In the earlier decades of captivity, the insular breeding by individual zoos led to inbreeding and reduced fertility. In 1979, several American zoos began a collaborative breeding-exchange program to maximize genetic diversity. Recent advances in equine reproductive science have also been used to preserve and expand the gene pool. Scientists at the Smithsonian Institution's National Zoo successfully reversed a vasectomy on a Przewalski's horse in 2007 — the first operation of its kind on this species, and possibly the first ever on any endangered species. While normally a vasectomy may be performed on an endangered animal under limited circumstances, particularly if an individual has already produced many offspring and its genes are overrepresented in the population, scientists realized the animal in question was one of the most genetically valuable Przewalski's horses in the North American breeding program. The first birth by artificial insemination occurred on 27 July 2013 at the Smithsonian Conservation Biology Institute.