Why reindeer eyes turn blue in winter
We are republishing this fascinating article for two reasons. The first is to honour the late Dr Catchpoole, a prolific author of many fine articles. The second is new information (see Update, 14 July 2022) that confirms even more the design features of the reindeer eye’s colour change.
The golden-coloured eyes of Arctic reindeer (caribou, Rangifer tarandus) turn deep blue in winter. A researcher involved with this 2001 discovery, neuroscientist Glen Jeffery, studies vision at University College London. Describing the colour difference as “dramatic”, he said, “It has taken us 12 years to slowly find out what is going on and why.”1,2
It turns out the colour change is because the reindeer seasonally change the wavelength reflection from their tapetum lucidum (TL)—the reflective surface commonly known as ‘cat’s eye’3 behind the retina.
In the Arctic summer, with up to 24 hours sunlight each day, the reflection from the TL is golden as most light is reflected back directly through the retina. By contrast, in winter, when the darkness lasts as long as summer’s light, the deep blue appearance of the eyes is associated with less light—and of shorter wavelengths—reflected out of the eye.
This change in the TL’s reflectivity is due to reduced spacing between its collagen fibres, likely from compression due to the increased eyeball pressure noted in winter animals. This in turn may be because of partial blockage to the eye’s fluid drainage as the pupil remains fully dilated during the long darkness (to maximize light entry). Additionally, the shift to blue may scatter light sideways through more photoreceptors rather than reflecting it directly—thus capturing more light and improving retina sensitivity (at the cost of visual acuity), also helpful in darkness.
The reindeer’s ability, the first of its kind discovered, is obviously an advantage in its unique environment. But establishing that something is useful is a far cry from showing that it evolved, rather than being designed; nor yet how it could have evolved.
Note that the adaptation here is not the same as when selection favours better-suited variants in a population. When that does happen,4 it is over generations, whereas this is an annual back-and-forth change within the individual’s lifetime.
Beyond calling this an “important adaptation”, the researchers make no attempt to explain how the mechanism enabling such a sophisticated seasonal adjustment could have arisen by Darwinian processes.
Such unsuspected layers of complexity make the evolutionary dilemma of eye origins5 even more intractable, and are strong evidence for creation and design, not evolution.
Update 14 July 2022
Another study in 2022 provided even more insight into the design features of the reindeer eye colouration.6 It is an adaptation to optimize vision during winter twilights, when both the reindeer and their predators are most active.
Just after the sun goes down, and just before the sun comes up, is a period called the “blue hour”. In the blue hour, the sky has quite a ‘pure’ blue hue, i.e. very little of the other light colours present. The blue hour is very different from the blue of the daytime sky that is caused by Rayleigh scattering of visible light, where blue scatters more strongly. The blue hour is caused by the ozone layer absorbing almost all light but blue. This effect is dominant when the sun is just below the horizon, so the light travels horizontally through the ozone layer, allowing maximal absorption. Most of the time, we don’t notice the blue hour, because our eyes have gradually adapted to the change.
In winter, this optimization to ‘twilight ozone blue’ is particularly important, because in polar regions, the twilight can last a third of a day. Also, the lichen eaten by the reindeer and the wolves that try to eat them reflect very little blue light, so appear dark against the snow. This greater contrast, and greater ability to detect motion, outweighs the disadvantage of lower visual sharpness mentioned in the main article.
In a popular-level article by one of the researchers, biomimetic implications were explored:
This finding may help engineers to make products that change reflected colour. The opportunities are endless. If you use a surface coated with a reflecting nano- structure, similar to that of the reindeer’s tapetum, rather than a pigment-based paint, you can change colour by changing the separation of the scaled-down “pencils” that reflect the light. For example, you could change the colour of your car by tuning the separation. Unlike many pigments, these structural paints do not fade over time.
So while reindeer have long inspired Christmas tradition told all over the world, now they could inspire technology and science.7
Re-posted on homepage: 20 July 2022
References and notes
- Choi, C., Reindeer eyes turn blue in the winter, livescience.com, 30 Oct 2013. Return to text.
- Stokkan, K-A, and 7 others, Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer, Proc. Royal Soc. B 280(1773), 22 Dec 2013 | doi:10.1098/rspb.2013.2451. Return to text.
- Like various other nocturnal creatures, cats’ eyes can seem to ‘glow in the dark’ when e.g. headlights illuminate their TL. Return to text.
- See creation.com/muddy. Return to text.
- Bergman, J., Did eyes evolve by Darwinian mechanisms?, J. of Creation 22(2):67–74, 2008; creation.com/eyes-evolve. Return to text.
- Fosbury, R.A.E. and Jeffery, G., Reindeer eyes seasonally adapt to ozone-blue Arctic twilight by tuning a photonic tapetum lucidum, Proc. Royal Soc. B 289(1977), 29 June 2022 | doi:10.1098/rspb.2022.1002. Return to text.
- Fosbury, R.A.E., How reindeer eyes transform in winter to give them twilight vision, theconversation.com, 1 Jul 2022. Thanks to Philip Bell for alerting me (JS) to this article. Return to text.