Captive Breeding and Reintroduction

The primary goal of captive breeding, also known as ex situ conservation, is to develop a self-sustaining or increasing population of an endangered species in captivity, without the need to capture additional individuals from the wild. Any surplus captive-bred individuals are available to support a program of release into the wild.

Another goal of captive breeding programs is to maintain an appropriate level of genetic diversity, which can allow the population be adaptable to conditions in the environment after release. Genetic diversity refers to the numerous alleles of genes in a population. (An allele is one of several forms of a gene, the latter being the unit inherited by offspring from their parents.) If all captive-bred individuals are offspring of the same parents, then the population is likely to have low genetic diversity because of the effects of inbreeding (or breeding between closely related individuals). This can also lead to a phenomenon known as inbreeding depression, a detrimental effect on offspring that can result from mating between close relatives. Inbreeding depression is due to an accumulation of deleterious recessive alleles, which can become expressed in a high frequency in inbred populations. Inbreeding depression can be manifested as lowered fecundity, smaller numbers of offspring produced, and decreased survival after birth.

If a highly inbred population was reintroduced into the wild, its chances of survival and reproduction are likely to be relatively low. In essence, genetic diversity helps to ensure that a released population will be able to survive and grow, despite natural selection against some of its individuals.

The size of released population is another important issue. A small population has a greater probability of extinction because of the potentially devastating effects of deaths caused by unpredictable environmental events or flaws in the reintroduction process. In addition, small populations may exhibit a phenomenon known as genetic drift, caused by the disappearance of certain alleles and fixation in the population of others. Genetic drift occurs readily in small populations, and results in a loss of genetic diversity.

It is also important that the alleles of the founder individuals, that is, the animals brought from the wild into the breeding program, are maintained so that the natural, "wild" alleles are not lost during years of captive breeding. Since the ultimate aim is to re-introduce animals back into a native habitat, maintenance of the original genetic diversity is crucial to the eventual survival of those individuals in the wild. Also, captive breeding over several generations may select for characteristics such as docility, which are not advantageous in the wild.

Many research programs are addressing these problems of captive breeding. At the Minnesota Zoo, for example, a program known as the International Species Inventory is keeping track of the pedigree of individual animals in zoos around the world. This information is used to help prevent mating among closely related individuals, and to thereby maintain the genetic diversity of captive populations.

Various methods are available for increasing the numbers of offspring that can be bred from a limited number of parents. One such method is artificial insemination, in which sperm is transferred to females by artificial means. This allows animals from different zoos to be mated without actually moving them from pace to place. Another enhancement technique involves the removal of eggs from nests of bird species that will subsequently lay replacement eggs. This allows more eggs to be produced by a female than would occur under natural conditions. Reproduction can also be enhanced by foster parenting the young of an endangered species by "parents" of a closely related one, thereby ensuring the rearing of young in a relatively natural, non-human environment. This method has been used to rear endangered whooping cranes, by foster-rearing captive-incubated nestlings by sandhill cranes.