Conservation programs specify different goals for breeding zoo animals, depending on the status of the species. For some species, zookeepers try to keep their population numbers healthy within captivity. For certain endangered species, biologists seek to make babies they can later introduce into the wild. For example, hundreds of zoo-born black-footed ferrets—an endangered American mammal that eats prairie dogs—have been reintroduced to the wild. Committees of experts meet as often as yearly to decide on AZA Species Survival Plans for each of about 600 species.

"The human population is growing so big that conservationists can never stop trying to maintain species either in captivity or in the wild," Moore says.  

The survival plans can include some surprising methods. A few years ago, the plan for black-footed ferrets included artificially inseminating some females with the cryogenically frozen sperm of a male that had died years ago. The current black-footed ferret population is descended from just 18 individuals discovered in Wyoming in 1980, after scientists thought the species had gone extinct. Such a small number of ancestors leaves the ferrets vulnerable to genetic diseases. But the male to whom the frozen sperm belonged was not closely related to living ferrets at the time. Scientists thought his DNA would be a valuable addition to the gene pool.

Another aspect of zoo breeding is animal matchmaking. During conferences, their keepers meet and try to set up their charges with compatible partners. "Animal curators know all of their animals, who is likely to breed with whom, who's behaviorally compatible," Moore says. "It's not only humans that have to go dating."