The situation was improved when the exchange of breeding animals among facilities increased genetic diversity and there was a consequent improvement in fertility, but the population experienced another genetic bottleneck when many of the horses failed to survive World War II. The most valuable group, in Askania Nova, Ukraine, was shot by German soldiers during World War II occupation, and the group in the United States had died out. Only two captive populations in zoos remained, in Munich and in Prague, and of the 31 remaining horses at war's end, only 9 became ancestors of the subsequent captive population. By the end of the 1950s, only 12 individual horses were left in the world's zoos.