# @science on Instagram

- **Type:** Image
- **Original URL:** https://www.instagram.com/p/DW3Iw9zj0y3
- **Gondola URL:** https://gondola.cc/posts/63606200-science-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/29b9edab05.jpg
- **Posted:** 2026-04-08T07:00:22.000+00:00
- **Account Owner:** Science 🧬 (@science) — https://gondola.cc/science

## Caption

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.⁠
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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 than hours. Over time, this constant effort blocks the small vessels that drain fluid out of the eyes. Pressure builds up inside the eyeballs, and they start to swell. “The animal’s moving towards glaucoma,” said Glen Jeffery, a neuroscientist from University College London who studies animal vision.⁠
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These events also change the tapetum. This layer is mostly made up of collagen, a protein whose long fibers are arranged in orderly rows. As the pressure inside the eye builds up, the fluid between the collagen fibers gets squeezed out, and they become more tightly packed. The spacing of these fibers affects the type of light they reflect. With the usual gaps between them, they reflect yellow wavelengths. When squeezed together, they reflect... blue wavelengths.⁠
So, as reindeer spend months of darkness, their permanently dilated pupils lead to swollen eyes, compressing the fibers in their tapetum and changing the color of light they reflect.⁠
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#reindeer #eyeball #science #biology

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## Tags

eyeball, science, reindeer, biology

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