The History of Zoos
The history of zoos is a mere testament of man’s enduring fascination with the natural world, wild exotic animals in particular. From domesticating, collecting and displaying (showing off) animals to establishing zoological institutions for education, conservation and research, while maintaining a certain entertainment factor.
Before the Common Era, BCE
First collections of wild animals found, Egypt - ≈3,000 BCE
When hunter-gatherers started to settle, domestication of animals appeared around 10,000 BCE. Holding wild animals in captivity is as old as these first attempts at domestication. The first 'collections' of wild animals, however, could only be traced back until the earliest urbanised civilization began about 3,000 BCE. These early collections, within the context of their times, were in effect the earliest zoological facilities, even though it would take several thousand years before people developed the concept of such facilities. Let alone that they would be called zoos.
Since about those days Before the Common Era (exotic) animals have been collected to satisfy man's curiosity for beautiful, dangerous, extraordinary, and human-like creatures of the animal kingdom. At first it were the world's rulers who, using their power and authority, collected animals and established menageries, either by having the animals sent from occupied territories or by receiving animals as gifts in return for favours. At the same time animals became part of the trade of luxury items between civil societies that prospered.
The oldest known zoological collection was revealed during excavations at Nekhen in 2009 of a menagerie that dates to about 3500 BCE. Nekhen was the religious and political capital of Upper Egypt at the end of prehistoric Egypt (3200–3100 BCE). The graves revealed remnants of 14 animals in total (May 2015), including a leopard, two crocodiles, hippopotami, hartebeest, two elephants, baboons, and African wildcats. The animals were found to have had broken bones.
Trade of exotic animals became common, Mesopotamia - ≈2,000 BCE
In the Mesopotamian region flourishing city-states developed along the Tigris and Euphrate rivers. This area was suitable for agriculture but lacked resources to sustain other technologies. Thus, trade developed over time, and with increasing prosperity the Mesopotamian merchants started trading wild exotic animals as well. These animals became part of animal collections of royalty and wealthy individuals, the one even more impressive than the other. Gardens and parks with fishponds became the standard of how the rich lived, with the larger parks containing their animal collections. Royalty kept lions and other animals, such as ibis, cranes, herons, pelicans, elephants, ostriches, gazelle and ibex, for exhibition purposes. Royal lions were kept in cages and pits during the Third Dynasty of Ur at the beginning of 2100 BCE.
During this period an Indus civilization existed along the Indus river in a region what is now Pakistan. Not much is known about their interest in animals, but animals - like the elephant - were domesticated. Furthermore, they traded items with Mesopotamian merchants. Including wild animals, such as elephants, tigers, rhinoceroses and antelope, because authenticating seals used on traded cargo depicting these wild animals give evidence of this.
Although no direct evidence exists, it is thought that animal collections have been part of the Indus society due to the above and the fact that wealthy people had access to other ancient societies that held animal collections.
First dedicated expeditions to collect exotic animal, Egypt
In Egypt Hatshepsut, the only female pharaoh (1508-1458 BCE), was responsible for expeditions to the Land of Punt, in present-day Somalia. Under her rule, the most ambitious and famous expeditions were made to Punt between 1510 and 1490 BCE. These expeditions were proudly recorded pictographically in her Deir el-Bahri temple. Many trade goods were bought in Punt, and Egypt had a new supply of myrrh, trees, ebony, ivory, gold, cinnamon, incense, eye paint, skins of southern panthers, and live animals. Like many civilizations that accumulate wealth, Hatshepsut took an interest in exotic animals and ordered to collect live animals, too. The animals brought to Egypt from Punt included apes, monkeys, birds, greyhounds, cattle, leopards, cheetahs, rhinoceroses, and giraffes. They formed the largest known animal collection in Egypt to that time. It is unknown how the animals were housed or maintained, despite the fact that the greatness of the acquisitions was depicted in murals.
Egyptian domestications attempts included many different kinds of native wild ruminants and carnivores. These animals were fattened on bread dough, as were many kinds of birds. A variety of birds were kept in domestic flocks, particularly geese and ducks, but also swans, doves and cranes. Wealthy Egyptians at all times kept menageries, in which they brought up the animals taken by the lasso or by the dogs in the desert, as well as those brought into Egypt by way of commerce or as tribute. Egyptians particularly liked to tame as many of these species as possible. Pharaoh Ramesses II (1301-1212 BCE) for instance had a tame lion that not only accompanied him into battle, but also guarded the royal tent at night. Ramesses II had like other pharaohs an extensive animal collection, primarily supplied with exotic African species that were brought back as result of the Punt expeditions.
First animal parks and reserves appear, China - ≈1,500 BCE
The Yellow River Basin in China is often considered the birthplace of ancient Chinese civilization. Animals were and remained through different time periods essential for the subsistence, ritual, and economic lifeways of past societies living within this Basin. It turned out that domesticated animals were sourced from a broad area around the Yellow River Basin to serve the needs of the societies in ancient China. It was during periods of unification (the dynasties) that significant animal collections were developed. On the one hand collections served the purpose of ritiual sacrifice as it was documented throughout the Zhou dynasty (1050-221 BCE) and into early imperial times that designated officials were in charge of animals destined for ritual purposes. These animals were kept in parks or breeding grounds. On the other hand imperial parks or parks owned by the wealthy class contained animal collections including exotic species just for the owner’s interest and advertise their power, as they did in in other ancient societies. Thus, the animal collections served practical and aesthetic purposes, starting with the Shang dynasty (1600-1050 BCE). The first well-known animal reserve that was created by the founder of the Zhou dynasty, Wen Wang. He established a park of 375 hectares, called the Garden of Intelligence, for hunting and fishing. These animal reserves or parks were large, walled-in natural areas that required their own staffs of administrators, keepers and veterinarians.
First ecosystem exhibit for exotic animals and plants created
In Mesopotamia Sennacherib's predecessors kept a variety of animals in parks. Herds of deer, gazelle and ibex for instance were transported from conquered territories to Assyria, and species like lions, apes, ostriches and falcons of which some species were never before seen in Assyria. Not only animals were imported, also foreign trees and fruiting plants. But, Sennacherib (704 – 681 BCE) was the first to create dedicated areas for these exotic animals and plants as an ecosystem exhibit. He is also believed to be the creator of the famous hanging gardens of Babylon, which were actually at the palace garden of Sennacherib located at Nineveh.
First scientific research interest in exotic animals, Greece - ≈500 BCE
The Greek and Roman societies existed alongside those of Mesopotamia and Egypt and eventually conquered them both (1100 BCE – 27 BCE). The Greek interest in science reflected their attitude towards animals which went beyond observational knowledge. Animals played a central role in their culture and mythology, and their animal collections during this period were not just about displaying power, but also about understanding the animal kingdom. Taking into account their travel and trade it was expected that large animal collections would be set up. The lack of wealth in the ruling city-states prohibited such developments of animal collections, but bears and lions were common among travelling showmen and animal trainers, and exhibited for entertainment. It was not until Alexander the Great that exotic animals set foot on Greek soil. Due to his military campaigns, the Greek not only expanded their territory significantly but their curiosity horizon as well. They came into contact with little-known lands and animals. Many of these animals were sent back by Alexander the Great to Aristotle. Philosophers like Aristotle studied animals for scientific purposes, and his writings indicate a rudimentary approach to zoology. Whether the animals remained alive and maintained in collections is still unknown.
From collection to entertainment and conscious abuse of exotic animals, Roman Republic
Meanwhile, the Roman Republic (509-27 BCE) was on the rise. It began with Rome as a city-state and eventually occupied the entire area what is called the Italian peninsula, nowadays. During this period they were content with their native soil and wildlife, but this changed when the Roman Empire saw the light of day. Roman imperialism spread throughout Europe, Africa and Asia, which brought the Romans into contact with exotic species, such as lions, leopards, ostriches, hippopotamuses, crocodiles, monkeys, rhinoceroses and elephants of course. Many of which were captured or diplomatic gifts and sent back to Rome.
The Roman view on nature was similar to the Greek attitude, practical and utilitarian. In other words use natural resources productively. Meaning that extensive agriculture, deforestation, mining and hunting had significant environmental consequences. So at the end of the reigning period of the Roman Empire (around 500 CE) they had depleted many of their natural resources, including wild animals.
Unfortunately, the fate of the captured animals was one of abuse and death. Already during the Roman Republic animals were used for popular ‘entertainment’ shows called baiting, where animals were used to attack another animal, for instance dogs versus bear. Elephants, together with their mahouts, that were taken in battle with Carthaginians were marched to Rome, exhibited along the route and then massacred in order to avenge the losses inflicted by these elephants during military conflict.
Read more in the Common Era
Common Era, CE
Roman Empire – use and abuse of animals
The animal abuse under the Romans ruling during the Republic (see Before the Common Era) got even worse during the Roman Empire (27 BCE – 476 CE). Early Republic baiting evolved into increasingly elaborate spectacles called venationes. These type of public spectacles featured contests between beasts or between men (prisoners) and beasts staged in an amphitheatre, usually in connection with gladiator shows. Lions, hyenas, leopards, giraffes, and rhinos entered the arena and were, in most cases, killed. The scale of this slaughter was enormous, with over 9,000 animals being ordered, transported and eventually killed for a single event: the inauguration of the Colosseum amphitheatre in Rome in 80 CE. The animals must have been sourced from animal collections, but not much has been written about how and where the animals were kept. When the animals were not scheduled for fighting, they were used in yearly parades honouring those who passed away.
The rich, emperors and civil administrators, showed that it was not all about violence and abuse when it concerned animals. A more humane way of keeping both native and exotic wild animals could be found in villa gardens, large parks and hunting reserves. The emperors had the largest collections, but it all ended with the end of the Roman Empire, especially the import of exotic animals into Europe stopped for centuries. Imports started again with the crusaders and the early explorers.
The disintegration of the Roman infrastructure led to the loss of the Roman animal collections, and fortunately the abuse during the venationes. Monarchs, monasteries and municipalities, however, continued to keep their wild animal collections. It were these collections, that continued to the Medieval period, and emerged as the menageries of the aristocracy during the Renaissance period. Again, or perhaps still, the animals were considered symbols of power and wealth.
Middle Ages – royalty, aristocracy and the Church ruled
During the Middle Ages in Europe, the keeping of exotic animals became a symbol of royal privilege. The first zoos emerged as private menageries, being spectacles of power and wealth enjoyed exclusively by royalty, the Church and the aristocracy. The most important monarchies, monasteries and municipalities maintained animal collections. The animals were seen as exotic collections to be displayed, rather than living creatures deserving of compassion. Tigers, lions, bears, and elephants were kept in cramped cages or chained up, with bare stone floors and minimal shelter from the elements. They often suffered from injury, stress, and early mortality due to the poor conditions and lack of medical care. But this was of little concern to the menagerie owners. The animals existed solely for the amusement of their high-status visitors. The Tower of London’s royal menagerie, established in 1235 CE, included a range of animals from lions to elephants, often received as gifts from other monarchs and explorers. But as said in other countries such menageries existed as well, such as Switzerland, Holland, Denmark, France, Italy and the Vatican. Pope Leo X kept tropical birds, lions, leopards, other cats and mammals.
Aztec and Inca civilizations were continuations of Americas’ earlier civilizations that may be as old as those of the Old World. They reached their heights and flourished between 1400 and 1500 CE. The Aztecs were well-structured and traded throughout the Americas. Extensive gardens and royal forests contained impressive animal collections. The Aztec emperor Montezuma II, in what is today Mexico, maintained one of the earliest animal collections in the Western Hemisphere. It was destroyed by Hernan Cortes during the Spanish conquest in 1520, who left in writing detailed descriptions of the beauty and value of what he destroyed. The Incas, like the Aztecs, had animal collections and gardens, both real and artificial. No detailed information on these collections was left by Pizarro, who conquered the Inca civilization.
Renaissance period – public access to zoos
In Europe, during the Renaissance period, a transition began when some of the rich elite occasionally allowed naive citizens to view their collections. Private curiosities slowly became public spectacles. The Medici family’s menagerie in Florence was one of the first to allow public access, setting the stage for the zoos that would follow. This introduction to exotic wildlife to the public accelerated when travelling menageries came into existence and zoos were founded which allowed the common people to visit their grounds. At first one day or one single morning a week, Sundays for instance, but soon this was increased until the animals were at display for everyone, year-round.
Age of Enlightenment – science became important
The Age of Enlightenment in Europe was an intellectual and philosophical movement in the 17th and 18th century during which new ideas and world views were developed. To satisfy the curiosity of the intellectuals research on many topics was initiated, including animals. This surge for intellectual and scientific progress, fundamentally changed the interaction of mankind with the natural world. It was during this era that the concept of the zoological garden (the zoo) became more than merely a public spectacle. There was more attention for the study and conservation of animals and nature in general. These type of zoos were first established in Europe in the late 18th century, with Tiergarten Schönbrunn (Vienna Zoo) as the world’s oldest extant zoo. The zoo was founded in 1752 by Emperor Franz I and his wife Maria Theresa, and initially part of the imperial menagerie at Schönbrunn Palace, intended for the scientific study of the animal kingdom. Another old still existing zoo driven by scientific curiosity is the Menagerie du Jardin des Plantes in Paris, established in 1793. London followed in the next centurywhen the Zoological Society of London established London Zoo in 1826, and opened its gates to the public in 1828. This marked a significant evolution in the concept of zoos, because unlike its predecessors, London Zoo was established with a clear scientific mandate.
Many cities in Europe and North America followed by establishing their own zoo, responding to people’s growing fascination with exotic places and animal, and often inspired by the model of London Zoo. Hence, the role of the zoo altered, and the zoo became a place where education, research, and leisure converged, allowing people from all backgrounds to engage with and learn about nature, specifically the animal world.
In the 1870s and 1880s it became common to display groups of indigenous peoples from Africa, Asia, South America and the Pacific Islands. These peoples were seen as scientifically "primitive" or "exotic". The so-called "ethnological expositions" were travelling exhibitions, but were also part of entertainment in zoos. These living displays often emphasised colonial propaganda, portraying the cultures as savage or uncivilized to justify imperial expansion. Individuals were coerced or lured into participating with promises of wages or travel, then confined in artificial villages. These human zoos, as demeaning as they were with people as objects, continued into the early 1900s, and only declined after World War I. It were different times but deeply troubling that this kind of entertainment was organised.
The Hagenbeck revolution – bar-less enclosures
Never mind the scientific attention for animals during this era, zoos were still designed to exhibit the animals in the best possible way. Kept in small cages, concrete and iron bars, the animals’ welfare was clearly impaired. But in the early 1900s matters changed when Carl Hagenbeck, a German animal entrepreneur from Hamburg, decided that his travelling exhibition of animals and indigenous people should be on display at a permanent home, the Hagenbeck Tierpark (Hamburg Zoo). More importantly, the animals had to be displayed with an unobstructed view in a natural setting. And so the bar-less, moated open-air enclosures with panoramic views were introduced. Hagenbeck was a well-known figure in the world of zoos. In fact, practically every major zoo, circus or private collector in the world bought animals from Hagenbeck. So, the way he designed his own zoo, opened to the public in 1907, had impact and became known as the Hagenbeck revolution of zoo design at the beginning of the 20th century.
At the same time it became clear that a dramatic number of species was decreasing in the wild at the beginning of the 20th century. This raised concern about the need to protect and conserve the planet’s biodiversity. Unfortunately, the still increasing number of zoos needed exotic animals urgently, while these exotic species often died in captivity without producing offspring. Therefore, wild animal traders sourced their animals in the wild. This had to change.
Post-World War zoos – a focus shift towards conservation, education and welfare
So, after the devastating period during both world wars, which were also challenging for zoos, zoos began to shift their focus from mere exhibition to conservation and education. This marked the beginning of ethical considerations, for the first time in the history of zoos.
This intensified during the 1970s, when growing interest in animal rights and criticism of welfare standards in zoos led to questioning the concept of zoos and a fall in visitor numbers - causing the many zoos and zoo organisations to re-evaluate their role. During the 1980s and 1990s many individual zoos started to put more emphasis on the role of conservation in their activities. Jersey Wildlife Park (now Durrell Zoo) was one of the pioneers of this new kind of zoo which kept wild animals primarily for conservation purposes, identifying a threefold purpose of biological study, captive breeding for reintroduction, and education and training, all focussed on actively supporting the survival of threatened species.
Next came the understanding that animal welfare was impaired when social animals were housed alone in small cages, and that continuation of endangered species being sourced from the wild would be counterproductive in the end. This caused a shift in zoo policy worldwide, knowing that diversity of animals in the wild was at stake made breeding of endangered species in zoos necessary and this relied on good animal husbandry systems that allowed social interaction. Maintaining the diversity of the animal kingdom became the main driver for many zoos and a requirement to be member of a continental zoo organisation such as EAZA and/or the global zoo organisation WAZA , established in 1935. Therefore, their mission is focused on sustainability in general, leading to objectives concerning welfare and care of animals, biodiversity conservation including ex-situ endangered species breeding programmes, environmental education and global sustainability.
This can be recognised in changes made in the management of modern zoos. The animals are offered an environment which is more or less similar to their natural environment, particularly to allow development and expression of natural behaviour. The result lead to enclosures that provide animals the opportunity to isolate themselves or hide, which is natural behaviour too and makes them less visible sometimes. This requires understanding of the public that primarily come to see the animals, and therefore needs explanation, guidance or a different approach. After all, visitors bring in the ever so needed money, thus a decreasing number of visitors is undesirable. Careful balancing between these two "benefits" is therefore essential.
In the field of biodiversity conservation, the zoo puts on a fight against the extinction of species based on two pillars. Conservation of species on a quantitative and qualitative level, and education of the public about the depletion of planet earth and its consequences. The latter is often enthusiastically addressed with various educational programmes, and information points in the zoos with volunteers as staff.
The introduction of endangered species breeding programmes
A few decades ago people realised that contribution of zoos to the conservation of species would only be effective if international co-operation between zoos could be established. The 'law of large(r) numbers' plays a significant role on gene, race and species level. Inbreeding, and thus the appearance of hereditary problems, should be avoided as much as possible. Therefore, targeted breeding programmes were developed which are centrally managed and directed, and make use of the gene pool of as many zoos as possible. In Europe this has led to the EEP and the European Stud Books (ESB). The management thereof is appointed, per species, to a specialised zoo and a dedicated employee who bears the responsibility for this task. He or she plays an important role in the maintenance of the collective species population in zoos, and possibly in the wild. For several species programmes are developed to try and return "artificially" (ex situ) bred animals to their natural habitat in the wild. This requires a still existing natural wild habitat of course, but several examples exist of the contribution of co-operating zoos preventing species from going extinct and even return captive bred animals in the wild, such as Przewalski horse, California condor, Arabian oryx and Socorro dove.
Developments in husbandry and care systems, conservation and education efforts
In zoos and umbrella organisations on regional and global level improvements are ongoing, due to advancing technlogy and knowledge. Money is used for hands-on research for developing better husbandry and care systems next to conservation projects on location for better understanding of the causes and consequences of the degradation of natural habitats on wildlife populations. The ultimate goal of research collaboration between zoos, research institutions and universities is to preserve the species at stake.
To get across the message on the current state of global and local nature, its biodiversity and the measures needed to preserve or improve this, it is important that zoo visitors are handed information in such a way that the message will stick. To enhance the educational outreach many zoos use mobile apps to inform the visitors and help them tour the zoo grounds. Zoos offer close encounters with the animals - via feeding sessions or via walk-through exhibits or other kind of immersive enclosure design - to make the experience of the zoo visit more impressive. However, current technology can enhance these experiences to another level. Zoos are experimenting with augmented reality and virtual reality to assist their educational efforts. Especially virtual reality, where people immerse in a fully digital world can create experiences beyond people’s imagination. It is very powerful to immerse in lifelike simulations of a natural environment and experience what the animals see, hear, smell and feel. Together with matching information about ecosystem degradation, land-use, and necessary conservation efforts this probably trumps the static signs at the enclosures with lots of text to read when it regards the take-home message.
By addressing biodiversity conservation via research and educational efforts, zoos become important players in the implementation of global biodiversity strategies. They contribute to the achievement of targets set by international agreements like the Convention on Biological Diversity (CBD) and the United Nations’ Sustainable Development Goals (SDGs). By participating in these global efforts, zoos help to address critical issues such as habitat loss, climate change, and species extinction.
Zoo legislation
Despite their long history the functions of zoos have only become subject to legislation during the Age of Engagement when new ideas were developed about the role of zoos in scientific research and stewardship of animals. Not surprisingly this led to the very first animal welfare law in England, the Cruelty to Animal Act of 1876. Other European countries followed more than 30 years later with Cyprus being the first in 1910. Zoo legislation is often tied to general animal welfare regulations. This legislation has changed over time, in line with the fundamental change in the growing public awareness and understanding of the need to protect wild animals and their habitats, and the concern for animal welfare. This has also had implications for the role expected of modern zoos, including the development of endangered species breeding programmes.
On the international level trade and transport of circa 5,000 endangered species, of which zoo animals form a large part, are regulated by the Convention on International Trade of Endangered Species of Wild Flora and Fauna (CITES). However, this only applies to countries that have signed the treaty. The treaty entered into force in 1973 and membership of countries is voluntary. In addition to CITES, safe transport by air of live animals, including zoo animals, is ensured by the standards set by the International Air Transport Association (IATA) in its Live Animals Regulations (LAR) - first published in 1969. Again and unfortunately from the animals’ point of view, IATA membership is also voluntary.
In many countries, most captive animals (including reptiles, fish, birds, and invertebrates) fall under the protection of national animal welfare regulations. In other countries, for instance the USA, welfare regulations cover only certain mammalian species. Over time the number of species protected by the regulations of each country increased while the standards found in these regulations became more strict. The objective of all basic animal protection regulation is to prevent cruelty to the animals and provide adequate care and living conditions.
In the European Union regulation was developed specifically addressing zoo animals. This Zoos Directive of 29 March 1999 demands the implementation of its requirements by the EU Member States not later than 9 April 2002. Its objective regards protection of wild fauna and conservation of biodiversity by setting requirements for licensing and inspection of zoos. This requires a challenging change in the role of zoos, and that of the regulatory bodies – the competent authorities – as well. Issues at stake where it concerns animal welfare are for instance the assurance that animals can express natural behaviour, and that the conditions of the animals’ captive situation guarantee safety of the animals, and visitors as well, while measures to prevent trauma and disease must be in place, and dedicated veterinary care when necessary.
Further to this, zoo animals’ health, and thus their welfare, will improve when legislation is in place to safeguard animal health in a country.
Finally, zoo associations such as WAZA, AZA, EAZA, ZAA, PAAZA, and SEAZA regulate the zoo industry through setting their own standards, that their members must abide by. Membership is voluntary but highly regarded in the world of zoos. These standards seem to be effective according the authors of research published in January 20221. The findings reveal that zoos accredited by AZA show improved compliance with the USA - Animal Welfare Act compared to non-accredited facilities having animals on display.
Still, when captive wild animal collections are not managed according to national or international legislation or by codes of ethics or professional best practice this may lead to a real concern for the welfare and husbandry of the animals within these facilities. Licensing and inspection by authorities remain a challenge.
1 Riedman, Kailer & Cunningham, Gregory & Divincenti, Louis. (2022). Does Accreditation by the Association of Zoos and Aquariums Correlate with Animal Welfare Act Compliance?. Journal of Applied Animal Welfare Science. 26. 1-8. 10.1080/10888705.2022.2028150.
(Source: B. Kohn, Zoo Animal Welfare, 1994; Animal Legal & Historical Center, Michigan State University; Council Directive 99/22/EC, relating to the keeping of wild animals in zoos; Welfare & Legislation, Wild Welfare)
In summary
Zoos have transitioned from ancient displays of power to centres for conservation and education, although the effect of the educational efforts is still questionable and a topic for further research. As in most fields the Age of Enlightenment initiated a shift towards scientific research, leading to the creation of the first modern zoos in the 19th century.
Enclosure design innovations like naturalistic habitats have replaced cages, focusing on animal welfare and enhancing the visitor experience. Post-World War conservation efforts have positioned zoos as important partners in species preservation and research.
Modern zoos integrate technology for enhanced animal care and visitor engagement. They are relevant in global conservation, aiming to sustain biodiversity and support wildlife in their natural environments. The evolution of zoos reflects a growing commitment to protecting and understanding our planet’s wildlife and ecosystems.1
The evolution of zoos depicted by "Rabb’s Arrow"2:
"Rabb’s Arrow" describes the evolution of zoos from 19th century menageries to 20th century Zoological Parks and subsequently 21st century Conservation Centres. The arrow invited multiple interpretations and hinted at the growing role of understanding human behaviour and social science to explore the relevancy of zoos and aquariums toward the 21st Century. Copyright 1994 Chicago Zoological Society.
1 Adapted from: History of Zoos: Past, Present & Future – website EcoCation
2 Rabb, G. B. (1994). The changing roles of zoological parks in conserving biological diversity. American Zoologist, 34, 159–164.
(Sources: Zoo and Aquarium History, 2001, edited by Vernon N. Kisling, Jr; ZOO, a history of zoological gardens in the West by Eric Baratay and Elisabeth Hardouin-Fugier; History of Zoos: Past, Present & Future – website EcoCation; Wikipedia; The Circulation of Ancient Animal Resources Across the Yellow River Basin by Xueye Wang et al. 2021; Animals through Chinese History - Earliest Times to 1911 (Ed. Roel Sterckx, Martina Siebert, Dagmar Schäfer); Brittanica.com;How can European Zoos help save the Cotton top Tamarin? by Claire Cardinal, 2015; The Dark History of Zoos by Tyler Muse, History Oasis; Savages and Beasts, the birth of the modern zoo by Nigel Rothfels, 2002)