The earliest zoos were stocked with wild captured animals and that avenue was open until nearly the middle of the 20th century. In the early parts of the 1900's there were individuals who became rather well known for capturing and selling African and Asian species to zoos in North America and Europe. Today zoo animals are acquired primarily through trade with other zoos from such government departments as fish and game agencies, the Society for the Prevention of cruelty to Animals shelters and by donation from the public. The American Zoological Association (AZA) publishes a monthly newsletter that lists animals available from zoos in other regions of the world. They also send out their lists of available animals from their own member institutions. New species for display generally come from other zoos, a few from animal dealers.

Because of expense, the availability of animals and stress related problems relating to transportation as well as permit requirements many zoos carry out their own breeding programs. Many zoos in Canada belong to the International Species Inventory System (ISIS) a computer-based information system for animals in captivity and more details of this program will be given later. By breeding animals in the zoo and having the ISIS resources available, zoos can maintain genetic diversity in the zoo animals. Breeding programs thus keep the captive populations viable and keeping the sexes together allows visitors to observe natural reproductive behavior. Reproductive biology is an area of considerable interest and an opportunity for research in many zoos.

Reproduction of zoo menagerie members is extremely important for several reasons. Diminishing numbers of wild animals are available for capture. Habitat destruction has reduced their numbers to the point many are listed as rare or endangered. Most countries of the western world are signatories to the Convention on International Trade in Endangered Species which places tight restrictions on either importing or exporting any species of wild animal or plant that is listed in the appendices CITES has published. Thus zoos find it extremely difficult to replace specimens native to other countries, and must rely on breeding their own animals and trading with other zoos. In the past the option of replenishing zoo stock from the wild was an option that today is largely absent.

CITES web site www.cites.org

A well-managed breeding program provides more than just a chance for visitors to observe the animals mating behavior. Newborn and young animals provide a focus for visitor interest, and excitement too, as they are often more active than the adults. Their presence helps encourage natural behavior, increases the value of the zoo collection and they may serve as a source of income through sales or serve as stock for trades. For many species that have become extirpated or endangered this is the only source. It needs to be emphasized that breeding programs fulfill a zoo's role in conservation and zoo born animals have been reintroduced into native habitats from which they were extirpated. Membership in ISIS helps ensure that genetic diversity is maintained so animals reintroduced into the wild carry a maximum assortment of alleles. This helps maintain a diverse gene pool in the species.

All zoos have limited space for displaying animals. The increasing success in breeding animals has frequently led to new problems relating to surplus animals. Some species that were once difficult or impossible to breed in captivity have become avid breeders when introduced into environmentally correct habitats. The result has been an overabundance of some animals thus decreasing their price value or making it difficult to sell or trade them. Thus zoos have had to engage in methods of population control. These have included castration or sterilization of male animals, chemical implants in females to prevent estrus, separation of males and females, culling herds of herbivores and feeding the carcasses to carnivores, or euthanasia of extra animals.

Most birds and those female mammals that have an estrus period breed at specific times of the year. Knowing the breeding season is important as it allows the keeper to separate males and females during the time of nesting or sexual receptivity and thus exercise control over whether or not reproduction will be allowed. There are many species however that may breed at any season and this strategy is inappropriate for them. From mating observations the date of birth can be estimated providing the gestation period of the species is known. The American Association of Zookeepers had as its first project the publication of the gestation period for various species of zoo mammals. This organization has also been active in producing an Infant Development Notebook that contains a wealth of information on mother reared young.

Males of certain species of mammals also become sexually active at certain times of the year and this may be associated with increased aggressiveness. This period is called the rut and antlered and horned hoofed species as well as male elephants may be very dangerous to be around when they are in rut. A keeper should be sensitive to when rut occurs and be especially careful then. More members of the deer family kill and/or injure zookeepers than do carnivores and many of these accidents are attributed to males in rut. This is a major reason for locking in hoofed stock daily while inspection, cleaning and other activity is taking place within the enclosure. Remember there is no substitute for safety.

Elk during rut can be dangerous and may attack visitors in Banff National Park.

For many birds the date incubation commences can serve as a guide to hatching dates. Knowing the date offspring are expected allows appropriate preparation for the new arrival. For some species of birds a diet change suitable for feeding chicks may be necessary. A dark den with suitable bedding needs to be available to many carnivores. Some female hoofed stock may be moved to birthing stalls or paddocks where they remain apart from the herd for some time. In many cases, especially in those carnivores that are solitary in nature, the female and young should be kept separate from the male.

Modifications to the environment of a pregnant mammal could potentially impact the development of her offspring as increased stress levels cause certain hormone levels to be elevated in the mother's blood. These increased hormone levels eventually reach the fetus and can affect its own hormone production and development. Thus all pregnant mammals should be closely monitored when changes that might be stressful are made to their environment.

The final success of any zoos breeding program depends upon the keepers. It is their knowledge, skills, care and attention that assure the program will be successful. Knowledge of breeding dates, incubation times and gestation periods means the new arrivals can be anticipated. This is far better than being unprepared for the first hours after birth or hatching are often most critical to survival of the infant. By carefully observing the animals and recording their behavior the keeper supplements information available for the individual animal as well as for the species. Sometimes obtaining this information will involve spending extra time with the animal conducting observations and recording the activity. Copulation will not always be seen so the keeper must be alert for a different posture, especially in female hoofed stock, that indicates breeding has occurred. In some cases behavioral changes and visible swelling indicates pregnancy and this should be recorded even if the breeding date is unknown.

Many species are easy to breed and zoos can expect regular additions to their population. Other species have been difficult to get to breed with any consistency. Better record keeping and exchange of information has occasionally resulted in brilliant successes. A few dozen years ago it was considered impossible to breed falcons or condors in captivity. Today it is rather routine. Recent successes in breeding amphibians and snakes of the boid family are noteworthy. Alligators are routinely bred in captivity today and large primates have responded to better enclosures by not only reproducing but also raising their young to adulthood. As knowledge of what is necessary to encourage successful breeding and raising young increases, and the standards of husbandry rise, further successes may occur but it will only happen when conscientious keepers observe behavior and record the conditions that encouraged birth and survival. The role of the animal's environment and captive conditions is undoubtedly critical to breeding success.