Zoos used to be primarily a place for people to see wild animals they had never seen. Now, however, zoos serve a new purpose – to be captive-breeding programs for endangered animals. There are less than a hundred Sumatran rhinos worldwide, and three were born in Cincinnati Zoo. When old enough, one of the males was sent to Way Kamabas National Park in Sumatra and this helped to start the process of re-populating his species in the wild and saving it from the extinction. Other zoos have started ...

Zoos used to be primarily a place for people to see wild animals they had never seen. Now, however, zoos serve a new purpose – to be captive-breeding programs for endangered animals. There are less than a hundred Sumatran rhinos worldwide, and three were born in Cincinnati Zoo. When old enough, one of the males was sent to Way Kamabas National Park in Sumatra and this helped to start the process of re-populating his species in the wild and saving it from the extinction.

Other zoos have started captive-breeding programs as well. Zoos have saved the Arabian onyx, the black-footed ferret, the red wolf, the Guam rail, and the California condor. But the cost of the programs for saving the threatened animals is not cheap. The condor program alone costs up to $2 million a year. While it is mostly large-city zoos involved in captive breeding, smaller zoos do their part. The Miller Park Zoo in Bloomington, Illinois, is working on breeding the endangered, and small. Mount Graham red squirrel.

Zoos have always been involved in animal conservation to some degree, but the question of which animals to save is a big one. Endangered animals may not be the most exciting to see in a zoo, and to di conservation work the zoos must survive financially. Space is a problem as well. A zoo might be able to save a small number of large animals or a large number of small ones. Currently, the most threatened group of animals is amphibians. Zoos realize that visitors may not want to see frogs, salamanders, or other small animals and may only pay to see rhinos, lions, and tigers. Some scientists believe zoos should be less tourist attractions and more conservation sanctuaries.

Thus, although zoos continue to do good work, they still confront tricky questions. There are a limited number of animals that can be cared for in zoos, and that means many species on the edge of extinction may not survive. The Sumatran orangutan and a thousand other threatened species have not made it into a captive-breeding program, but the species will continue to exist. How? In the Frozen Zoo. Cells from these animals are stored in the San Diego Zoo Institute for Conservation Research in liquid nitrogen. Only one animal in the Frozen Zoo is presently extinct, but many others are at dire risk. In the Frozen Zoo, however, the cells of animals will be preserved for study through their genetic material.

(Adapted from Reading Explorer 5, Nancy Douglas et al., 2912)

What is the main idea of this passage?