 In addition, in at least one instance the progeny of interbreeding with a domestic horse was bred back into the captive Przewalski's horse population, though recent studies have shown only minimal genetic contribution of this domestic horse to the captive population.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.Only 9 of the 31 remaining horses at war's end became ancestors of the subsequent captive population, which did not return to its pre-war size until a decade later.