The bit that actually changes color is the tapetum lucidum or “cat’s eye”—a mirrored layer that sits behind the retina. It helps animals to see in dim conditions by reflecting any light that passes through the retina back onto it, allowing its light-detecting cells a second chance to intercept the stray photons. The tapetum is the reason why mammal eyes often glow yellow if you photograph them at night—you’re seeing the camera’s flash reflecting back at you.⁠ Most mammals have a golden tapetum, and so do the reindeer in summer. So why does this layer become blue in winter? Through years of dissections and measurements, Jeffrey’s team thinks that they have the answer. And it begins in darkness.⁠ ⁠ In dark conditions, muscles in your irises contract to dilate your pupils and allow more light into your eyes. When it’s bright again, the irises widen, and the pupils shrink. The same thing happens in reindeer, but the interminable Arctic winter forces their pupils to dilate for months rather...