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The goal for the Cold War rivals was the same: to prove that animals could survive in orbit so that people could, too. But why did the Soviets use dogs, while the Americans used primates?

The story of ZIB illustrates one rather pragmatic reason: Dogs were everywhere. The Moscow streets were crowded with stray dogs—free, if unwilling, volunteers.

The Soviet Union already had a long tradition of using dogs as research participants, says Amy Nelson, a history professor at Virginia Tech who has studied the Soviet space dogs. At the turn of the century Ivan Pavlov’s work with canines uncovered the learning process known as classical conditioning, a reflexive behavior that ties together a stimulus and a response. Pavlov had been studying canine digestion when he noticed that his pup subjects drooled before he even gave them meat, a hint that something had tipped them off that a delicious treat was coming.

Primates were more difficult to acquire. The chimpanzees were brought over from the Congo region in Africa, Britz says. The Air Force, which conducted some of the earliest primate flights before NASA was established in 1958, paid catchers in African nations to collect dozens of young chimps. Many of the test subjects arrived at the headquarters of the program, Holloman Air Force Base in New Mexico, in poor shape. Out of nine veterinarians, eight contracted hepatitis from the chimps. Britz was the only one who didn’t get sick.

The chimps were one to two years old. “They were just like kids,” Britz says. “We would play with them.” He remembers getting a call at home about flickering lights inside one of Holloman’s buildings. When he arrived, he found that one chimp had opened a padlock, escaped his cage, and helped another chimp out of his own. They were running around the lab, flipping light switches, and pulling chemical wipes from their container, one by one, as if they were Kleenex tissues.