I.

Introduction

Zoo, also known as zoological garden or zoological park, institution devoted to the exhibition, preservation, and study of animals. Although most people visit zoos for entertainment, zoos also educate the public about animal behavior, natural habitats, and the plight of animals in danger of extinction. Zoos play a role in the conservation of threatened or endangered animals by developing innovative breeding programs to maintain populations of species that are in decline. Zoos also conduct scientific research on animal diseases, nutrition, reproduction, and behavior.

At the beginning of the 20th century most zoos captured animal specimens in the wild and displayed these animals like museum exhibits, enclosing them in small cages that gave the animals few opportunities to behave in a natural way. Today most zoos make animal welfare their highest priority. Whenever possible, animals are housed in specially designed enclosures that mimic their natural habitats. Zoo officials also use care when building up their animal collections. To avoid decimating animal populations in the wild, zoos no longer rely on newly captured specimens to stock their collections. Instead they now try to breed a significant proportion of their animals using captive animals.

Zoos employ several kinds of workers to care for animals. Zoologists (scientists who study animals) arrange captive breeding programs and make decisions about the types of species put on public display and the housing conditions they need, while veterinarians are concerned with animal health (see Veterinary Medicine). Zookeepers tend to the day-to-day welfare of the animals in their charge, including feeding and grooming animals and maintaining enclosures. They often deal with questions from visitors. All of these positions require at least a bachelor’s degree in biology, zoology, or another animal-related field, as well as firsthand experience with animals.

Most large zoos house a wide variety of species to attract visitors, but some also focus on species that are in special need of protection. In the United States, the Cincinnati Zoo in Ohio has been particularly successful in breeding lowland gorillas, whose populations in their natural habitat of Africa have declined in recent years due to poaching and human encroachment. Both the Los Angeles Zoo and the San Diego Zoo in California have used novel captive-breeding techniques to prevent the extinction of the California condor, a large vulture with dark gray or brown plumage. In 1987 fewer than two dozen condors were left in the wild. By 2000, after more than a decade of zoo-based breeding, the total population had risen to about 170 birds.

The earliest zoos date back more than 4,000 years, but the first zoo that closely resembles modern zoos was the Imperial Menagerie, established in 1752. Set in the gardens of Schönbrunn Palace in Vienna, Austria, this zoo is still in operation today. The first zoo to open in the United States is a matter of debate. The Philadelphia Zoological Society was established in Pennsylvania in 1859, but the Civil War delayed the opening of this zoo to the public until 1874. In the meantime the New York Central Park Zoo opened to the public in 1861. Canada’s first zoo was the Riverdale Zoo, which opened in Toronto, Ontario, in 1887. This zoo was closed in 1974, and the animals were transferred to the new Metropolitan Toronto Zoo. The oldest Canadian zoo still in existence is the Assiniboine Park Zoo in Winnipeg, Manitoba, which opened in 1904.

Today at least 600 million people a year visit more than 1,000 zoos around the world. The Berlin Zoological Gardens in Germany is home to the world’s largest animal collection, with more than 15,000 animals belonging to nearly 1,700 species. The United States and Canada have more than 200 zoos, including the Bronx Zoo in New York City and the Metropolitan Toronto Zoo. The 107-hectare (265-acre) Bronx Zoo cares for more than 835 different species of animals, one of the most diverse animal collections in the world. The Metropolitan Toronto Zoo covers 290 hectares (710 acres), one of the largest zoos in the world. Most zoos house land animals, but others, known as aquariums or oceanariums, focus entirely on animals that live in fresh water or the sea. Aquariums such as the John G. Shedd Aquarium in Chicago, Illinois, are among the most popular animal attractions today.

II.

Zoo Operation

Zoos require specialized buildings and skilled staff, and as a result zoos are expensive to run and to maintain. Obtaining sufficient funds is essential for a zoo's survival. In the United States and in most other developed countries, zoos are usually financed by funds from cities or states, and by entry fees paid by the visiting public. Zoos also encourage supporters to help finance their research programs.

A number of national and international bodies help zoos coordinate their work. On a global level, the World Zoo Organization in Apple Valley, Minnesota, enables zoos in different countries to exchange expertise and animals that can be used for breeding. In the United States, the American Zoo and Aquarium Association (AZA), headquartered in Silver Spring, Maryland, coordinates the work of zoos and requires that its members maintain the highest levels of animal care. The AZA also manages the Species Survival Plan (SSP), which currently protects more than 120 animal species that are endangered or threatened in the wild. The SSP organizes captive breeding among genetically diverse animals from different zoos around North America as an insurance policy against extinction. They also arrange the release of captive-bred animals back into the wild.

A.

Zoo Exhibits

In the early 20th century, zoos kept large animals in cages with steel bars. These sterile environments, coupled with ever-present gawking visitors, prevented animals from performing natural acts such as mating and hunting for food. By the second half of the 20th century zoos had begun to create exhibits designed more for the benefit of the animals. Zoos constructed large enclosures in which landscaping and terrain simulated the natural environments of the African savanna, North American prairie, or Amazon rain forests. Large mammals, such as elephants, rhinoceroses, and bears, are now often kept in these open-air enclosures ringed by deep ditches.

Many of the world's leading zoos now have giant aviaries—large enclosures that visitors can walk through to watch birds nesting and flying overhead. At the San Diego Zoo, for example, the Scripps Aviary contains more than 200 species of birds from the African tropics. The birds are housed in a huge enclosure complete with rain forest plants.

On a smaller scale, some zoos have created climate-controlled butterfly houses where visitors can view tropical species. At the Perth Zoo in Australia, visitors step into an 800 sq m (1,000 sq yd) enclosure to view hundreds of butterflies from more than 28 species feeding on nectar from flowers or on sugary juices from fruit. Often these insects become so accustomed to people that they will even settle on clothing, something that rarely happens in the wild.

Many animals, including over two-thirds of the world's mammal species, are nocturnal, sleeping by day and becoming active at night. Watching these animals under normal daytime conditions can be a disappointing experience. To get around this difficulty, many zoos now have nocturnal houses, in which special lighting is used to reverse night and day. During the night, the interior of the house is brightly lit, while during the day, a dim light mimics the nighttime but still enables daytime visitors to see the animals without disrupting their nocturnal activity. This day-for-night swap gives visitors a chance to watch nocturnal hunters, such as members of the cat family and civets, as well as fruit-eating bats.

A.1.

Animal Parks

Most zoos were founded in cities where they were easily accessible to visitors. These city-center sites proved to have drawbacks, however. Many large animals—particularly herding species—need space in order to breed, and in cities that space can be hard to find. Several leading zoos solved this problem either by moving or by obtaining open land where animals could be given the room they need. In 1931 the London Zoo pioneered this approach when it acquired nearly 200 hectares (500 acres) at Whipsnade Park, an outdoor reserve that has since become one of the world's prime breeding centers for rare crocodiles, black rhinoceroses, and Przewalski's horses. The San Diego Zoo, which opened on a 40-hectare (100-acre) site in 1916, followed the strategy on an even bigger scale when it created the 730-hectare (1,800-acre) San Diego Wild Animal Park, located in Escondido, California. The park allows visitors to see animals roaming free in a variety of landscapes that look uncannily like their natural habitats. Parks like these have become immensely popular attractions, and the income they generate helps zoos meet the expense involved in raising endangered animals.

In constructing animal parks, zoo architects create a large protected area that appears limitless but in fact has carefully concealed fences and boundaries to protect animals. A variety of methods are used in an animal park to mimic the terrain of a far-off habitat. For instance, the San Diego Wild Animal Park is nestled in the San Pasqual Valley, an area in California that is similar to the African savanna. Zoo architects further enhance the feeling of an African plain by using extensive landscaping and hidden irrigation systems.

Zoo architects consider the needs of animals in the wild before making any environmental enhancements to the animal park. Tall trees are provided to accommodate the browsing needs of the tall-necked giraffes. Tree stumps are left in the ground for rhinoceroses to use as rubbing posts. Foliage in some areas provides shelter or privacy for animals, while other areas are left barren of grass or other plants to enable animals such as zebras to roll in the dirt to repel insects.

When animals feel comfortable in their habitat, they are able to perform natural courtship, mating, and nurturing. As a result, in many cases animal parks maintain the best success rate for captive breeding and newborn survival. Often animals that have been raised in a traditional zoo are released into an animal park as a test to determine whether they will be successful candidates for reintroduction into the wild.

A.2.

Exhibit Design

When developing the way animals are housed and exhibited, zoo planners make the safety of animals and humans the highest priority. Getting close to animals is part of the fun of visiting a zoo, but from a design perspective, it has to be managed with care. Exhibits that allow visitors to come too close to a wild animal may cause the animal to become alarmed or act aggressively. In nature, most animals keep a safe distance between themselves and potential danger, and their instincts tell them to flee the moment this safety zone is breached. In general, zoo animals are much more tolerant of being approached than their wild counterparts, particularly if they have been born in captivity. But zoo animals also have their limits, and zoo planners keep these in mind when they design zoo exhibits. In animal parks visitors can closely observe animals in their “wild” habitat from the safety of monorails or railroad trams, a method that protects animals and visitors alike.

For zoo planners, making animals easy to view is just one aspect of design. Equally important is providing a stimulating environment for the animals themselves. Animal needs vary enormously. Snakes, for example, thrive with few distractions apart from an occasional meal. Other animals, such as pigs, monkeys, and bears, need complex surroundings that give them a chance to play, explore, and forage for food. If they cannot perform these activities, they begin to behave in an abnormal way, often grooming themselves for hours on end or pacing around in the same limited part of their quarters. Repeated actions such as these—known as stereotyped behaviors—are a sign of mental deterioration.

Zookeepers provide mammals with stimulation in many ways. Primates enjoy playing with toys just as much as humans do, and many animals benefit from having to search for their food. In the past, zoo animals were also trained to perform for food rewards, a practice that still continues at some aquariums that keep whales, dolphins, and porpoises. However, zookeepers today are generally cautious about teaching animals to perform tricks for the simple purpose of entertainment unless it can be shown that the animals are benefiting from the experience. At the Woodland Park Zoo in Seattle, Washington, for example, zoo experts train elephants to move logs and perform other work. Visitors may be entertained watching elephants pick up huge logs with their trunks, but zookeepers believe that the training and work involved in these tasks also provide the elephants with enjoyable intellectual activity, which keeps the animals happy.

In zoo design, animals that have precise requirements pose some of the most interesting challenges. Leafcutter ants, for example, snip pieces of leaves from the treetops and then carry them to a large nest underground. In the nest the ants use the leaf pieces to grow a fungus that provides them with their food. To successfully raise these insects, zookeepers must provide the ants with acceptable food plants, a suitable nest site, and climatic conditions in which the fungus can thrive.

Zookeepers must also meet the temperature requirements of a variety of zoo animals. At some zoos, the climate is mild enough for most tropical species to stay outside year round, but other zoos require heated areas to keep resting animals warm. In some parts of the world, the winter cold is so severe that animals must seek shelter indoors. At the Metropolitan Toronto Zoo in Canada, zebras and white rhinos live outdoors during the summer, but in winter—when temperatures can drop to –30°C (-20°F)—these animals have to spend several months indoors. Polar bears have the opposite problem—they have such good insulation that they easily overheat. When designing zoos in warm climates, zoo planners must devise enclosures for polar bears that maintain icy Arctic temperatures.

B.

Animal Care

Zookeepers work with a wide range of animals, but they often specialize in the care of particular animal groups. Some animals quickly become comfortable with their keeper’s presence in or near their enclosure and grow accustomed to their keeper's routine. In turn, the keeper develops a detailed knowledge of individual animals, their health-care needs, and the way they normally behave. Zookeepers are usually the first to notice any changes in behavior or eating habits that might be an indication that an animal is ill or injured.

Over the years, some animals become so accustomed to their keepers that the animals act very much like household pets. However, zookeepers constantly bear in mind that the animals in their care are not domesticated. This is particularly important when animals are under unusual stress—for example if they have recently been moved, if other animals have just joined their group, or if they have newly born young. In these conditions, keepers have to be careful not to trigger defensive behavior in animals that could place the keeper or other animals in danger.

C.

Diet

Keeping zoo animals properly nourished involves many people. Zookeepers give animals their food rations, but veterinarians and nutritionists work behind the scenes to ensure that each animal receives a balanced diet. Zoo kitchens must stock a wide variety of meat, fish, insects, grains, and plants to meet the needs of different animals. Kitchen workers chop and grind food, add vitamin supplements, and sometimes cook food to make it appetizing and nutritious for each animal. Some animals, such as deer, will thrive on plant matter of almost any kind, while many carnivorous mammals and birds will succeed on almost any kind of meat.

Other animals have specific dietary requirements that can be harder to match. For example, in the wild, the giant anteater and aardwolf feed exclusively on termites and ants—foods difficult for a zoo to supply in large quantities. For these animals, a substitute diet based on cooked egg and finely ground meat is used instead. Other insect-eaters, such as chameleons, present even greater problems because they will only eat insects that move. To feed them, zoos often have their own insect-breeding units, where they raise crickets, beetles, and flies on an almost industrial scale.

If zoo animals receive too much food, they can easily become overweight. Apes are particularly prone to this problem. The largest zoo gorilla was a male that weighed 290 kg (630 lb), which is about 60 percent more than the weight of an average male in the wild. This gorilla lived in the San Diego Zoo in the 1930s. Since that time, overweight zoo animals have become rarer. This is partly because zoo animals now are housed in enclosures that permit them to exercise more, and partly because zookeepers have a better understanding of animal nutrition requirements.

D.

Medical Care

If zoo animals become ill, veterinarians are on hand to diagnose and treat the problem. Veterinary care may include dental work, such as removing teeth or abscesses; vaccinating animals to protect them from infectious disease; and managing captive-breeding programs and supervising in the birth of new animals. Veterinarians also perform major internal operations. When the patient is as large as a bear or an elephant, surgery calls for considerable expertise on the part of the veterinarian and the medical support staff to properly anesthetize the animal.

The effectiveness of attentive medical care—and of zoo care in general—is reflected in animal life spans. Captive animals generally have a longer average life span than do animals in the wild. Captive orangutans have been known to live into their late 50s, whereas wild orangutans live to around 30 to 40 years of age. Captive elephants have lived into their late 70s, but their wild counterparts generally do not live beyond the age of 60.

III.

Importance of Zoos

Zoos are important for a number of reasons. Zoos offer a variety of educational opportunities that teach about the behavior and natural history of zoo animals and their counterparts in the wild. Zoo scientists participate in programs that bring animals from different zoos together to breed; in addition they work to protect animal habitats in the wild and they conduct research that advances knowledge of animal biology.

A.

Education

Modern zoos act as centers of education and are designed to make learning about animals an interesting and enjoyable experience. They provide visitors with the chance to see rare creatures, such as tigers, penguins, and giant pandas, that come from many different parts of the globe. Zoos also introduce people to their own local wildlife, offering a valuable learning experience in an increasingly urban world.

Most zoos provide educational programs aimed at a wide range of ages. For young children, a visit to a children's zoo provides an opportunity to see and touch animals other than household pets. Children can learn about the work involved in keeping animals and about the differences between domesticated and wild animals.

For older schoolchildren, many zoos conduct classes or day trips that focus on different aspects of animal biology and the impact of humans on animal life. These educational visits may include discussions about zoo management, the feeding and care of different species, and the methods used to encourage animals to breed in captivity. High school students learn about the complex issues that zoo professionals must consider on a daily basis, including strategies for controlling captive animal numbers so that animal populations do not outgrow the available space in a zoo. Students also learn about the need to strike the right balance between keeping species that attract visitors and keeping those in greatest need of protection.

A zoo’s staff may include scientists who conduct research in the wild. These experts often give lectures describing their experiences in remote places and, in many cases, under difficult conditions. In addition, many leading zoos post details of current research programs on their Web sites, enabling anyone with access to the Internet to keep up to date with new research.

B.

Captive Breeding

For zoos, encouraging animals to breed, particularly animals threatened with extinction, is work of prime importance. Some species, such as lions, breed almost too well in captivity, creating difficulties when their numerous offspring have to be fed and housed. But many other animals, such as cheetahs and giant pandas, are not nearly so adaptable. When kept in zoos, they often fail to breed. The world’s leading zoos now collaborate in captive-breeding programs to enable zoo animal populations to thrive and reproduce outside of their wild habitats, forestalling extinction and eliminating the need to capture wild animals to replenish captive populations. Ultimately scientists hope that animals that have been bred in captivity can be restored to the wild.

Captive breeding also guards against the loss of genetic diversity (the hereditary differences among individuals within a species). Genetic diversity enables species to adapt to and survive changes in their environment. In the wild, genetic diversity is maintained partly by the sheer number of animals in a particular population: A large breeding population means that animals have a wide choice of partners when they breed. In zoos, however, animals have a limited choice of partners, increasing the chance for inbreeding (mating between closely related animals). The genetic diversity of zoo populations steadily declines as a result of inbreeding. This growing genetic uniformity can have serious consequences. Because inbred animals share so many genes, they are likely to be equally vulnerable to disease and other forms of stress. If a potentially lethal disease strikes an animal population with a similar genetic makeup, the entire group may die.

To counteract this danger, scientists at the International Species Information System (ISIS), based in Apple Valley, Minnesota, maintain a database of genetic information about more than 250,000 living zoo animals from more than 50 countries. They also keep data on a further 1.4 million animals that have bred in captivity in the past. Zookeepers use this database to identify potential mating partners that are genetically distinct.

Scientists use a variety of captive-breeding techniques to boost animal populations. In a process called artificial insemination, scientists transfer sperm from a male animal into the uterus of a female animal. Sperm can be frozen and shipped to other locations, enabling scientists to breed animals from different zoos without the need to transport animals. In another captive-breeding procedure, called embryo transplantation, an embryo of a donor female is transferred to another female, known as the surrogate, of a similar but genetically different species. The surrogate mother carries the embryo through a full pregnancy and live birth. At present, embryo transplantation still has a high failure rate, although scientists have successfully used this technique to breed the Przewalski’s horse, a short, stocky horse that now exists only in captivity, and the gaur ox, an endangered type of wild cattle. Scientists hope that in the near future this technique can be used to increase breeding rates for endangered animals dozens of times over, reversing years of population decline.

In many cases, advanced technologies are not necessary to encourage breeding. Instead, zookeepers rely on research performed on the animals’ reproductive habits in the wild to improve captive-breeding success. Using this information, a zookeeper can tailor the way the animal is cared for so that the prospects of breeding improve. For example, in the case of cheetahs in the wild, research shows that courting adults stay apart except during the period immediately before and after mating. When they are ready to mate, cheetahs mark their territories with urine to leave a signature scent for prospective mates. Acting on this research, many zoos now keep prospective cheetah partners separate until both are ready to mate—a simple change that has produced some positive breeding results.

Scientists consider captive breeding a success when they create an animal population that can become self-sustaining in the wild. An example of a successful reintroduction to the wild is the Père David’s deer. This Asian deer was native to marshlands in northeast China before its habitat was destroyed more than 800 years ago and it became extinct in the wild. Since that time the species has been bred in zoos in China and later in Europe. In 1985 four Père David’s deer were released to the wild in a protected animal reserve in China, where they are now breeding successfully.

Other efforts to reintroduce captive-bred animals into the wild have had mixed success. The United States Fish and Wildlife Service initiated a captive-breeding program for the endangered red wolf in 1973. Since 1986 the red wolf has been gradually returned to the wild, first in the Alligator River National Wildlife Refuge in northeastern North Carolina and later in the Great Smoky Mountains National Park in western North Carolina and eastern Tennessee. While the red wolf population has thrived in the Alligator River National Wildlife Refuge, red wolves in the Great Smoky Mountains were unable to find enough food within the park, and malnutrition, disease, and predation contributed to poor survival rates for pups. Likewise, a third of the California condors that were bred in captivity and released to the wild have died. The condors reared in captivity were too tame to cope with natural dangers. In addition, many of the birds were poisoned by lead when they fed on animal carcasses that had been killed by lead buckshot.

C.

Preservation in the Wild

Although zoos deal largely with captive animals, zoo scientists believe that captive breeding should not be considered a substitute for protecting species in the wild. In order to help threatened species, zoo scientists also work to safeguard natural habitats and educate local people about the value of their native wildlife.

The Wildlife Conservation Society (WCS) is one of many zoological institutions that carry out research programs across the world. WCS programs have included efforts to protect the chiru, an antelope that lives on the mountain plateaus of Tibet and Ladakh. The chiru sports a thick brown coat that is used to make fine wool. Although the sale of this wool is illegal under the Convention on International Trade in Endangered Species (CITES), the heavy demand for this wool on the black market has decimated the chiru’s population. The WCS has helped curtail the illegal trade of this wool by alerting consumers and retail stores to avoid buying these illegal wool products. The WCS also sponsored research into the behavior of the mandrill, a type of baboon that lives in West Africa's tropical forest. As part of this work, researchers fitted a small number of mandrills with collars that contain tiny radio transmitters. The scientists tracked the radio signals emitted from the collars to learn the location of the mandrills’ favored feeding habitats. Scientists hope to establish protected areas within these habitats to help the mandrills survive in the wild. In Peru WCS scientists sought to protect macaws and other tropical forest birds. Along with fieldwork to gather information about these birds, this program promoted ecotourism, attracting tourists to explore the natural wildlife. The money spent by ecotourists encouraged local people to protect wild birds instead of trapping and selling them as pets.

In Nepal, staff from the Zoological Society of London conducted similar work to protect the Asian rhinoceros. Nepal's Royal Chitwan National Park holds about 700 Asian rhinoceroses—nearly a third of the world's total population—and their security is vital for the future of the species. However, farmers surrounding the park saw the protected area as a source of additional grazing land, firewood, and other resources in an area where farmland is in short supply. Farmers also treated rhinos as pests when they strayed onto farmland and destroyed crops. Veterinary scientists from the Zoological Society set up clinics to help Nepalese farmers learn to manage smaller numbers of farm animals and increase their productivity. This program enabled farmers to use their small farmlands more economically so that they did not need to expand into protected parklands. The scientists also helped farmers recognize the value of wildlife reserves as a source of income for the area.

Despite their successes, these field programs sometimes encounter situations in which the needs of humans and wild animals are difficult to reconcile. In Central Africa, the fate of the mountain gorilla is a particularly disturbing case. Since the late 1970s, WCS scientists have been involved in a program to protect mountain gorillas by combating poaching and encouraging ecotourism. This work has been stymied by the continuing social unrest in Rwanda, Uganda, and Democratic Republic of the Congo. As a result, no more than 500 of these beleaguered primates are thought to survive.

D.

Scientific Research

In addition to carrying out research in the wild, zoo veterinarians and other scientists study animals in zoos themselves. Among other research, zoo scientists study animal diets and the way animals normally grow in order to identify problems when animals fail to thrive. Research also focuses on animal behavior, particularly with species that live in groups. Zoo scientists study animals that have died in their care to identify any health problems that could be addressed by changing the way animals are looked after. Other scientific research performed on zoo animals has helped scientists develop new drugs, medical equipment, and surgical techniques that improve animal health.

IV.

History of Zoos

The earliest known collections of captive animals date back more than 4,000 years, to the time of the first civilizations in North Africa and the Middle East. In ancient Egypt, the pharaohs acquired giraffes, monkeys, and gazelles from the southern edges of the Sahara. From India they imported chickens—at the time considered exotic birds—which proved to have a very practical value. In the Middle East, the kings of Assyria collected animals in the course of their conquests, keeping them in landscaped zoological gardens. Among them were Asian elephants and Bactrian camels, as well as crocodiles and hippopotamuses, which were kept in specially constructed pools.

In 1490 bc Queen Hatshepsut of Egypt launched what is thought to be the first animal collecting expedition. Five ships ventured as far as Somalia and returned with leopards, exotic birds, monkeys, and a giraffe. The animals were all housed in the queen’s palace menagerie. Two centuries later another Egyptian ruler, Ramses II, took pride in his collection of animals, particularly a lion that is said to have followed his master everywhere, even into battle. In 1000 bc Chinese emperor Wen Wang established a 600-hectare (1,500-acre) animal preserve. Called the Garden of Intelligence, this animal collection was established to study and learn from wildlife. These early menageries were kept purely for amusement and to exhibit royal wealth and power, and they were rarely open to the public.

Around the 4th century bc the ancient Greeks collected animals primarily for scientific education. During this time, increased interest in the natural sciences led to the development of a discipline for the study of animals, known as zoology. Alexander the Great, king of Macedonia, brought back exotic animals from his military conquests. Greek scientist Aristotle, who was Alexander’s teacher, studied these captive animals and compiled his observations in his Historia Animalium (350 bc), considered the first encyclopedia of zoology.

In ancient Rome animal collections were primarily used in brutal contests in large amphitheaters in which animals were forced to fight each other, or humans, to the death. In the 1st century bc Roman emperor Titus once put on a spectacle in which 5,000 animals were killed in one day, and 9,000 more were slaughtered over the next three months. The number of animals slaughtered during these spectacles was so massive that the populations of some animals, including leopards, became endangered. These animal games continued to flourish well into the 12th century, at which time their popularity declined.

In England early in the 12th century, during the reign of King Henry I, a royal menagerie was collected in a mansion in Woodstock in Oxfordshire. The menagerie included camels, lions, and lynx. A century later King Henry III moved the collection to the Tower of London. An animal collection remained in the tower for the next 500 years. The public was invited to view these animals for a small fee, and those who brought a dog or cat to feed to the lions were admitted for free.

When Spanish explorer Hernán Cortés entered Tenochtitlán (now Mexico City) in 1519, he found an enormous zoo. The Aztec leader Montezuma II had gathered a spectacular collection of animals. Ten ponds were home to flocks of waterfowl; hundreds of vivariums (glass enclosures) housed reptiles; and huge cages with bronze bars secured pumas, jaguars, sloths, and monkeys. More than 500 keepers attended the animals, including a team of nurses who tended sick animals and kitchen workers who prepared food for the animals.

With the start of European exploration and colonization, from the late 15th century onward, the first major collections of animals featuring species from several continents were established. When the explorers returned home from their travels, they brought many animals with them. These early animal collections, which included ornamental species such as parrots and peacocks, were assembled largely as curiosities.

By the 18th century, however, zoos began to reflect the growing scientific interest in the natural world. In 1793 a zoo was established in connection with the Jardin des Plantes (Botanical Garden) in Paris, France, fostered by figures such as French zoologist Georges Cuvier, the leading animal anatomist of his day. By the time the London Zoo opened in 1828, scientific study had begun to rival entertainment as a reason for collecting animals. During the mid- to late 1800s, thousands of species were brought back to Europe, where they were examined and classified by biologists. This flood of living animals—or their preserved remains—demonstrated the unsuspected vastness of the animal kingdom, particularly in the tropics.

Modern zoo design dates back to the early 1900s, when animal trainer Carl Hagenbeck opened a zoo in Stellingen, Germany. Hagenbeck housed animals in outdoor enclosures that used ditches and moats instead of bars. Many of these enclosures were built to imitate natural habitats, such as rocky mountainsides or open plains.

The first modern zoos founded in the United States were the Philadelphia Zoological Gardens in Pennsylvania, established in 1859; Central Park Zoo in New York, which opened in 1861; and Lincoln Park Zoo in Chicago, Illinois, which opened in 1868. By the early 1900s American zoos started to become involved in conservation work, as the growing human impact on North America's wildlife became steadily more apparent. Many zoos established collections of threatened native animals, but not all of the species succeeded in captivity. Most prominent among the failures was the passenger pigeon, once the most abundant bird in the world. As a result of overhunting, the species died out in the wild in the late 1800s, and the last living specimen, a 29-year-old captive-bred female named Martha, perished at the Cincinnati Zoo in 1914.

Most of today's zoos exhibit both common animals and ones that are much rarer. More recently, however, some zoos have been founded to concentrate solely on animals that are at risk. In India, for example, the Madras Crocodile Bank Trust breeds the endangered fish-eating gavial. In Australia, the Marsupial Cooperative Research Centre at the Perth Zoo raises endangered marsupials in captivity. Writer and naturalist Gerald Durrell established the Jersey Wildlife Preservation Trust in 1963 on the island of Jersey off the French coast. The trust has successfully raised a number of exotic animal species, such as the Mauritius pink pigeon, Bali mynah, and Rodriguez fruit bat, all of which are on the brink of extinction in the wild.

V.

Current Issues

With so much of the world's animal life currently under threat, scientists are working to improve information technology to help with captive breeding. DNA fingerprinting, in which an animal’s unique genetic makeup is used for identification, is fast becoming a matter of routine. Advances in computer databases and the Internet will enable an expanding pool of genetic information about captive animals to be made available to zoos all over the world. This will decrease the prospects of further inbreeding, as threatened species fight to survive.

In looking ahead, some experts warn against placing too much reliance on zoos as biological 'arks,' where species can be protected regardless of environmental changes outside. Critics warn that zoos, like all human institutions, are vulnerable to unforeseen events, such as disease, fires, and financial or social upheaval. More significantly, the number of species that zoos can protect is strictly limited. Today's zoos are home to no more than a few thousand species—only a fraction of the world's animal life. Zoos already have to perform a delicate balancing act between large, crowd-attracting animals and threatened animals that have less visitor appeal. During the 21st century, zoo experts will face even harder decisions about which species they can afford to save.