flunkey makes great gliding leaps between the crowns
of the highest trees to feed on fruit and termites.
Among the tree-living reptiles of the African rain forest perhaps the most
specialized is the anchorwhip, Flagellanguis viridis - an extremely long
and thin tree snake. Its broad, grasping tail, the most muscular part of
its body, is used to anchor it to a tree while it lies coiled and camouflaged
among the leaves of the tallest crowns in wait for an unwary passing bird.
The snake is capable of darting out three metres, equivalent to about four-fifths
of its body length, and seizing its prey while still retaining a tight hold
on the branch with its tail.
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The evolution of life under threat
The clatta is found in the lower branches of trees.
When attacked, the clatta drops down, presenting the predator with an impenetrable horny-plated tail.
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Female and young male khiffahs possess neither armour nor claws. They are the colony's principal food gatherers.
The khiffah's nest is divided into two levels containing separate storage and living spaces. The whole structure is roofed with a full thatch of leaves and twigs. |
During most of the Age of Mammals the apes and monkeys enjoyed a degree
of security among the tree tops. For even though there were some predators,
none was adapted to prey on them specifically - but that was before the striger.
This fierce little creature, Saevitia feliforme, developed from the last
of the true cats about 30 million years ago and spread throughout the rain
forests of Africa and Asia, its success hinging on the fact that it was as
well adapted to life in the trees as its prey. The striger even adopted the
bodily shape of the monkeys on which it fed; a long, slender body, forelimbs
that could swing apart to an angle of 180°, a prehensile tail and opposable
fingers and toes that allowed it to grasp the branches.
With the coming of the striger the arboreal mammal fauna of the tropical
forest underwent considerable change. Some of the slow-moving leaf- and fruit-eating
animals were wiped out completely. Others, however, were able to adapt in
the face of this new menace. As usual, when an environmental factor as radical
as this is introduced, evolution takes place in a rapid leap, because now
quite different physical attributes are advantageous.
The clatta, Testudicaudatus tardus, a lemur-like prosimian with a heavily
armoured tail protected by a series of overlapping horny plates, demonstrates
this principle. Before the arrival of the tree-living predators, such a tail
would have been a disadvantage, interfering with the efficiency of food gathering.
Any tendency for such a cumbersome structure to evolve would have been quashed
rapidly by natural selection. But faced with constant danger the efficiency
of food gathering would have taken on an importance secondary to defence
and would have therefore created the correct conditions for it to evolve.
The animal itself is a leaf-eater and moves slowly, upside down, along the
boughs. When a striger attacks, it drops down and hangs from a branch by
its tail. The clatta is now safe - the only part within reach of the predator
is too heavily armoured to be vulnerable.
The khiffah, Armasenex aedificator, is a monkey whose defence is
based on its social organization. It lives in tribes of up to twenty individuals
and
builds defensive citadels in the boughs of trees. These large, hollow nests,
woven from branches and creepers and roofed with a rainproof thatch of leaves,
have several entrances, usually situated where the main branches of the tree
thrust through the structure. Most of the work of food gathering and building
is carried out by females and young males. The adult males remain behind
to defend the citadel and have developed a unique set of features to carry
out their highly specialized role; horny armour over the face and chest and
vicious claws on the thumb and forefinger.
It is not unknown for a female to taunt a passing striger and allow herself
to be pursued back to the citadel, dashing to safety while the striger finds
its way barred by a powerful male capable of disembowelling it with a swipe
from its terrible claws. This apparently senseless behaviour, however, provides
the colony with fresh meat, a welcome supplement to their basic vegetarian
diet of roots and berries. Only young and inexperienced strigers are caught
this way.
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The twilight zone of woodland life
Clinging to the trunks of trees with their clawed fingers, trovamps are ideally placed to leap, dart-like, on-to their prey.
On each jaw the trovamp has two barbs formed from the canine teeth. When the jaw is closed they protrude to give the appearance of tusks. |
A single animal may be parasitized by as many as ten trovamps at one time. Each trovamp is held fast to the creature's side by barbed teeth and curved front claws. Male pittas guard their garem of females throughout the breeding season. Each female occupies a separate nest.
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Compared with the canopy layer the floor of the tropical rain forest is
a dark, humid place. Little light penetrates through from the tree tops,
and
although there are many shrubs and herbs they nowhere present a thick, impenetrable
barrier. Despite a steady fall of dead leaves from above, the soil cover
is very shallow. The vegetable material on the ground is under constant attack
from micro-organisms and from the ubiquitous termites that perform a broadly
similar function to that of the earthworms in temperate latitudes by keeping
the debris circulating.
These termites are the principal food source of the turmi, Formicederus
paladens,
one of the few large mammals found on the African rain forest floor. It is
descended from the pigs that were once common in this environment. In the
turmi the tusks of the upper jaw are projected forwards, elongating the snout
still further, and have turned outwards to produce strong pick-like instruments
with which it digs into termite mounds. The lower jaw has lost all its teeth
and musculature and the mouth has diminished to a tiny hole through which
sticks out its ribbon-like tongue to gather termites.
From the same ancestral pig stock as the turmi comes the zarander, Procerosus
elephanasus. This much larger vegetarian animal lives on the sparse herbs
and shrubs found in less dense areas of the forest floor. Its long trunk,
developed from a snout similar to the trunk of the ancient elephant, enables
the zarander to reach leafy branches 4 metres above the ground, where it
can snip branches and vines from the trees by the scissor action of its upper
and lower tusks. Despite its long nose, the zarander has little sense of
smell. Like other mammals of the forest floor, the lack of wind and general
circulation among the dense trees means that scents do not travel far. Relying
on its keen hearing to warn it of the approach of an enemy, it takes off
into the thicker parts of the forest at the arrival of a predator, squeezing
its narrow body between the tree trunks, and remaining motionless, camouflaged
by its stripes and dark body colour.
One of the smaller mammals of the African tropical forest is the trovamp,
Hirudatherium saltans, a parasite which sucks the blood of larger mammals.
The trovamp is built rather like an insectivore or one of the smaller prosimians.
It is very agile, climbing about, usually in packs, among the trunks and
the branches of the shrubs. The trovamp is a prodigious jumper and can leap
3 metres from a branch to bury its needle-like jaws into the hide of a passing
animal. Its protruding canine teeth act as barbs and prevent it from being
dislodged from its host until it is finished feeding. As many as ten trovamps
may parasitize one host and will remain feeding until the animal is severely
weakened.
A large number of birds inhabit these regions. The most remarkable from the
point-of view of social behaviour is the giant pitta, Gallopitta polygyna.
The male pitta, unusually for a bird, is about three times the size of the
female and each year takes a harem of three or four females. Each female
builds a separate nest in the same vicinity and relies on the male to provide
her with food during the breeding season. The male also provides protection
from predators as well as defending the harem against rivals.
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Creatures of the tropical wetlands
The mud-gulper lives largely on water plants which it dredges from the muddy bottom of rivers and lakes. On land the mud-gulper tucks its tail under its body. As a signal to the opposite sex, the toothed kingfisher's beak changes colour early in the breeding season.
Normal coloration.
Breeding coloration. |
Although of recent aquatic origin the tree duck lives mainly on land.
The toothed kingfisher is not a swimming bird in the usual sense. It uses its wings rather than its feet - a method that is particularly successful under water.
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The largest aquatic mammal of the African swamplands is the mud-gulper,
Phocapotamus lutuphagus. Although it is derived from a water-dwelling
rodent it shows adaptations that closely parallel those of the extinct ungulate
hippopotamus. Its head is broad and its eyes, ears and nostrils are located
on bumps on the top so that they can still operate even when the animal is
totally submerged. The mud-gulper eats only water plants, which it scoops
up in its wide mouth or scrapes up from the mud with its tusks. Its body
is long and its hind feet are fused to form a flipper, giving it a seal-like
appearance. Even though it is very clumsy out of the water it spends much
of its time on mudbanks, where it breeds and rears its young in noisy colonies
at the water's edge.
Less well adapted but nevertheless efficient in the water is the swimming
monkey, Natopithecus ranapes. Descended from the swamp monkey, Allenopithecus
nigraviridis, of the Age of Man, this creature has developed a frog-like
body with webbed hind feet, long, clawed fingers for catching fish and a
ridge down its back to give it stability in the water. Like the mud-gulper,
its sensory organs are placed high up on its head. It lives in riverside
trees, from which it dives to catch the fish that are its staple diet.
Land-dwelling animals that have taken to an aquatic mode of life have usually
done so initially to escape land-dwelling predators. This is probably why
the water ant has taken to building its huge nest on rafts in swamps and
quiet backwaters. Each nest is made of twigs and fibrous vegetable material,
waterproofed by a plaster of mud and bodily excretions. It is connected to
the banks and to floating foodstores by a network of bridges and ramps. However,
in their new mode of life the ants are still vulnerable to the swimming ant-eater,
Myrmevenarius amphibius, which has evolved in parallel with it. The ant-eater
lives solely on the water ants, and to reach them undetected it attacks the
nest from below, ripping through the waterproof shell with its clawed paddles.
Since below the waterline the nest is made of discrete chambers that can
rapidly be made watertight in an emergency, little damage is done to the
colony as a whole. The ants drowned in an attack, however, are enough to
feed the ant-eater.
Fish-eating birds, like the toothed kingfisher, Halcyonova aquatica, are
frequently found along the water courses of the tropical swamps. The bill
of the kingfisher is strongly serrated with tooth-like points that help it
to spear fish. Although it cannot fly as well as its ancestors, nor can it
hover or dive as they did, it has become adept at "underwater flight",
pursuing its prey in their own medium. After catching a fish, the kingfisher
brings it to the surface and gulps it into its throat pouch before taking
it back to the nest.
The tree duck, Dendrocygna volubaris, is a water-living creature that almost
seems to have changed its mind about its preferred habitat and appears to
be in the process of undergoing a change back to the more arboreal lifestyle
of its remote ancestors. Although it is still duck-like, the webs on its
feet are now degenerate and its rounded beak more suitable for feeding on
insects, lizards and fruit than on water organisms. The tree duck still takes
to the water to escape predators and its young do not venture on to land
until they are nearly adult.
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Marsupial climbers and marsupial predators
The chuckaboo, a marsupial monkey, is a communal tree-dweller.
It has a prehensile tail with a hairless gripping pad on the tip. |
The female has two pouches, one on either side of her abdomen, so that her young do not get in the way when she is climbing.
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Beyond the mountains of the Far East - the most extensive and the highest
chain in the world, greater even than the Himalayas at their zenith 50 million
years ago - lies the great Australian sub-continent.
The conditions in this area today - lush tropical forests occupying vast
river basins - make it difficult to believe that a mere 100 million years
ago this landmass was part of the Antarctic continent. When at this time
Australia split off and began drifting northwards, the Age of Mammals was
well under way and the continent already had its own mammal population. These
mammals were nearly all marsupials - mammals that nursed their young in a
pouch on their abdomen - and because of Australia's long history of isolation
have largely remained so. In the rest of the world, however, the marsupials
were gradually superseded by the placentals - mammals not giving birth until
their young are more fully developed.
By the Age of Man, Australia had reached the desert and tropical grassland
latitudes, where the conditions provided the evolutionary impetus for the
development of running and burrowing animals such as the kangaroo, Macropus spp., and the wombat, Vombatidae. After man the continent continued its drift
northwards until, sometime in the last ten million years, it collided with
the mainland, throwing up the great barrier mountains that exist today. Although
some diffusion of animals has taken place between Australia and the rest
of the Northern Continent, the mountains have kept this cross-traffic to
a minimum and the sub-continent still has a predominantly marsupial fauna
- albeit one adapted to the tropical forests.
As in previous ages the Australian marsupials have developed forms that are
superficially very similar to those of placental creatures existing in similar
environments in other parts of the world. A prime example of this is the
chuckaboo, Thylapithecus rufus — essentially a marsupial monkey with grasping
arms and legs, opposable digits and a prehensile tail. Its bodily form, similar
to many of the true monkeys in other parts of the world, is well suited to
life in the trees.
A less energetic tree-dweller, the slobber, Reteostium cortepellium, can
be thought of as a kind of marsupial sloth that spends nearly all of its
life hanging upside down from trees and creepers. It is totally blind and
subsists entirely on insects that it catches in the flowers of its home creeper
by entangling them in long strands of mucus dangled from its mouth. Its large
downturned ears and sensory whiskers alert it to an insect's arrival and
tell it when to drop the mucus, which it aims at the flower's scent. As the
slobber's hair grows in spiral tufts and is pervaded by a parasitic algae,
it is completely camouflaged against the background of creepers, and when
totally motionless can escape the attention of predators.
A marsupial predator that the slobber takes pains to avoid is the hiri-hiri,
Carnophilius ophicaudatus, which, despite the fact that it is a tree-dweller,
is also highly efficient in preying on ground-living animals. Lying in wait
on a low branch, it dangles its strong prehensile tail down like an innocent
vine. When some unsuspecting animal trots by, the hiri-hiri seizes it swiftly
with its tail and strangles it. The hiri-hiri is descended from the Tasmanian
devil, Sarcophilus harrisii.
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Life on the forest floor
The poisonous fatsnake can strike out at prey 5 to 10 metres away from where it is lying.
The fatsnake's body is heavy and slug-like.
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The male hawkbower is more lightly built than the female.
The male skewers its prey outside the bower to attract flies.
The termite burrower is a wingless bird. Its feathers are fine and hair-like, and its long claws and shovel-shaped beak are designed for digging into termite mounds.
Its tongue has a bristle-like tip. |
The floor of the great rain forest of the Australian sub-continent is home
for a number of marsupial mammals. One of the most generalized and successful
of these is the omnivorous posset, Thylasus virgatus, the marsupial equivalent
of the tapir. Like its placental counterpart, it wanders through the gloomy
undergrowth in small herds, snuffling and scraping for food in the thin soil
with its flexible, sensitive snout and protruding tusks. Cryptic coloration
helps to conceal it from its enemies.
The largest animal of the Australian forest, and in fact the largest animal
found in any of the world's tropical forests, is the giantala, Silfrangerus
giganteus. This animal has evolved from the plains-dwelling kangaroos and
wallabies that were common when much of the continent was semi-arid grassland,
and betrays its ancestry by its upright stance and peculiar loping motion.
The giantala is so large that it seems at first sight ill-adapted to life
in the confined conditions of the tropical forest floor. However, its great
height does give it the advantage that it can feed on leaves and shoots that
are well out of reach of the other forest inhabitants and its bulk means
that shrubs and small trees do not impede it. As the giantala crashes through
the thickets, it leaves behind well-marked trails, which, until they are
reclaimed by the natural growth of forest, are used as trackways by smaller
animals such as the posset.
Convergent evolution on the Australian sub-continent is not solely characteristic
of the marsupials. The fatsnake, Pingophis viperaforme, descended from one
of the many elapid snakes that have always been a feature of Australian fauna,
has adopted many of the characteristics of forest ground-dwelling viper snakes
such as the gaboon viper and puff adder of the long-lived genus Bitis that
are found in other pans of the Northern Continent. These include a fat, slow-moving
body and a coloration that renders it totally invisible in the leaf litter
of the forest floor. The fatsnake's neck is very long and slender and allows
its head almost to forage independently of its body. Its main method of catching
prey is to deal it a poisonous bite from where it lies hidden. Only later,
when its venom has finally killed it and begun its digestive function, does
the fatsnake finally catch up with it and eat it.
Australian bower birds have always been noted for the fantastic structures
built by the male for the purpose of wooing a female. The hawkbower, Dimorphoptilornis
iniquitus, is no exception. The bower itself is quite a modest affair, housing
the permanent nest and a small altar-like structure at the entrance. While
the female incubates the eggs, the male, a rather hawk-like bird, catches
a small mammal or reptile and sets it on the altar. The offering is never
eaten but serves as bait to attract flies, which are then caught by the female
and fed to the male to ensure his continuing attention during the long incubation
period. Once the eggs have hatched the chicks are fed on the fly larvae that
have developed in the rotting carrion.
Another curious bird is the termite burrower, Neopardalotus subterrestris.
This mole-like bird lives entirely underground in termite nests, where it
digs nesting chambers with its huge feet and feeds on the termites with its
long and sticky tongue.
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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
The Grass-eaters : Giants of the Plains : The Meat-eaters
TROPICAL FORESTS 86
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
