Limits to captive breeding of mammals in zoos.

Captive breeding of mammals in zoos is the last hope for many of the best-known endangered species and has succeeded in saving some from certain extinction. However, the number of managed species selected is relatively small and focused on large-bodied, charismatic mammals that are not necessarily under strong threat and not always good candidates for reintroduction into the wild. Two interrelated and more fundamental questions go unanswered: have the major breeding programs succeeded at the basic level of maintaining and expanding populations, and is there room to expand them? I used published counts of births and deaths from 1970 to 2011 to quantify rates of growth of 118 captive-bred mammalian populations. These rates did not vary with body mass, contrary to strong predictions made in the ecological literature. Most of the larger managed mammalian populations expanded consistently and very few programs failed. However, growth rates have declined dramatically. The decline was predicted by changes in the ratio of the number of individuals within programs to the number of mammal populations held in major zoos. Rates decreased as the ratio of individuals in programs to populations increased. In other words, most of the programs that could exist already do exist. It therefore appears that debates over the general need for captive-breeding programs and the best selection of species are moot. Only a concerted effort could create room to manage a substantially larger number of endangered mammals.