Behind the retina of many mammals sits the tapetum lucidum, a mirror that gives light a second pass and makes eyes glow in a torch beam. In reindeer that mirror is not fixed. Eyes collected in summer reflect gold; eyes collected in Arctic winter reflect deep blue.
The layer is built from collagen fibres in fluid. Work published by researchers in Tromso and London linked the winter change to sustained pupil dilation through months of dim light, which raises pressure inside the eye, squeezes fluid out from between the fibres and shortens the spacing that sets the reflected wavelength. The blue state scatters light more widely inside the eye, so less is lost and dim shapes become detectable, while fine detail suffers.
Reindeer also see into the ultraviolet, which makes lichen and urine-marked snow stand out against a bright field. How much either trait changes real foraging or predator detection in the field has not been measured directly.