How does a reindeer see the world?

What the research shows

Hogg et al. (2011) established reindeer ultraviolet vision with two kinds of evidence: optical measurements showing that the cornea and lens transmit near-ultraviolet light, and electrophysiological recordings in which both rods and cones responded to it, down to roughly 320–350 nm. The cone peaks shown here are not reindeer measurements. They are borrowed from other deer, since Jacobs et al. (1994) measured white-tailed and fallow deer, so the blue–yellow dichromacy is supported by analogy rather than measured directly.

The seasonal tapetum is the best-documented oddity. Stokkan et al. (2013) found that the reflective layer shifts from gold in summer to deep blue in winter, and in scotopic electroretinograms winter eyes responded to flashes up to three log units, about 1,000 times, dimmer than the summer threshold, with a b-wave more than four times larger. That is the source of the thousandfold figure on this page; it describes a retinal threshold, not measured behaviour, and the authors considered the blue tapetum the likely cause while noting it was not proved. Dominy et al. (2022) linked the colour change to altered spacing within a collagen reflector. Acuity has never been measured in reindeer; the 6 cpd used here is an estimate taken from other deer, which a review by Newman & D'Angelo (2024) puts at about 3–6 cpd. A phone camera does not record UV, so any UV view illustrates the effect without capturing it.