Don’t set your watch to the migration timetable of the Galápagos giant tortoise—it doesn’t follow a predictable schedule the way so many other animal migrations do. Scientists first tracked the migration of giant tortoises in the Galápagos Islands in 2013, and they’ve discovered that not only is it marvelously slow, it’s kind of erratic, and flies in the face of human understanding as to why and how most animals migrate. Only the older tortoises make the roughly 6-mile climb out of the soggy jungle up into the hills—in this case, the slopes of Alcedo Volcano on Isabela Island. The journey is loosely related to mating, but researchers think there may be many other unknown variables at play. Whatever compelled these two lumbering giants up here, in about six months, they’ll start the slow climb back down to the jungle.