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Monday, November 25, 2013

Reindeers’ color-changing eyes

If you live above the Arctic Circle (or if you became the first permanent resident of Antarctica), you have to cope with seasons of light and dark. Humans haven’t evolved to handle these extreme environments, but Arctic reindeer have, and the way they’ve adapted to long stretches of day and night may amaze you.

Like many animals, reindeer have a tapetum lucidum (TL), essentially a mirror that reflects light forward from the retina, giving neurons a second chance to capture that light. It’s the TL that gives cats their eerie glowing stare. However, there’s something unusual about these particular TLs, as shown by Karl-Arne Stokkan from the University of Tromsø. Reindeer TLs are not the same in winter as they are in summer.

Left: eye taken from a reindeer in winter
Right: eye taken from a reindeer in summer
Credit: Glen Jeffery.

During the summer, the reindeers’ TLs appear to be golden, which is how most ungulates’ TLs look. The golden TL reflects almost all the light out of the eye. In contrast, the blue TL reflects 60% less light. Experiments with live animals showed that winter adapted reindeer were far more sensitive to light.

To be clear, we’re not talking about two subsets of reindeer. The same reindeer have golden eyes in summer and blue eyes in winter. What’s causing them to change? The answer seems to be eyeball pressure and collagen spacing. During the winter, the intra-ocular pressure is greater, probably due to pupil dilation, leading to more tightly spaced collagen in the TL.

The scientists tested this by putting heavy clear coverslips on top of sections of the golden summer TL. As you can see, the compressed TL is now blue.

Figure 8.
(a) Central TL from summer animals, revealing a golden appearance.
(b) The same specimen after coverslipping with an 8 g weight. 

This is the first documentation of a color-changing eye strategy for coping with extreme environments. 

Stokkan KA, Folkow L, Dukes J, Neveu M, Hogg C, Siefken S, Dakin SC, & Jeffery G (2013). Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer. Proceedings. Biological sciences / The Royal Society, 280 (1773) PMID: 24174115.