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TUNDRA AND THE POLAR REGIONS
Tundra and Arctic habitats are found at both polar extremities of the globe and at the tops of high mountains. Conditions in these localities are broadly similar and the habitats differ only in that one is an effect of latitude and the other of altitude.
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The bleakest places on the surface of the earth are found around the North
and South Poles - regions of constant ice and snow where no plants grow. Because
of the tilt of the earth, at certain times of the year no sunlight whatsoever
reaches these regions and night lasts for months at a time. Even during summer,
when daylight is continuous, the sun's rays hit the ground at such a shallow
angle that very little warmth is felt. These conditions prevail both on the
Southern Continent of Antarctica and on the ice-mass that covers the northern
Polar Ocean.
The extent of the ice on the Polar Ocean is dependent on the low salinity of
the Arctic waters - a saltier sea would not freeze over to such a degree. The
Polar Ocean is separated from the Atlantic Ocean by a barrier of islands that
inhibits their intercirculation. This island chain is formed from what was once
a single island known as Iceland. It consisted of lavas that erupted from the
mid-Atlantic ridge as the crustal plates of Europe and North America moved away
from one another. As this movement continued, enlarging the Atlantic Ocean, Iceland,
straddling the mid-oceanic ridge, split into two parts, each moving in opposite
directions. The continuing volcanic activity spawned a string of new islands
in the growing gap between the two parts. Almost 180° away, at the opposite side
of the Arctic Ocean, the same crustal movements were responsible for closing
the Bering Strait, the gap between North America and Asia, and fusing the two
areas into one vast Northern Continent. As a result the Polar Ocean is now practically
landlocked, and is fed by the rivers of the surrounding supercontinent.
The generally low-lying areas fringing the polar ice sheets comprise the tundra.
During winter they are as cold and bleak as the Arctic wastes, but in summer
the temperature rises above freezing and may reach an average daily temperature
of 10°C. In summer the snow melts, but because of the permafrost - the layer
of perpetually frozen soil beneath the surface - the water cannot drain away
and floods the numerous hollows and depressions.
Spring on the tundra is a time of spectacular change. A sudden bloom of vegetation
bursts forth to take advantage of the brief growing season. Much of the vegetation
reproduces asexually rather than by producing seeds, as is the case in warmer
regions. Vegetative reproduction is much faster and therefore, because of the
short summers, much more successful. Those plants that do reproduce sexually
produce seeds that are highly resistant to frost. Mosses, lichens and low bushy
herbs are typical of tundra plants. The tundra vegetation's sudden summer flourish
is accompanied by a bloom of insects and in spring a veritable plague of flying
creatures emerges to take advantage of the short period of warmth and sunlight.
The seasonably of plant and insect life on the tundra means that for most mammals
and birds food is only available during part of the year and most of the larger
animals are consequently migrant, spending the harsh winters to the south. In
the Southern Hemisphere there are no equivalent large landmasses at latitudes
that would produce a covering of tundra vegetation. The tundra that exists is
found scattered on the islands of the Southern Ocean and on mountains just below
the snow line.
The wandering herds and their predators
The pilofile's bristles expand its insect-catching area beyond the region of its head.
When its beak is closed the bristles drop down, allowing it interrupted forward vision.
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In summer the pilofile feeds on the wing.
In winter it migrates south, shedding its bristles and growing at their place long probing beak.
The pilofile's green-and-brown-blotched egg is perfectly camouflaged in the tundra vegetation. |
In comparison with other parts of the world, the animal and plant life of
the tundra consists of a rather small number of species, each of which
contains
a relatively large number of individuals - a situation which is diametrically
opposite to that found in the tropics. The low species count is entirely
due to the region's inhospitable conditions. All tundra animals have evolved
from
creatures found in more temperate areas; their ancestors probably colonized
the tundra only because they were driven to do so by fierce territorial
competition. Life has to be unusually unpleasant elsewhere for a group
of animals to venture
into the tundra in the first place.
During the summer months the tundra is literally infested with flying insects
and has a large population of insect-eating birds. Many, like the pilofile,
Phalorus phalorus, have bristled beaks - a ring of stiff hair-like feathers
surrounding the beak that forms a cone and deflects insects into its mouth.
The bristles in effect widen the bird's potential capture area and increase
its food supply.
For many large animals the tundra is only habitable during the summer months
and in winter they migrate southwards into the coniferous forests, where conditions
are less austere. The largest of these animals is the woolly gigantelope, Megalodorcas
borealis, a close relation of the tropical gigan-telope. It differs mainly
in size and in the possession of a large, fatty hump, which provides it with
nourishment during the hungry winter months. It has a long, shaggy winter coat
and broad hooves, which prevent it from sinking into soft snow. It uses its
enormous horns as snow ploughs to expose the mosses, lichens and herbaceous
plants on which it feeds. Its eyes are small to avoid being frost-bitten and
its nostrils are bordered by blood vessels that warm the air before it reaches
the lungs.
In early summer, the woolly gigantelope loses its shaggy coat and takes on
a much sleeker appearance. The hump which sustained it through the winter months
is now entirely depleted and it spends much of the time eating to rebuild its
energy store for the long trek back south in the autumn.
Because of the woolly gigantelope's size - three metres at the shoulder without
the hump - there are very few predators powerful enough to threaten it. Its
only real enemy, the bardelot, Smilomys atrox, is a creature that would have
been very much at home back in the first half of the Age of Mammals. At that
time elephants, animals of comparable size to the gigantelopes, were preyed
on by sabre tooths. These creatures, members of the cat family, had long, stabbing
canine teeth with which they inflicted deep, stabbing wounds on their quarry.
After an attack the sabre tooths would wait until the elephants bled to death
before moving in to feed. This successful arrangement was even evolved independently
among the marsupials. However, during the Age of Man the elephants declined
and the sabre tooths, being entirely dependent on them, died out completely.
With the advent of the gigantelopes the sabre tooth pattern reappeared, but
this time among the predator rats. The bardelot, unlike other members of the
group, exhibits sexual dimorphism in that only the female is equipped with
sabre teeth and hunts the gigantelopes. The male, having none, resembles more
the polar bears that once inhabited these latitudes.
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A compact ecosystem
1 year |
2 years |
3 years |
4 years |
5 years Initially a four- or five-chambered nest the meaching's fortress grows exponentially, reaching its maximum size after about five years.
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The lesser ptarmigan nests exclusively in meaching fortresses. |
Winter coat Summer coat In early autumn the polar ravene moults its dull summer coat and grows a thicker creamy-brown covering of fur. |
Although the polar ravene is larger than the ravene of the temperate woodlands, it has smaller facial features.
Polar ravene
Temperate ravene |
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Life in the northern seas
Height 60 cm height 45 cm
The flightless auks exist as a chain of subspecies around the Polar Ocean capable of breeding to their neighbours excepting at the ends of the chain, where differences in size and physiology make it impossible.
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The pytheron, although totally unrelated, has an appearance similar to the ancient seals and sea-lions.
Spread-eagled on an ice-floe the pytheron appears lumpish and ungainly.
In the water it is swift and graceful, swimming like a penguin. |
The northern polar sea is almost landlocked and contains a permanent icecap,
which has a considerable influence on the environment of the surrounding continent
and contributes substantially to the stability of the region's cold climate.
The ice-cap is maintained only because the Polar Ocean is fed by enormous quantities
of fresh water by the rivers of the surrounding continent. This gives the sea
an unusually low salinity and therefore a strong tendency to freeze over.
In winter the Polar Ocean is largely barren. In spring, however, the sunlight
produces a bloom of unicellular algae near the surface, which provides food for
the microscopic animal life that forms the basis of the oceanic food chain. In
spring, shoals of pelagic fish come northwards through the northern island barrier
to feed on the zooplankton, bringing with them countless numbers of seabirds.
The first species to arrive is the flightless auk, Nataralces maritimus, a totally
aquatic creature with paddlelike wings. In this respect they resemble the penguins,
which were so successful in the southern oceans in earlier times. Except during
winter the flightless auks rarely come ashore or climb on to the ice, where they
are quite defenceless. They retain their eggs until they are almost ready to
hatch and lay them in the open water.
The flightless auks first evolved at the northernmost tip of the Northern Continent
and, as they became established, spread both east and west, forming a chain of
subspecies in a ring around the Polar Ocean. Throughout most of the ring each
subspecies is able to breed with the neighbouring ones, but where the ends of
the chain overlap the differences are so great that no interbreeding is possible
and these populations must be regarded as separate species.
Preying on the flightless auks, and also on the fish, are the pytherons, Thalassomus
piscivorus, a group of aquatic carnivorous mammals related to predator rats.
They occupy the same ecological niche that the seals occupied earlier in the
Age of Mammals and like them have developed streamlined blubbery bodies and fin-shaped
limbs.
Among the organic detritus on shallower areas of the ocean bed are found banks
of shellfish. Living on these shellfish is the distarterops, Scinderedens
solungulus,
by far the most massive aquatic relative of the predator rats. It reaches a length
of about four metres and has an insulating coat of matted hair made up of a mosiac
of solid plates, giving it a lumpy rather than streamlined appearance.
Its most unusual feature is its teeth; the upper incisors form long, pointed
tusks — the left-hand one projects forward, whereas the right-hand one points
straight down and is used as a pick for removing shells from the sea bottom.
This asymmetry is also found in the limbs; the left foreflipper only is equipped
with a strong claw, which it uses to dislodge particularly stubborn shells. Because
the distarterop's evolutionary line separated from the predator rat's when they
were both still comparatively small rodent-like creatures, it would appear therefore
that the predator rat's double-pointed incisor teeth, from which the distarterops
tusks (and also the bardelot's sabre teeth) have evolved, were a comparatively
early development.
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The origins and ancestry of the vortex
Skerns are found mainly around the volcanic islands of the Southern Ocean. They have oily-green plumage, large feet and legs, but no wings.
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Skerns cannot walk, but use their legs to push themselves along on their bellies.
Swimming at the surface, they sit very low in the water.
When hunting fish underwater they become graceful and agile swimmers. |
In contrast with the vast Southern Continent, which supports life only
around the edges, the surrounding ocean teems with life. Among its most
notable inhabitants
is the vortex, Balenornis vivipera, the largest animal found anywhere
in the world. Resembling many of the sea creatures of the past, it has
a long, tapering,
neckless body, a powerful paddle-shaped tail and long stabilizing fins
- an ideal arrangement for efficient movement through water. Similar
shapes can
be seen in the great arthrodires of the Age of Fishes, in the pliosaurs
of the Age of Reptiles and in the whales of the first half of the Age
of Mammals
- the last creatures to occupy this ecological niche before the vortex.
The vortex is in fact descended from the penguins, which, although they
were birds, had long since lost the power of flight and were totally
adapted to
an aquatic life excepting for one thing - they always had to come on
shore to lay eggs. This remained so until, shortly after the extinction
of the whales,
one species of penguin developed the ability to retain its single egg
internally until it was ready to hatch and gave birth to live young in
the open ocean.
Freed of the necessity to come ashore, this species became completely
marine and ultimately gave rise to a completely new order of marine birds,
the Pelagornids,
of which the porpin, Stenavis piscivora, is the commonest surviving example.
The Pelagornids are unique in the aquatic world in that, like their ancestors,
they are both warm-blooded and egglayers, albeit that their eggs are
retained within the body until the moment of hatching. In this respect
they resemble
the mammals and some reptiles. However, it is important to note that
Pelagornids do not possess mammary glands with which to feed their young,
as do mammals,
and are warm-blooded as reptiles are not.
The porpin, like most of its class, is a fish-eater. Its distinguishing
feature is a long, serrated beak that enables it to catch larger fish
than would otherwise
be possible. So successful has it been that it has remained virtually
unchanged for the last 40 million years.
Although a plankton-eater and very much larger, the vortex is also a
member of the order Pelagornid. Its beak has developed into a large plankton
sieve,
which consists of a very fine mesh of bone plates instead of coalesced
hair, as in the case of the whales' baleen plates.
Around the volcanic islands of the Southern Ocean are found the skern,
a species of flightless seabirds that have evolved a unique behavioural
quirk in response
to the problem of incubating eggs in this hostile environment. As well
as being a hazardous time for the embryo chicks the parent birds also
run the risk of
exposure. The skern has solved the problem by laying its eggs in the
warm volcanic sands of the islands and deserting them immediately afterwards.
It is able
to delay the time of laying until the temperature of the sand is exactly
right. When a volcano shows signs of activity it immediately becomes
the scene of
frenzied activity. The birds scramble ashore, and with the aid of their
temperature-sensitive beaks probe the sand for areas with the right condition
for incubation. After
laying their eggs ten to twenty centimetres deep and covering them with
sand they return to the sea, seeing neither their eggs nor their offspring
again.
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The effect of altitude on animal communities
Shaggy hair on the undersides of its legs and on its feet give it a booted appearance.
The ruffle is sure-footed over boulders and loose scree.
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The young parashrew's single migration flight may last for up to 24 hours
The parashute tail is present only during adolescence and is moulted when the parashrew becomes sexually mature.
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The flora of the mountains has much in common with that of tundra regions
because of the similarity in climatic conditions found there - low
temperature, high
precipitation and short growing season in both habitats.
Although the mountain areas of the world are so isolated and widely
distributed that they can largely be regarded as separate faunal provinces,
the fauna
of the fold-mountain belt between Africa and Europe show characteristics
that
are typical of mountain life the world over. The ruffle, Rupesaltor
villupes, a descendant of the rabbit, exhibits many of these features.
It has a
rounded head and body, and disc-like ears - adaptions that guard it
against cold. It
has long hair under the neck and body to protect its legs from the
cold and its teeth are well adapted for grazing mosses and lichens.
The upper
incisors
are set at an angle and are used for scraping the patchy vegetation
from the surfaces of rocks and boulders.
The groath, Hebecephalus montanus, a variety of small hornhead frequently
found grazing on grassy, south-facing slopes, lives in small herds
of four or five
females, guarded jealously by a male. The most apparent difference
between males and females is in their horn structure. The males have
flat, bony
plate-like horns which they use to buffet one another in their frequent
fights for herd
dominance. The females' pointed pyramidal horns are much more deadly
and are used to defend themselves and their young against predators.
While the herd
grazes, the male normally stands on a promontory watching for signs
of danger. When it sees an intruder the male signals by erecting its
long
flag-like tail
and the herd makes for the shelter of a nearby crag or cave.
One of the deadliest predators found in the African-European mountains
is the shurrack, Oromustela altifera, a carnivore related
to the weasel-like pamthret,
Vulpemustela, of the northern coniferous forests. Sure-footed
over difficult rocky terrain and well camouflaged by its mottled grey
fur
it is the
groath's principal enemy. The shurracks hunt in packs, surrounding
their prey, or cornering
them in ravines, sharing the kill among themselves.
Perhaps the strangest mammal found in these regions is the parashrew,
Pennatacaudus volitarius. The adults are unremarkable small shrew-like
creatures, but the
juveniles possess one of the strangest devices found in the animal
kingdom. At the end of their tails, they have a fantastic parachute
structure
formed of interwoven hair, which they normally use only once before
discarding. When
the time comes to leave the parental nest, they launch themselves into
the air, relying on the thermal currents that rise from these bare
rocky slopes
in summer to carry them to a fresh habitat, in some cases several kilometres
away. As a means of dispersal this is a bit hit-and-miss, but the inevitable
high death rate that this behaviour produces among young parashrews
is more than compensated for by the large numbers of offspring produced
by each adult
breeding pair.
The evolution of the parashrew's parachute tail is primarily due to
the creature's insectivorous ancestry. It is thought that these early
creatures
used their
tails as balancing organs when leaping to catch insects in mid-air.
The parachute consists of soft, curled hairs hooked together to form
a mat
and held in shape
by a series of bristles growing from the tip of the tail.
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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
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
