WHY BIRDS SING

Birds sing for many reasons: to advertise their presence to a mate, to warn intruding birds away from their territory, and to convey

other kinds of important information to one another.

KEY FACTS ----------------------------------------------------~

HOW BIRDS MAKE SOUNDS

Humans and other vertebrates produce sounds from the lar­

ynx, which is at the top of the windpipe. A bird's vocal organ, the syrinx, is at the fork leading from the windpipe to two bron­chial tubes. The syrinx produces higher-frequency sounds than the human voice.

The structure of the syrinx varies with the species. In some songbirds it has five to nine pairs of muscles attached to the bron­chial tubes, regulating and con­trolling sound. Sound is also modified by three vibrating membranes, as well as by the windpipe, the mouth, and an air sac in front of the windpipe.

The bird's size and shape, too, can affect the sound it makes. The common crane, for exam­ple, can be heard about a mile away, partly because its wind­pipe can be up to five feet long.

Right: The common crane's calf is loudest when the male is displaying in mating season.

COMPLEXITY OF BIRD SONGS

Birds and people hear sounds of similar frequencies, but birds are probably less adept at hearing low-frequency sounds. But birds are better at distinguish­ing sounds that are very close together. What humans hear as a single note of a bird's song can actually be as many as 10 differ­ent notes, each of which can be heard distinctly by another bird. Even the simplest song conveys more information to birds than a human listener could imagine.

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Left: The song of the white­throat and other warblers sounds very intricate to human ears. When the song is slowed down mechanicalfYt we hear the many notes contained in even a short song burst.

0160200481 PACKET 48

A birds song consists of a phrase or series of phrases

that is repeated over and over again in order to

convey a message. The musical patterns are so

complex that the human ear cannot detect their

intricacies. The sound may seem musical, like

the song of the blackbird, or harsh, like

the booming sound of the bittern.

~ KEEPING IN TOUCH The most important way birds communicate among them­selves is by song, particularly in forests, where they cannot easi­ly see each other. One bird may sing, "I'm over here. Where are you?" and be answered byoth­ers of its species. This type of song helps keep a large flock together. A different call will rouse birds of the same species to follow the caller.

Song also helps to distinguish two species that look alike and share the same habitat. For example, the willow warbler's

musical song is made up of de­scending notes, while the chiff­chaff warbler has a monotonous two-note call. These different songs bring together birds of the same species, ensuring suc­cessful breeding.

A bird's alarm call is essential. Some alarm calls of separate spe­cies are alike, especially when they warn of a predator that threatens more than just one species. Alarm calls in response to a ground predator like a fox differ from those that warn of an airborne predator like a hawk.

During the breeding season, birds sing to stake out their ter­ritories and to attract mates. Usually it is the male that sings, but females of some species sing in order to maintain a hold on their own territories.

Many species sing only as the breeding season approaches, while others sing throughout the season to reassure their mates and to warn off rivals from their territories. A court­ship song can indicate the sex of the singer and sometimes

left: To some people, the yellow­hammer's song sounds like" a-little­bit-of-bread-and-no-cheese. "

whether or not it has mated. The male's song is often an­

swered by another male, and people listening sometimes mis­take it for a love duet between male and female. Males and females of certain tropical spe­cies such as the boubou shrike do sing to each other, however.

Song is more effective over a larger area than a visual display. Sometimes the two are com­bined in a spectacular courtship ritual such as that of the superb Iyrebird of Australia.

Right: The male superb Iyrebird puts on an elaborate visual and aural courtship display.

left: Even when reared by another species such as the willow warbler, the cuckoo retains its distinctive song so other cuckoos can identify it.

~ HEARD BUT NOT SEEN Many species, such as the black­bird and the yellowhammer, have song posts-high perches or exposed branches from which they can be seen and heard sing­ing at intervals throughout the day. Other birds, like the lark, sing in flight.

Singing out in the open, as the lark does, seems risky be­cause the bird's location is then revealed to predators. Some naturalists think that the best

left: The usually quiet male bittern "booms" noisily for a mate in mat­ing season.

lark singers are making their strength known to predatory hawks with a message that says "I'm too tough to chase."

Other birds, perhaps more aware of danger, hide in thick vegetation while singing, or, like the whippoorwill, they sing at night. The songs of these con­cealed birds are often loud to act as sound beacons.

Front cover: Geese keep in touch with loud, honking calls when fly­ing in formation . Front insets: Robin chicks (left) call noisily for food. A starling (right) sings on a song post.

ANIMAL ADAPTATIONS TO LIFE IN THE DARK

Humans and many other species are active primarily during daylight hours and spend much of the night asleep. In contrast, noctumal

creatures are active mainly during the hours of darkness.

____ ~~K~EYFAC~T~S------------------------------~

NOCTURNAL ANIMALS DURING THE DAY

Most nocturnal animals rest during the day, but species like

the badger may emerge from cover for short periods . Some nocturnal animals, such as the

h~use mouse, may feed during the day if food is scarce. In hot climates animals stay in bur­rows, rock crevices, or thick

NIGHT SIGHT

Nocturnal animals usually

have large eyes to collect as much light as possible. Owls and nocturnal bush babies

have tubular eyes with a large lens (the eye part that focuses

received light) and retina (a light-sensitive membrane at the back of the eye). The

owl's eyes face forward and move only slightly in their

sockets, but the owl can

rotate its head almost 360 degrees to compensate.

A nocturnal animal retracts

I the iris (the eye's colored part) and dilates the pupil (the dark

part at the center of the iris) to

let in light. The light is reflected back through the retina by the

tapetum, a layer of tissue that

undergrowth to escape the sun and come out to feed during the cool, moist dusk.

After grazing at night, the hippopotamus keeps cool dur­ing the day by staying in water, returning to land briefly at mid­

day to bask in the sun . Its eyes, ears, and nostrils are set on top

acts like a mirror. The reflected light stimulates sense cells.

There are two types of sense cells in the eye: rods and cones. Rods are sensitive to low levels

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of its head, so the hippo can remain almost submerged in

water for long periods. The eyes of nocturnal ani­

mals are very sensitive to day­light. Nocturnal reptiles and amphibians have a nictitating membrane, or third eyelid, which draws across the eye for protection. Cats and other animals have a pupil that nar­

rows to a sl it to restrict light. The huge eyes of the South American douroucouli, or night monkey, are so sensitive to light that the animal can

go blind if it opens its eyes in daylight for long periods.

Left: The hippopotamus wallows throughout most of the day in order to stay cool.

of light, while cones pick out details in bright light and see

in color. The retinas of noctur­nal animals such as bats have no cones.

Left: The bush baby sleeps during the daylight hours and searches for food at night. By being active at night, it avoids com­petition with monkeys and other primates in its African habitat.

0160200491 PACKET 49

Examples of creatures that are active in the dark

can be found among all forms of animal life, from

deep-sea fish to birds and bats. Various land-dwelling

animals have developed special adaptations so that they

can exploit the cool conditions, moisture, and protective

concealment that darkness brings. A few fish and insects

even have the ability to generate their own light.

~ NIGHTTIME SENSES Animals that are active at night

have adapted to living in dim

light by improving their vision

or by developing other senses

to compensate for the low levels

of light. Even in species with

keen night vision, other senses

are important. The serval, a cat

with night-sensitive eyes, uses

its excellent hearing to detect

movement in the undergrowth

and to pinpoint prey. In addi­

tion to using their eyes to fly

and hunt in the dark, bats use

echolocatio~sensing objects

by emitting high-frequency

Front cover: The greater horsehoe bat hunts prey at night using radarlike echolocation.

Front inset left: The barn owl has large eyes that give it exceptional night vision.

Front inset right: The edi­ble, or Roman, mail lives in moist areas and usually emerges at night to feed.

sound pulses and detecting the

echo. Snakes of the pit viper

family rely on two small organs

on their snouts to detect heat

given off by any warm-blooded

prey. Additional heat organs in

their mouths guide their strike.

The piranha uses its infrared

vision to locate prey in murky

rivers. Sharks and rays have sen­

sors that detect the electric im­

pulses generated by their prey's

muscle movements.

Below: The servaIs long, sensitive whiskers help it to avoid obstacles in the dark.

Left: Using its keen vision, the night jar catches its insect prey at dusk or dawn. During the day it lies motion­less on the ground, cam­ouflaged by its mottled plumage.

~ CREATING LIGHT IN THE DARK In the dark ocean depths there

are creatures that can give off a

glow of light. Both the angler­

fish and the viperfish dangle a

luminous lure in front of them

to attract prey. The viperfish also

has well-developed eyes and an

array of lights along its flanks.

Fireflies and glowworms can

activate a chemical process in

the rear of their abdomens to

produce an effective light. The

female glowworm shines her

"torch" at night to attract males

during the breeding season.

Left: The firefly, a night-flying bee­tle, can use its glow to signal a type of Morse code.

Right: In the dark ocean depths the squid can give off rippling patterns of light and color.

~ WHY ANIMALS ARE NOCTURNAL Some species find night condi­

tions preferable to those during

the day. Desert animals tend to

avoid the hot, dry daytime cli­

mate and feed at night when

the air is cooler and compara­

tively humid. They use dew to

counteract daytime water loss.

Moist air is also vital for soft-

bodied animals such as snails,

slugs, and worms. At night

they can come out to feed

without the danger of having

their body fluids evaporate.

The nocturnal habits of many

species are closely linked to feed­

ing. Birds such as woodcocks

and snipes feed mainly after

dusk, when their prey is most

active. They search for worms

and insects just below the soil's

surface with their long, probing

bills. But when the ground is

frozen in winter, the birds also

feed in daylight.

Left: The western tarsier is a twi­light and nocturnal hunter with keen sight and hearing.

Low levels of light at dawn and

dusk give species such as zebras

some protection from predators.

The animals' striped and spotted

coats break up their silhouettes

in dim light. But lions and other

big cats have adapted to hunting

when zebras and antelopes go

out into the open.

The presence of humans has

forced some animals to become

nocturnal. The European otter is

usually regarded as a nocturnal

species, but it is active during the

day in some remote islands off

Scotland, where it is not dis­

turbed by people.

By being active at different

times, animals are in less com­

petition for food, and resources

are exploited 24 hours a day.

Nocturnal and day-active ani­

mals usually have different food

sources, but there are excep­

tions. The great horned owl

hunts hares by night, while the

red-tailed hawk preys on the

same species during the day.

HOW REPTILES HUNT

Reptiles use a variety of skills when hunting for food. Some lie in wait for hours until prey passes by. Others are active hunters,

stalking and catching prey in surprise attacks.

KEY FACTS

THE SNAKE AS HUNTER

All snakes hunt by themselves, and most are passive hu nters . Species with camouflaged skin lie in wait in locations where prey is likely to pass by and then strike before their pres­ence is detected. Most snakes are not fast enough to chase prey. Species with good eye­sight, such as wh ipsnakes and sand snakes, are the excep­tion . They flush out prey and pursue it for a short distance before killing it.

Most snakes use smell to hunt. They have unique sen­sory abilities that enable them to "taste" the ai r. The snake's flickering forked tongue car­ries particles of scent from the air to a pouch in the roof of

the mouth. Known as Jacob-

Below: The venomous night adder strikes its prey before swallowing it headfirst.

son's organ, this pouch lets the snake analyze scents.

Pit vipers and some pythons have heat-sensitive pits on the snout that can sense tempera­ture changes of only a fraction of a degree. These snakes can detect the location of warm­blooded prey and strike accu­rately, even in total darkness.

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Above: The Indian python is a con­strictor that kills prey as Jarge as a hog deer by suffocation.

Venomous snakes usually strike their prey and then fol­low the dying animal until the poison takes effect. Constric­tors suffocate their victims, while other snakes simply swallow live prey whole.

0160200531 PACKET 53

In order to find food, reptiles put together information

from their various sensory organs. With this collected

information, they are able to judge both the distance

and direction of prey. Some reptiles, like chameleons,

have well-developed eyesight. Other reptiles, such as

pit vipers, possess heat-sensitive organs that can

detect the approach of warm-blooded prey.

~ CHOOSING PREY Two main factors influence any reptile's choice of prey. First, prey must be readily available. Second, the reptile must be able to obtain it without too much danger of being preyed upon itself.

The relationship between predator and prey reflects a balance among the species in an area. A predator rarely

Front cover: A chameleon has excellent vision that makes it an adept insect hunter. Its eyes swivel and focus indepen­dently of each other.

Front inset left: The komo­do dragon senses the pres­ence of prey by " tasting" the air with its forked tongue.

Front inset right: As it moves through the wate" a caiman finds food by simply half-opening its mouth and trapping fish.

benefits from becoming so successful that it eliminates all the breeding adults of a prey species. The Puerto Rican liz­ard is an exception to this rule. It feeds voraciously on a wide variety of prey and covers a large range. Although it has depleted some populations of land snails, it has left other prey species to thrive .

~ HUNTING IN WATER

Left: A gecko uses fairly un­sophisticated methods of hunting. It either sits and waits until an insect passes close enough to catch, or it stalks its prey.

Members of the crocodilian prey in a powerful lunge, the family-alligators, caimans, crocodile drags it into the wa-crocodiles, and gavials-have ter, jams the carcass under a fat deposits along their backs submerged log and lets it rot and tails that let them go for before eating it. long periods without eating. Flesh-eating turtles catch fish

Crocodilians that have nar- and crustaceans in the water. row snouts, such as the Afri- The alligator snapping turtle can slender-snouted crocodile, has a fleshy lure in its mouth. It hunt by moving slowly through wiggles the lure to entice fish the water with their mouths into its mouth and then snaps open to catch fish and crabs. its jaws shut. Species with heavy snouts, like Rear-fanged snakes are well the Nile crocodile, ambush adapted for hunting in water. their prey. They have strong They have nostrils on top of jaws and can subdue large ani- the snout that enable them to mals, even buffaloes and ze- swim half-submerged. When bras. They prey mainly on the snakes dive, valves in the mammals that drink at the nose close so they can swim river's edge. After seizing the

Left: The alligator snapping turtle has a pinkish, wormlike lure in its mouth that entices fish.

underwater.

Right: The fer-de-lance uses heat­sensitive pits in its snout to detect warm-blooded prey.

~ LIZARD HUNTERS Many lizards, including sand lizards and geckos, prey on insects.They may hunt both passively and actively. As a pas­sive hunter, a gecko visits sites where insects are likely to gath­er, especially sites where flow­ering plants are plentifu l. It simply waits for prey to pass, then snaps it up.

Lizards also hunt actively, especially when food is scarce. A lizard sees or smells prey and then stalks it. Geckos usually approach prey slowly and then make a quick strike when they get close enough.

The chameleon has excellent eyesight and can seize moving prey with speed and accuracy. While resting on a branch, it

Left: A chameleon takes only four­hundredths of a second to extend and retract its tongue.

will spot an insect, small bird, or lizard and then stalk it, mov­ing carefully along the branch. Its feet are adapted for grasp­ing, and its tail coils around the branch for balance. The cha­meleon can swivel its eyes in­dependently of each other, both up and down and side­ways. So it can keep one eye on its prey and watch for pred­ators at the same time.

The chameleon can catch prey at considerable distances from its body. Once it is within striking distance, the chame­leon focuses both eyes on the prey, calculates the prey's posi­tion, then shoots out its tongue with lightning speed. A sticky pad at the tip of its tongue grasps and holds the victim while the tongue retracts and hauls in the meal.

Left: The Nile crocodile first drags its prey into deep water to drown it. Then it tears the body into small pieces by twisting the carcass in its massive jaws.

THE TRUE SEAL FAMILY

The true seal family contains 10 genera with 19 species. Unlike the closely related fur seals, true seals do not have external ears, and

their flippers cannot support their body weight on land.

KEY FACTS

DISTINGUISHING FEATURES OF TRUE SEALS --------------------------------------------------~

Build: A true seal has a torpedo­shaped body with backward­pointing forelimbs and flippers for forefeet. Its flippers have five clawed digits. It has no ex­ternal ear flaps but can close its ear openings underwater.

Coat: A true seal's skin is cov­ered with fine hair. Underneath is a layer of fat several inches thick to protect against cold.

Right: The southern elephant seal's backward-pointing rear flippers are of little use on land.

Mobility: A true seal propels it­self easily through water but is clumsy on land. Some species drag themselves along the land with their forelimbs.

Sensory perception: A true seal has good eyesight and hearing but a poor sense of smell.

Left: The com­mon, or harbor, seal has five digits on its fore flippers. Each digit possesses a claw to im­prove the seal's grip when it moves about on ice.

DIFFERENCES BETWEEN TRUE SEALS AND FUR SEALS

All seals belong to the suborder true and fur seals is in their skel-Pinnipedia. True seals are classi- etons. True seals have short front fied in the family Phocidae, while flippers and backward-pointing fur seals and sea lions are in the hind limbs that cannot propel family Otariidae. The two seal them on land. Most species families are easy to distinguish move clumsily on land, anchor-because fur seals have small ex- ing the front of the body and ternal ears and true seals do not. drawing the rear toward it, then

The main difference between anchoring the hindquarters and

DID YOU KNOW? • Like a land mammal, a seal • The Baikal and Caspian seals has four limbs, but all bones are probably descended from above the ankle are hidden the ringed seal, which entered inside its body. Lake Baikal and the Caspian Sea • Most seals have a number of when they were linked to the stones in their stomachs. The ocean, millions of years ago. stones seem to be swallowed • The Baikal is the smallest true deliberately, but their function seal, growing to only four and a is not known. half feet.

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Teeth: The number and shape of the teeth vary with the spe­cies. Most pups develop their permanent teeth by three months of age.

Communication: Seals use cries to communicate. Males threaten each other with barks and roars.

squeezing the front forward. Fur seals move more easily on land, bringing their hind limbs forward beneath their body as they shift their weight forward.

As its name implies, a fur seal has a thick fur coat protected by guard hairs, whereas a true seal has a shorter coat.

• One Weddell seal dove over 1,900 feet-the deepest dive on record by a true seal. Most seals hunt for food at less than 300 feet. • A ringed seal that was cap-tured off Baffin Island in Cana-da in 1954 is known to have lived 43 years.

0160200651 PACKET 65

The true seals, which belong to the family Phocidae,

spend most of their life in the sea. Some live in coastal

waters, others inhabit the deep oceans, and one species

even lives in a freshwater lake. Seals are perfectly adapted

to an aquatic environment, but at the same time they

are air-breathing, warm-blooded mammals.

~ EVOLUTION The evolution of seals is a mys­tery. The earliest known fossils of these pinnipeds (fin-footed ani­mals) are from the Miocene pe­riod, 5 million to 22.5 million years ago. Most fossil skeletons are similar to those of modern seals, but one species had a tail

~ HABITS True seals spend most of their time at sea, often out of sight of land. A young seal can swim soon after birth. Adults come ashore to molt, breed, or doze.

Adult seals live in small groups that may increase when food is

Front cover: Until a month after its birth, a harp seal pup has a white coat for which it is hunted.

Front inset left: Unlike fur seals, true seals do not have external ears, but their hear­ing is just as acute.

Front inset right: 5eals occasionally give birth to twins.

and limbs long enough to walk. Seals probably descended

from the creodonts, predatory mammals that flourished over 65 million years ago. True seals may then have descended from the earliest otters and fur seals from the earliest bears.

plentiful. Females are grouped in harems in the breeding season.

Most seals live in cold waters, although a few, including the northern elephant seal and some monk seal species, live in tropi­cal waters.

In many true seal species, the male has one mate, but in some seal species the male mates with more than one female. A sexual­ly mature elephant seal or gray seal establishes a territory that is inhabited by a harem of females.

A female seal has one pup each year. She gives birth on the same beach every year, often within 30 feet of the previous year's spot. Breeding times vary from species to species, but breeding in a particular species always oc­curs at the same time of year.

Left: The southern elephant seal is the largest seal. The bull can grow to 20 feet in length.

Left: Like oth­er true seals, the Hawaiian monk seal is a very good swimmer, but it moves slowly and clumsily on land.

The mother cares for the pup until it can catch food for itself. Seal milk is rich in fat, and after feeding on its mother's milk, a pup can survive for two weeks while she is away catching food.

A female mates again two to six weeks after giving birth. The fertilized egg lies dormant and does not develop for about three months After the egg starts to grow, the pregnancy lasts about nine months, so each birth occurs a year after the previous one.

Right: A mother may identify her pup by smelt even though most seals have a poor sense of smell.

~ FOOD &: FEEDING Seals eat a variety of seafood. Fish, octopus, and shrimp are popular prey, but different spe­cies have their own preferences. Leopard seals catch penguins and also eat other seals. Ele­phant seals, the giants of the family, prefer to feed on rays and small sharks.

Seals may dive deeply to find their food . While a scuba div­er can go down only 1 30 feet in safety, seals can catch fish at depths of 300 feet or more. They avoid the illness called the

Left: During the mating season, the male Weddell seal establishes a territory under ice.

Left: The crab­eater seal feeds mostly on tiny shrimplike ani­mals called krill, although it also eats small crabs. Found mostly near pack ice, it has the largest population of all seals.

bends (a buildup of nitrogen in the blood caused by pressure when diving) by descending with a minimum of air in their lungs. Also, a seal's heart slows to as little as 10 percent of its normal rate when it dives. This conserves oxygen in the blood for the brain and other organs.

On land, seals drink fresh wa­ter, but some scientists believe that seals may drink salt water when at sea and that their kid­neys help eliminate the salt. It is also possible that true seals do not drink seawater and in­stead obtain moisture from their food.

THE BEAR FAMIL V

~ ~ - - -- ~- --

'" .

The bear family contains the largest land-dwelling meat eater. Yet, as a group, these well-known animals depend more on

fruits and vegetables than they do on animal flesh.

I

KEY FACTS

DISTINGUISHING FEATURES OF THE BEAR FAMILY

TEETH & CLAWS

A bears teeth reflect its mixed diet of plants and meat. Its cheek teeth, or carnassia/s, have lost the ability to rip flesh. Its large, flat back teeth are used for crushing.

A bear has sharp, nonretract­able claws on the five toes of each foot. It frequently uses these claws to dig out insect nests or small mammals hid­ing in burrows.

SENSES

A bear is short-sighted, and its sense of hearing is poor. But it has a highly developed sense

of smell. A bear's nose is simi­lar to a dog's, but it is always wet. A bear gets information about its surroundings by sniff­ing the air.

COMMUNICATION

A bear communicates with voice and body posture. The position of a bear's ears may also give a clue to its mood.

When one bear confronts another bear, it lifts its head aggressively and opens its mouth to growl. It may also stand on its hind legs to look more imposing.

THE BEAR'S BUILD, COAT, AND MOVEMENT

DID YOU KNOW? • Plants make up 80 percent of the grizzly bear's diet. • The polar bear is found far­ther north than any other land mammal except the Arctic fox. Of all land mammals, it is the best-equipped to stand cold. • The Kodiak bear, a North American subspecies of the

Bears vary greatly in size, from the 1 ~O-pound sun bear to the l,300-pound grizzly bear. But all bears have heavily built bod­ies with short, muscular legs.

All bears have fur, but the tex­ture varies among different ani­mals. For example, the polar bear has dense, insulating, yel­lowish white fur, while the fur of the sloth bear is long and

Left: The brown bear is good at catching fish and often stands up while fishing.

brown bear, measures up to 1 0 feet when standing. • Polar bears have been hunted for their fur and meat since pre­historic times. • At least nine bear genera and dozens of bear species have become extinct. • Although brown bears have

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Above: The brown bear's combi­nation of sharp and flat teeth suits its varied diet.

shaggy. Most bears are a shade of brown that blends in with their wooded habitat.

Bears usually walk on the soles of all four feet. Because they are all bow-legged, bears have an ambling, shuffling gait. When necessary, a bear may break into a trot or even a gallop. Bear cubs and many adult bears can climb trees and swim. The polar bear is unique among mammals in using only its forelegs to propel itself through water.

been almost eliminated from Europe, they are still the most widespread of all bears. • Bears usually move slowly, but they can reach speeds of 30 miles an hour. • Polar bears have been seen in open seas 50 miles from the nearest ice floe or land. ~

0160200501 PACKET 50

There are seven species of bears, grouped in three genera.

With the exception of the polar bear, which lives

in freezing arctic waters and on ice floes, bears live

mostly in the northern temperate regions of the world.

They are usually found in forested areas, where they

meet with a minimum of disturbance from

humans-their only enemy.

~ EVOLUTION Bears are the most recently evolved carnivores (meat eaters). The dawn bear first appeared about 20 million years ago. The size of a small dog, it lived in areas of Europe that had a sub­tropical climate at the time. Like foxes, raccoons, and dogs, the dawn bear evolved from small tree-climbing mammals called

1. Spectacled bear, Tremarcios ornatus: 4 to 7 feet long. Lives in the northern Andes Mountains of South America.

miacids that lived 30 to 40 mil­lion years ago.

As more species appeared, bears gradually increased in size. Some species became extinct, notably the cave bear, which was much larger than today's bears. The most recently evolved bear is the polar bear, which probably appeared 70,000 years ago.

2. Sun bear, Helarctos malayanus: 3 to 4~ feet. The smallest bear, this rare species lives in tropical forests in Southeast Asia.

~ BREEDING Bears are normally solitary, but they come together to mate. The mating season varies with the species. Some species, such as the sun bear, can breed at any time. After mating, male and female separate. The father does not help raise the cubs.

Gestation lasts six to eight months, depending on the

~ HABITS Bears usually have a home range but are not territorial. They may feed close together on a rich food source such as salmon or berries. They spend their active periods foraging. In some species the active period is daytime; in others it is night. Bears rest in a simple nest in a tree or in a sheltered lair such

3. Sloth bear, Ursus ursinus: 3 to 6 feet. Lives in tropical forests of India and Sri Lanka. Feeds pri­marily on plants. Has long , mobile lips and tongue for eating termites and insects.

species. This time includes a delayed implantation of the egg. Once the embryo begins to develop, its growth is rapid.

Bears that have a winter sleep give birth during this time. Usu­ally two cubs are born, and each is about the size of a rat. Blind, naked, and helpless, they stay in the den for the winter, suckling

as a hollow in a dense thicket. Bears in colder regions sleep

during the winter for up to five months. The bear finds a den or digs out a hollow in a steep slope and lines it with grasses, leaves, and moss for warmth. The winter sleep coincides with the seasonal shortage of food. Since polar

4. Polar bear, Ursus maritimus: 6 to 8 feet. Lives in the seas of the north­ern polar region. It feeds mainly on seals.

from their mother as she dozes. When the mother emerges in

spring, the cubs are fully furred and active. They learn survival skills by accompanying her on foraging trips. The cubs usually spend the next winter with their mother and leave her in spring. They become sexually mature between three and six years old.

bears do not suffer from such shortages, they are less likely to make sleeping dens. But pregnant females or mothers with young cubs may have dens beneath the snow.

Although they do not truly hibernate, bears save energy by staying warm and sheltered during winter.

6. Black bear, Ursus americanus: 4 to 6 feet. Inhabits any type of wood­

land in North America. This species is largely vegetarian , but it may eat

any food it comes across.

7. Asian black bear, Ursus thibetanus: 4~ to 6~ feet. This forest

inhabitant spends much of its time in trees. Eats insects such as ants, as

well as fruit and berries.

5. Brown bear, Ursus arctos: 6~ to 10 feet. Several subspecies, including griz­zly, Kodiak, and European brown.

~ FOOD &: FEEDING Only the polar bear has a diet that is mainly animal flesh. All other bears eat a variety of foods, including seeds, roots, nuts, and berries. A bear may dig with its paws or gather food with its lips.

Most bears eat small rodents and mammals, but few actively hunt them. Polar bears are the most active hunters, usually seeking out and killing seals.

All bears like honey and may break open a bees' nest to find it. The sloth bear raids termite nests, sucking up the insects and larvae with its funnellike lips.

Front cover: A cub stays with its mother for at least two years. Front inset left: The polar bear's feet are adapted for swimming. Front inset right: The sun bear is also called the Malayan sun bear.

THE DEVELOPMENT OF AMPHIBIANS

Millions of years ago, amphibians were the first vertebrates to leave the water and live on dry land. Today they still have many

characteristics of their water-dwelling ancestors.

KEY FACTS

BREEDING ADAPTATIONS

Frogs and salamanders are typ­

ical amphibians that lay their eggs in water. Their tadpolelike larvae metamorphose into adults that can live on land.

Other amphibian species have developed different ways

of breeding, reflecting habitat and the degree to which they have adapted to life on land.

For example, for amphibians living in mountainous areas, fast-flowing rivers are often

NEOTENY

Some salamanders whose lar­

vae develop in water may lack the hormone thyroxine, which

triggers metamorphosis, and they become trapped in the

larval stage. This situation is called neoteny (pronounced

nee-AH-ten-ee) or the "Peter

Pan Syndrome." For example, the Mexican

axolotl can reach sexual matu-

DID YOU KNOW? • The paradoxical frog tad­pole can be up to four times as long as the adult.

• Mudpuppy larvae can take five years to metamorphose.

Right: Fiji tree frogs lay eggs from which a fully formed adult hatches.

unsafe for the development of

young, so they may not have a larval stage in the water.

Some tailed amphibians lay

eggs on land. Others do not even lay eggs. The eggs devel­op inside the female alpine sal­

amander, and she gives birth to fully formed miniature adults. Gastric (stomach) brooding

frogs and ovoviviparous (Iive-

left: Newborn alpine sala­manders look like tiny ver­sions of their parents.

Right: The gas­tric brooding frog "vomits" its live young.

rity as a larva and does not need to develop further. But if

it is fed thyroxine, the axolotl

• Instead of drinking water, most amphibians absorb it through their skin.

• Many amphibians tunnel underground to avoid temper-

© MCMXCI IMP BV/ IMP INC WILDLIFE FACT FILETM PRINTED IN U.S.A

bearing) toads also give birth to live young that resemble fully metamorphosed adults.

will metamorphose into an adult form that resembles the tiger salamander.

Left: If it is fed the growth hormone thy­roxine, the tadpolelike Mexican axolotl grows to resemble a salamander.

ature extremes. One Siberian salamander was found alive

46 feet underground.

• All amphibians are freshwa­ter species; none are marine.

0160200471 PACKET 47

Amphibians reproduce and their young develop

in a variety of ways. Some salamanders give birth

to miniature adults, while most frogs and toads

undergo a complete bodily transformation from larva

to adult, called metamorphosis. Other frogs carry eggs

or tadpoles on their backs. Some frogs even "vomit"

live young that have developed in their stomachs.

~ AMPHIBIAN EVOLUTION Early amphibians, such as Ich­thyostega, first appeared 400 million years ago. It is generally accepted that they evolved from lobe-finned fish, as these fish had fleshy fins containing bones simi­lar to those found in the fore­limbs of early amphibians.

Lobe-finned fish probably lived in warm, shallow waters that were low in oxygen, which made it advantageous to gain the ability to breathe air through the skin or through primitive lungs. Once these "fish" could survive on dry land, they also gained freedom from aquatic predators.

In their new habitat the early amphibians quickly increased

Right: The poison-arrow frog carries its tadpoles to a moist site to develop.

Front inset left: The tiger salamander lives in water or on land.

Front inset right: A male midwife toad carries eggs on his back.

their numbers, reaching a peak around 300 million years ago. The first branchiosaurs (fossil tad­poles) date back to this period.

As more specialized land ani­mals evolved, amphibians de­clined, and today only about 4,100 species remain. These are contained within three orders: tailed amphibians, frogs and toads, and caecilians (tropical limbless amphibians resembl­ing worms).

Modern amphibians usually live on land, but the larvae of many species still develop in water. All amphibians have soft, nonscaly skin from which mois­ture may be quickly lost, so most live in moist habitats near water.

left: Fossil evi­dence suggests that caecilians once had limbs and a tail used for swimming. They evolved wormlike bod­ies as an adap­tation to life on dry land.

left: The mudpuppy has large external gills that help it breathe underwater.

Right: The female marsu­pial frog has a birth pouch on her back that opens to release several live young.

~ M ETAMORPHOSIS In the northern hemisphere, the spawn (eggs) of most frogs and toads are laid in water, and the larvae devel­op there. The female lays the spawn in a large mass or in strings, which the male then fertilizes. Some amphibians lay thousands of eggs, since most larvae fall prey to larg­er amphibians, fish, and even other tadpoles .

Each egg contains yolk to nourish the developing em­bryo and is usually covered in jelly to prevent the egg from drying out and to discourage predators. When sufficiently developed, the larva hatches into the water. Its muscle structure is similar to that of a fish, and it swims by sweep­ing its tail from side to side.

A larva that feeds on aquatic plants breathes through inter­nal gills like those of fish. But a flesh-eating larva that swal­lows its prey whole often has external gills on the back and

left: A female tree frog Jays her eggs on a leaf while the smaller male fertilizes them.

sides of its head, where they are less likely to be damaged. All amphibian larvae also breathe through their skin and the mucous membranes in their mouths.

Once the larva reaches a certain size, a growth hor­mone called thyroxine is pro­duced by the thyroid gland and triggers metamorphosis. In the common frog tadpole the first visible change is the development of hind legs, fol­lowed by forelegs. Then the tail begins to shrink. The tad­pole loses its horny teeth, its mouth widens, and its eyes enlarge so it can see on land . . Internally the changes are just as radical: the tadpole's gills are replaced by lungs and its intestines shrink, ready for the more flesh-based diet of an adult frog .

The length of metamorpho­sis depends on the species and on external factors such as the availability of food and water temperature. The frog tadpole usually takes about 12 weeks to develop into an adult frog.

THE DOG FAMILY

The dog family, Canidae, includes over 30 different species. The gray wolf and the red fox are two typical species. They are among the

most widespread and successful of all flesh-eating mammals.

KEY FACTS

Communication: Canids use

postures and facial expressions

to maintain pack hierarchies.

Their calls range from whines to

howls. They also communicate

with scent: By urinating on a

rock or bush, a dog leaves infor­

mation about species, sex, ma­

turity, and the time it passed by.

Forepaw: Five toes, including a thumb­like dew claw. High-mounted stopper pad .

Hind paw:

Right: Like most can ids, the American coyote uses growls, high­pitched whines, or loud howls to communi­cate with other individuals.

Four claws.

TEETH AND SENSORY PERCEPTION

Teeth: Most canids have small incisors

and large, sharp canine teeth. The

front cheek teeth are flattish for grind­

ing. The sharp back teeth can cut

bone and flesh. This combination

of teeth permits a varied diet.

Sense of smell: Excellent and

vital. In most species, it is at least

twice as good as that of humans. It is

used to track prey, find partners, and

identify individuals and territories.

DID YOU KNOW?

Paws: Cushioned by leathery

pads. Nail-like, non retractable

claws help to provide grip.

They also enable certain can ids to

burrow and others to dig out prey.

• The raccoon dog is the only

dog species that hibernates. It

spends the harsh winters of its

east Asian homeland alone in

its den with its body tempera­

ture reduced to conserve ener­

gy. It is also the only species of

dog that never seems to bark.

coyotes, gray wolves, and do­

mestic dogs, can interbreed and

produce fertile offspring.

• Some dog species, including

• Pups known as poojas have

been bred from a jackal and a

poodle. In one litter, some poo­

jas inherited the jackal's "lan­

guage," while others knew only

the language of the domestic

©MCMXCVIIMP BV/IMP INC. WILDLIFE FACT FILETM PRINTED IN U.S.A.

Build: The smallest canid is the

fennec fox, which is only nine

inches high. At the other end of

the range, the gray wolf may

reach a height of three feet at

the shoulder. These variations in

build reflect the different canids'

physical adaptations to their di­

verse habitats.

Hearing: Very sharp; can detect

high-pitched sound. Usually the

ear flaps can be angled to lo­

cate a sound source. They

help give off body heat.

Eyesight: Keen, but inferi­

or to smell and hearing.

Can ids probably have some

color vision. The eye whites are

usually covered by the lids, ex­

posing only the colored iris.

dog. As a result, these pups

from the same litter were not

able to communicate.

• There is some debate about

when the dog was first domes­

ticated. The oldest verifiable

remains of domesticated dogs

are 9,500 years old and were

found in England.

US P 6001 12 052 PACKET 52

The dog family is characterized by species that ~ HABITS are intelligent, adaptable, and opportunistic. These

Members of the dog family, known as can ids, are found worldwide. Dogs are active at all times of day. Although they are adapted for pursuing prey through open country, most exploit many food sources.

animals' flexibility is one reason for their success in a

wide range of habitats. Although some dogs are solitary

animals, many are very sociable. By banding together

and living in well-structured societies, dogs are able to Some canid species have very flexible social organizations. The gray wolf, for example, may be tackle large prey and provide security for their young.

~ ORIGINS Canids first appeared in North America 36 million years ago. Over the next 20 million years, 42 genera evolved, spreading to Eurasia. Dogs started to inhabit Southeast Asia, Africa, and South America only about 600,000 years ago. Settlers brought their domesticated dogs with them

to Australia, New Guinea, and Madagascar. The ancestors of dogs like the Australian dingo soon became wild and spread through the land.

Although dogs are still wide­spread, the number of genera has fallen to 10. The largest genus is Vulpes, which includes

the red fox. The next largest, Canis, includes the wolf, coyote, and domestic dog.

The dog family also has seven single-species genera. These in­clude the badgerlike bush dog and the raccoon dog, which­true to its name-looks more like a raccoon than a dog.

1. African hunting dog, Lycaon pictus: This pack-living dog is re­nowned for its prowess in hunting prey such as gazelles. It can sustain a speed of 30 miles an hour for sev­eral minutes. This species is ex­tremely social , but the sexes establish separate hierarchies.

2. Gray wolf, Canis /upus:The adapt­ability of the gray wolf has prevented it from being wiped out by humans. Although it is very social , it can also

live in single pairs if warranted by the terrain and other conditions. It is

thought that all domestic dogs are descended from the gray wolf.

3. Domestic dog, Canis familiaris: There are several breeds worldwide. The Shetland

sheepdog, or "Sheltie," is shown here. This breed was first used as

a work dog in the Shetland Islands, where its size suited the rugged envi­ronment. It is now bred as a pet.

solitary or live in a highly ordered pack of five to eight animals de­pending on the terrain, season, and food supply. This flexibility has enabled the wolf to survive despite persecution by humans. For golden jackals, pair-bonding is crucial to survival, especially when there are pups. The adults hunt together and pair for life.

~ BREEDING Wild can ids breed once a year, while domestic dogs are in estrus (breeding condition) twice a year. Females use scent and body pos­ture to show their readiness to breed. Scent is most valuable to solitary can ids, who must find partners in the breeding season.

In a dog pack, usually only the

~ FOOD &: HUNTING Although diet varies depending on the species, all dogs seek ani­mal prey. African wild dogs hunt in packs of up to 30, ambushing prey like impalas and gazelles. They can bring down an animal as large as a zebra by chasing it until it is exhausted.

By contrast, the more solitary red fox may live almost entirely

dominant pair breeds. In some packs, other females do not even come into estrus. Nonbreeding dogs provide food and take care of the young.

Females give birth after seven to ten weeks, often in under­ground dens. Litters typically contain two to six young, but

4. Bush dog, Speothos venaticus: This social species looks more like a bear than a dog. It lives in bush and tropical rainforests, where its com­pact body lets it move through dense vegetation. It hunts in groups and is a good swimmer. It is threatened by habitat destruction.

5. Red fox, Vu/pes vu/pes:The most adaptable of all the foxes, it has accli­

mated itself to a vast range of habi­tats, including urban areas. It lives

with others but hunts alone.

on apples, berries, and rosehips during the fall. The bat-eared fox eats harvester termites, lo­cating them by sound with its huge ears.

Front cover: The gray wolf may be solitary or social.

Front insets: The red fox (left) has a vast distribution. The raccoon dog (right) is a primitive canid.

some species have up to 20 young. The young are usually weaned in a few months.

The young of solitary species are well cared for by both the male and female. Having one well-nurtured litter each year en­sures a high survival rate and is key to the dog family's success.

6. Crab-eating fox, Dusicyon vetu/us: Also known as the savannah fox, this canid has a wide distribution in northern South America. Its varied

diet includes fruit, insects, frogs, carrion, refuse­

and crabs.

,\\:CARD38

WHY MAMMALS FIGHT

Mammals usually fight as a last resort. They prefer to settle disputes with a ritualized show of aggression, intended to deter

adversaries from engaging in physical combat.

KEY FACTS

THE RHINO'S RITUALIZED FIGHT

Most mammals try to settle con­

flicts without fighting, which is a

waste of their energy. Conflicts

between males frequently occur

among large grazing animals like

the white rhino that have well­

defined territories. But these ani­

mals have herd structures and

ritualized threat displays that

help them avoid actual combat.

Male rhinos fall into two cate­

gories: dominant bulls and sub­

ordinate bulls. A dominant bull

possesses a territory of up to one

square mile. He tolerates several

subordinate males in his territory

but not another dominant male.

If two dominant males meet

on the edges of their territories,

they defend their ground with

DID YOU KNOW? • Brown rats live in groups

that have a common scent.

Group members are not very

aggressive toward each other,

but they can identify an out­

sider by its different scent, and

they attack the intruding rat.

• Hyenas fight over prey. They

will attack and even eat young

hyenas that try to snatch food.

• On the inside of his ankle,

a harmless, ritualized display.

Facing each other with lowered

heads, they swing their horns,

slash at plants, pace back and

forth, and spray the ground with

urine. Usually the display ends

Left: Hissing, growling, and­spitting, along with a show of teeth, are usu­ally enough to settle disputes between tigers, which are soli-tary, unsociable creatures.

the male duckbill platypus has a

spur that is connected to a poi­

son gland. The spur is used to

subdue prey. It may also be

used in defense and, perhaps,

to arouse the female.

• Aggression is not associated

only with males. In a troop of

talapoin monkeys, the domi­

nant females are most likely

to mate and reproduce. These

© MCMXCII IMP BVIIMP INC WILDLIFE FACT FILE'M PRINTED IN U.S.A.

Above: White rhinos use a ritual­ized and usually harmless display to establish social rank.

with one male retreating, espe­

cially if he realizes from the scent

of his opponent's urine that the

latter is a particularly dominant

male. If neither male retreats, the

rhinos may lock horns and wres­

tle. In extreme cases, they charge

and strike horns with a crash.

If a dominant male enters an­

other rhino's territory, a more se­

rious confrontation occurs. The

resident male charges at the in­

truder and drives him off, usually

with little resistance.

high-ranking females harass

junior females, causing such

stress that the subordinates

fail to ovulate and have fewer

young. Young animals may

be driven from the group as

a result of this aggression.

• Lions are the only social cat

species, but they often fight to

the death over possession of a

mate or a territory.

0160200591 PACKET 59

Often a mammal must fight to establish rank, defend

a territory, or win possession of a mate. Such conflicts

may be essential elements of the animal's lifestyle.

Usually, the fights occur between members of the same

species. Fights that occur between mammals of different

species are generally over competition for food. Or they

are fights for survivaJ...-between prey and predator.

""IIIIIIIIIIII WEAPONS & PHYSIQUE An animal's teeth and claws may

serve as weapons in a fight, but

they are primarily used for other

purposes such as grooming, dig­

ging, and tearing food. For ex­

ample, a lion's claws, teeth, and

strong jaws are adapted for tear­

ing food rather than fighting.

Nonpredatory animals have

horns, tusks, or antlers that seem

like real weapons. But these too

are used primarily for other pur­

poses. Antelope use their horns

to fend off predators, and feed­

ing elephants break off branches

with their tusks. The walrus's

tusks can pierce an opponent's

Front cover: Hippos fight to defend mating territories or in self-defence against a per­ceived threat.

Front inset left: A lion displays its canine teeth to threaten an aggressor.

Front inset right: Elephants rarely engage in physical com­bat, since they could suffer fatal injuries.

blubber, but they are used main­

ly to get shellfish from seabeds.

Most deer use their antlers to

establish territorial or mating

rights. A stag's intimidating look­

ing antlers may discourage a rival

from fighting, but they are used

in combat if a fight breaks out.

The bodies of many mammals

are adapted for fighting. Animals

that clash head-on, such as the

bighorn sheep and the musk ox,

have thickened foreheads to pro­

tect their brains. Pig species have

facial warts or shieldlike hides on

their forequarters in order to de­

flect tusk blows.

""IIIIIIIIIIII TERRITORY & MATES At the onset of the breeding sea­

son, the largest, most dominant

red deer stags take possession

of a territory and a group of fe­

males. The stag's neck thickens

and he acquires a deep, roaring

voice with which to challenge

rivals that intrude on his territory.

If roaring and posturing do not

deter a rival, a fight will develop.

Since the resident stag is on his

home ground, he has a psycho­

logical advantage over the new­

comer and usually wins the fight,

unless he is very old or injured.

The bull elephant seal also de­

fends a territory and a harem of

females. He rears up to his full

height of 8 to 1 0 feet, roars, and

lunges at his opponent.

""IIIIIIIIIIII THE PECKING ORDER Mammals that live in groups

must establish their place in

the society. Some species use

aggressive challenges to estab­

lish social hierarchy, and the

winner then dominates the

loser. These challenges are usu­

ally ritualized and rarely fatal.

When a conflict arises in a

pack of wolves, the challengers

snarl, raise their hackles, and

curl their lips to display their

teeth. One usually backs off,

but if a fight occurs, it is usually

Left: An elephant seal's teeth are capable of tearing hunks of flesh from an opponent.

fairly brief. The loser rolls on his

back to present his vulnerable

underbelly. Rather than attack­

ing, the winner accepts this ges­

ture of submission and ends

the fight.

A wolf may also show sub­

mission by licking and nuzzling

the lips of a dominant wolf. All

wolves rely on other pack mem­

bers for survival, so these rituals

help to reinforce the pecking

order without the wolves hav­

ing to resort to violence.

Right: Bull elephants frequently engage in gentle combat to estab­lish the herd hierarchy.

""IIIIIIIIIIII SELF-DEFENSE For swift-running or climbing

mammals, flight is often the best

defense. But when flight is not

possible, weapons are used.

The oryx points its spearlike

horns at an attacking big cat or

a hyena. Surrounded by lions, a

buffalo strikes out with its horns

and hooves, which are deadly

weapons. Skunks and zorillas de­

ter attackers with a foul-smelling

spray so that they can make a

quick escape.

Close-knit groups come to the

Left: The fight is over once prey has been caught, since a predator is quick to subdue its victim.

Left: The red deer's antlers can inflict a fatal wound, but a contest usually turns into a test of strength. The outcome of this shoving match will decide who wins the right to mate.

aid of their members. A baboon

caught by a predator such as a

leopard may be released when

the baboon troop comes to the

victim's defense. The troop will

shriek, bark, and even bite the

leopard until it releases its prey.

Adults with defenseless young

can be very aggressive. It is not

unusual for a female brown bear

to defend her cubs against an

attack by the father, who sees

them as a meal. If attacked by

wolves, adult musk oxen form

a protective ring around their

young and lower their massive

heads as a barricade.

"'CARD 39 BIRD SOCIETIES

Bird societies may contain hundreds, even thousands, of individual birds. Although the societies differ greatly, they all benefit the

many birds that make up the community.

KEY FACTS

SOCIAL VARIATION AMONG AFRICAN WEAVERBIRDS

Many African weavers are very

social, often living in mixed flocks

of several weaver species. Out­

side the breeding season, mixed

flocks may congregate by the

thousands to feed. During t his

I period, the male and female

have almost identical nonde­

script brown plumage. The indi­

vidual's safety within the society

depends on not being noticed.

Mixed colonies of several weav­

er species are established at the

onset of the breeding season.

The individual's needs within the

bird society now change. The

male can no longer afford to be

inconspicuous because he must

attract a mate. In order to do so,

he acquires a striking breeding

plumage-usually black and

yellow. He must now take his

chances with predators that are

~ -I ?nl,? YOU KNOW? I -Birds tend to imitate their

neighbors when they see them

I preening, mating, or engaging

I in displays.

I-The male greater rhea from

South America gathers a har­

em of 15 females. The females

I lay their eggs in a single nest.

attracted to the nesting site. Ad­

vertising a nest, forming a pair,

and rearing young become pri­

orities in the male's life.

Some African weaverbird col-

onies have more than a thou-

Left: The red­billed quelea is a species of weaver that forms huge flocks outside the breeding season and large colonies when nesting.

The male then tends the nest,

while the females move on to

another male.

- According to records, 25 roosts

in Tennessee and Kentucky were

found to contain over one mil­

lion birds each.

- The most abundant bird in the

© MCMXCII IMP BV/IMP INC WILDLIFE FACT FILETM PRINTED IN U.S.A.

Left: Outside the breeding season, a male African weaver­bird has fairly nondescript plumage. But at the onset of the breeding season, he acquires a strik­ing set of feath­ers in order to attract a mate. The courtship displays used by the males vary with the species.

sand nests-all very close togeth­

er. The different species within a

colony often look similar. To en­

sure that they mate with their

own species, males have evolved

various behavior patterns in or­

der to attract females.

In some species the male weav­

er builds the nest and advertises

it with a song flight or by hang­

ing upside down in the entrance.

In other species the male chases

the female, then builds the nest

after mating. In still other species

males gather at a communal dis­

play area, or lek, to compete for

the females' attention.

world is probably the red-billed

quelea of Africa. Feeding flocks

regularly number more than

one mill ion. Attempts to con-

trol this agricultural pest have

killed over 1 00 million birds in a

single year, but the overall pop­

ulation remains unaffected.

0160200571 PACKET 57

The benefits of living within a compact society vary

from one species of bird to the next. Some birds find

it advantageous to spend their entire lives within a

close-knit community. Other birds go off on their own

but gather at certain times of the day to feed or roost.

Still other species come together only at particular

seasons in order to mate or to migrate.

~ NESTING About 15 percent of all bird spe-

cies nest in colonies. In some

cases colonies form in locations

where suitable habitat is limited,

but there is enough food near­

by for many birds. The gannet

nests in huge colonies on rocky

islands in the North Atlantic,

where the rich ocean waters

provide the colonies with an

abundance of food.

Mixed colonies of different

species may be advantageous

because stronger species can

protect weaker individuals. Col-

Right: Great blue herons nest in treetops-often at heights of up to 730 feet.

Front cover: Gannets make the best use of limited space by nesting in dense colonies.

Front insets: It is beneficial for griffon vultures (left) to feed together. If one finds carrion, they all eat. The emperor penguin (right) lives in huge colonies in Antarctica.

onies also help provide protec­

tion from predators in instances

where a single bird might be

unable to drive off an intruder.

In a treetop colony of herons or

rooks, the older breeding pairs

frequently nest in the center of

the colony, where they are pro­

tected by the younger birds in

the outer ring.

~ FOOD & FEEDING Many bird species feed in groups

when there is an abundance of

food in a limited space. Curlews

gather on the tidal mud to feed

on shellfish during the relatively

short period of low tide. In the

fall thrushes may congregate

around fallen fruit in an orchard.

At times group activity makes

it easier to obtain food. Flocks of

Starlings, blackbirds, and other

species that feed in groups usu­

ally roost communally when

they are not breeding. Mixed

roosts of thrushes and finches

are often found in woodland

sites. Roosting flocks generally

find safety in numbers, while

birds that roost alone are more

likely to be preyed upon.

Birds that feed alone frequent­

ly gather at dusk to roost in a

group. Some birds feed at wide-

Left: Large numbers of oystercatch­ers gather on coastal flats and estuaries to feed on shellfish.

pelicans swim in a line with their

bills below the water, driving

fish into the shallows in order

feed on them.

Vultures watch each other in

flight. When one spots a dead

animal and swoops down on it,

the others follow, and a large,

squabbling group of birds gath­

ers around the carcass to feed.

Iy scattered sites and then come

together to roost in another

place. Pied wagtails that feed

beside reservoirs and rivers of­

ten roost together at warm

sites such as greenhouses.

A spectacular sight is a mass

roost of starlings congregating

at dusk, then descending into a

small wood to feed. Trees used

for roosting may die from the

accumulated droppings of the

starling flocks.

Right: The male black grouse en­gages in competitive displays with other males to attract a mate.

~ MIGRATING Birds that migrate at night tend

to fly alone but may maintain

contact through calls. However,

most birds migrate in flocks dur­

ing the day. Working as a group,

a flock can detect and respond

to the threat of a predator.

Left: The social cattle egret nests in large colonies and also roosts with other birds.

Left: White pel­icans have a group feeding strategy. Work­ing as a team, they force fish into shallow water, where the flock is able to feed more easily.

Birds come together in fall to

form migrating flocks. In Sep­

tember swallows and martins

congregate on telephone wires

for several days, then suddenly

disappear when the assembled

flock is ready to migrate. Large,

broad-winged soaring birds

meet at points of rising air cur­

rents before they move across

open water. Buzzards, eagles,

and storks gather in spiraling

flocks to migrate when leaving

Sweden, crossing the Bospho­

rus, or setting out from Africa.

Left: House martins gather in large flocks before migrating to warmer climates.

THE SPIDER FAMIL V

The spider "family" is not a family at all. It is a huge and diverse order of more than 30,000 known species. Only about 30

of these are harmful to people.

KEY FACTS

HOW THE ORB SPIDER BUILDS ITS WEB

1. The spider spins a thread that floats between two points. Once the trailing end sticks,

the spider crawls across, spin­ning a thick suspension line for the web it will build.

4. Spiraling out from the hub, the spider spins dry silk. Then

the spider works back in, add­ing sticky silk.

5. When the web is completed (right), the spider stays in the dry hub or walks on the dry silk to avoid getting stuck itself.

DID YOU KNOW? • Few of the large tropical spiders are dangerous to humans. They rely on power to subdue their victims, and their venom is fairly weak.

• Contrary to popular belief, a spider in the bathtub did

2. The spider adds another thread along the suspension line, crawls to the hub (center),

and drops down to make a Y shape. It climbs the Y and spins a thread downward.

not get there by crawling up the drain. It would drown in the water in the plumbing. Usually the spider has fallen off a wall, and it cannot climb

the slick tub surface.

• One species of trapdoor spi-

© MCMXCI IMP BV/ IMP INC WILDLIFE FACT FILETM PRINTED IN U.S.A.

3. The spider makes a more secure framework by spinning the outer frame threads. It adds more radial threads (like the spokes of a wheel) to tighten

the structure.

der-which is named for the tight-fitting lid it builds at the entrance to its burrow-has a toughened abdomen. If it is attacked, this spider retreats

into its hole and presents only

its shieldlike abdomen. :=::!J 0160200481 PACKET 48

Spiders play an important role in nature as efficient

consumers of insects and pests, but they are generally

feared and misunderstood by humans. These highly

specialized predators can sense and trap prey in many

different ways. Spiders are best known for laying traps with

their webs, but some spiders actively hunt, ambush, or fish

for their prey. A few even cast nets over their victims.

~ SENSES Most spiders do not have good eyesight. They often have eight eyes, but these are simple, inef­ficient organs compared with an insect's compound eyes. The wolf and jumping spiders are exceptions: they have two large forward-facing eyes that pro­vide keen binocular (overlap­ping) vision . A jumping spider actually jumps on its prey, so it must judge distances well.

Front cover: The female golden orb­weaving spi­derdwarfs the male.

Front inset left: The jumping spi­derhas two large forward­facing eyes.

Front inset right: The female wolf spi­der carries an egg sac and her tiny young when they hatch.

Right: The fish­ing spider can catch small fish.

A web-building spider relies on touch rather than sight. The many sensory hairs on its legs pick up vibrations in the web. The spider can sense what it has caught and where. Most can even analyze tiny changes in air pressure caused by the movement of an insect's wings.

Right: Raft spiders find prey in the water by feeling for vibrotions on the surface.

~ WEB PRODUCTION Spider silk is strong, sticky, and elastic-excellent for building traps. The silk oozes out of spin­nerets (special nozzles in the spider's abdomen) as a liquid and hardens on contact with the air. As soon as the spider forces out a small amount, it uses its hind legs to pull out more. The process seems to

~ FOOD & FEEDING Spiders feed exclusively on the flesh of animals (mostly insects) that they kill. They use a variety of tactics for capturing prey. Wolf and jumping spiders stalk their victims. Well-camouflaged crab spiders lurk in one place, waiting to ambush insects that stray too close to them. Web­building spiders spin sticky, silk­en traps for their prey.

A spider quickly kills or para­lyzes its prey with poison from its fangs. To digest the prey, it pumps salivary juices over and into the prey's body. This action reduces the flesh to a thick liq­uid that the spider sucks up, using its powerful stomach pump. All that remains of the victim is an empty shell, which may be left near the spider's lair.

stretch and strengthen the silk. The end is anchored, and the spider begins to weave.

The web acts as a 24-hour trap. All the spider has to do is wait. Any insect that blunders into the web is stunned with an injection of venom and bound up. The spider repairs the web, then returns to eat its victim.

TYPICAL FEATURES OF A SPIDER

Jaws: Basically a pair of highly modified pincers, called chelicerae. Each has a hollow fang that poison flows through.

Legs: Always 8. Attached to the cephalothorax.

Abdomen: Relatively soft. Contains such organs as the heart, gut, genitals, and silk glands.

Spinnerets: Organs through which silk for web is released .

Mating can be dangerous for a male spider. Not only is the female usually larger, but she spends her life preying on ani­mals that come near. The male must convince her that he is not another meal.

Jumping spiders have excel­lent eyesight, so the male can make coded gestures to show the female that he is of the same species. Most other spi-

Left: The female net­casting spider spins a small, sticky web and holds it at the tips of her four front legs. She waits for an insect to pass by, opens the net wide, and throws it over the insect.

ders rely on touch. The male may stroke the female, tweak her web, or offer her a bound insect as a gift. If his ploys are successful, the female falls into a kind of trance and lets him mate. If not, he must move quickly to avoid being eaten.

The female lays her eggs in a silken sac that she may guard closely. Some species carry the egg sacs on their backs.

Eyes: Usually 8. Some spi­

ders have 6, 4, 2, or even 1.

Palps: Jointed "feelers" resembling an extra pair of short legs. In male spiders the palps

have large clublike structures on the tips that playa part

in mating.

Cephalothorax. Body segments of head and thorax fused together (an insect always has a head, thorax, and

abdomen). It is protected on its top ~~~~~~~side by a hard carapace (covering)

~~~~~ . similar to the shell of a crab.

Tarsal claws: 2 on each "foot " for gripping surfaces.

Web-building spiders have a third , middle claw that helps them hang

on to threads of the web.