late onset of reproduction (7 yr for females is the youngest ever recorded in the NIH breeding program; 10-12 yr is more typical). Reproduction can be sustained by at least some individuals beyond the age of 40 yr. Chimpanzees almost always produce one offspring per pregnancy.
These life-history characteristics mean that the growth potential of chimpanzee populations is much less than that of populations of all other species used in research, but growth in captivity can be achieved. The NIH breeding program averaged 10 live births per 100 animals during the peak years of 1987-1993. The birth rate was reduced later because of a potential surplus of chimpanzees for research.
The age structure of a population is important for understanding population dynamics. The biomedical research chimpanzee population in the United States now has a generally stable age structure with captive-bred animals filling in the lower age classes as a result of the ban on importation of wild chimpanzees imposed by CITES in 1975. The NIH breeding-program population has a similar structure.
Because of the success of the breeding program during a period when relatively few animals were being used, many younger animals are now in the population. Life expectancies of animals of various age classes can be calculated from the existing records. Life-table models can be used to forecast how rapidly a population will decline as a result of the background rate of mortality under existing captive-management conditions.
The large number of animals available for research is ample for reasonable projections of research needs; hence, the recommendation of a 5-yr breeding moratorium. Thereafter, the ChiMP office can assess up-to-date research-need forecasts, the number of animals that have been moved to sanctuaries, and attrition due to natural mortality and then recommend needed changes to management.