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.