Iran claimed last month to have sent a monkey into space. The country previously launched smaller animals into the final frontier, including a rat, worms and two turtles. What do space programs look for in animal astronauts?

Portability, experience in the lab and coolness under pressure. For more than 60 years, space programs have sent animals into space for the same reason coal miners sent canaries into the coal mine: to test for dangerous conditions. To select which species to send, scientists have long looked for a few key traits. First, the animal astronauts should be small, to fit in a spacecraft’s necessarily compact quarters. Second, they should be light, to avoid burdening the rocket. Third, scientists choose animals that they’re already used to studying.

The Soviets chose to launch many of their most important test flights with dogs because they had experimented on them since the beginning of the 20th century (most famously in the experiments of Ivan Pavlov). They also thought that dogs would be less fidgety in confined spaces. The Americans chose to work with monkeys and chimpanzees for the same reason – they were accustomed to working with them in the lab – though they also valued monkeys’ and chimpanzees’ many physiological similarities to humans.

Once rocket scientists have settled on their species, they often run tests to determine a standout member of the pack. After the Soviets settled on launching canine cosmonauts, their recruits were subjected to a series of “Right Stuff”-style tests to find the top dog. Only female dogs were eligible, because they were an easier fit with the sanitation devices, and only stray mutts were tested, reportedly because they thought the street-tough animals would fare better in extreme conditions. It was after scoring high in these tests that Laika, who was thought to be particularly easygoing, was selected to be the first animal to orbit Earth. On Nov. 3, 1957, Laika was blasted into Earth’s orbit on the Sputnik 2, never to return.

The field of primates trying out for the American side was whittled down through a similar process.

The first cat was launched in 1963, when the French wanted to give space flight a try. It’s unclear why, exactly, the Iranians had previously launched turtles and worms – it may be that Iranian scientists just are used to studying those creatures.