Reindeer eyes have a seasonal setting: a reflective layer behind the retina shifts from golden tones in summer to deep blue in winter. It is the layer inside the eye that changes, not the iris.

The layer reflects light back through the light-sensitive cells. This seasonal shift helps reindeer adapt their vision to Arctic summers with long days and winters marked by darkness and blue twilight.

Reindeer have a winter setting for their eyes

Research specimens: winter reflective tissue on the left, summer on the right; eye structures removed to reveal the layer — Stokkan et al., Proceedings of the Royal Society B (2013), Figure 1. Research specimens; winter left, summer right in top pair. Scale bars retained.

Simplified eye diagram showing a reflective layer behind the retina — Pandi explanatory eye schematic based on Stokkan et al. Simplified; not to scale.

The original research figure showing blue winter and golden summer reflective tissue — Stokkan et al., Proceedings of the Royal Society B (2013), Figure 1. Research specimens; winter left, summer right in top pair. Scale bars retained.

Reindeer have a winter setting for their eyes

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