After five days of daily injections of a newly developed anti-stress hormone, the balding mice were returned to their habitats. Three months later researchers went to gather up the mice for follow-up studies, but their hair-less subjects were gone.

“It was completely unexpected,” Jean Rivier, with the Salk Institute for Biological Studies in La Jolla, Calif., told Discovery News. “They couldn’t identify the mice except for their ear tags. That’s when they realized that they’d grown hair.”

Follow-up studies on younger mice that hadn’t yet lost their fur showed the anti-stress hormone actually prevented hair loss.