It’s unknown how many head back north during breeding season, and how widespread this behavior is, says study author Marta Wayne, a biologist at the University of Florida. But the technique they used could help them find these answers. Originally, Wayne says, the team planned to use genetic analysis on the butterflies. However, there isn’t much genetic variability in the monarch population, she says, and stable isotope analysis, which looks at the different chemical elements fixed in the monarch’s wings as they developed, can reveal where they were as caterpillars. Different places in the country have different isotopes, and they are contained in the milkweed that the monarchs ate as caterpillars. Monarch wings don’t regenerate, so the isotopes they picked up when they grew their wings remain fixed.