After hatching on beaches around the world, baby sea turtles take to the water. Males never return to land again, while females faithfully come back to the beach of their birth years later to lay their own eggs. Where the turtles go in the meantime and how they manage to navigate the ocean have largely remained mysteries.

Now, researchers have discovered a handy physiological structure that likely keeps sea turtles in rhythm with the seasons: a pink spot on the top of the animals' heads that scientists are referring to as a "skylight." If their hypothesis is correct, this spot acts as a sunlight detector for the turtles, which use information about lengthening and shortening days to help them synch up with the seasons to optimize for food and mating.

Researchers from Ireland and Hawaii studied these structures on four leatherback sea turtles. (All were found already dead, victims of long-line fisheries.) The bone and cartilage layers below the spot, ScienceNow describes, turned out to be much thinner than the rest of the turtles' skulls. The pink spot was so thin, in fact, that the team thinks it allows sunlight to shine directly into the pineal gland—which is responsible for establishing circadian rhythms—in the turtles' brains.

To corroborate this hypothesis, the team also studied an extensive database of turtle sightings in British waters. Water temperature, they found, was too slow to change to explain the sudden shift of turtles from one feeding ground to another. Day length, the think, is a more likely trigger. While many questions remain, the finding shines a bit of light on one more aspect of those beloved but mysterious animals' lives.