[coprogram]

I read in Arctic Dreams (Barry Lopez) that polar bears are hard to detect in the visual (white on white) and infrared (very good thermal insulation) spectrums, but that they are easily detectible in the ultraviolet spectrum due to the particular qualities of their fur. I'd suggest this is a reason that UV vision has developed in reindeers. Polar bears are the largest land predator in that region, and being able to see it in the only spectrum it's really detectable in would be a major benefit for reindeer.

[chimpanzee]

The abstract mentions a similar hypothesis in regard to wolves and their prey:

> White wolves are well camouflaged on white landscapes; unless the prey are sensitive to UV light. It is an appealing hypothesis, but wolves in circum-Arctic habitats subsist on a variety of ungulate species––moose (Alces alces), muskoxen (Ovibos moschatus), roe deer (Capreolus capreolus)––which predicts similar selective pressures on their visual systems (Dominy & Harris, 2022). Studying the eyes of these animals may prove rewarding but, to date, there is little indication they share reindeers’ distinctive visual abilities.

[thaumasiotes]

This hypothesis depends on the predator-prey system not yet having reached equilibrium. Animals are white in the snow because that makes them hard to see, not by coincidence. (Where snow isn't present year-round, the same animal will usually be white in the snow and brown in the summer!) If wolves benefit from being white in the visible spectrum, and their prey starts detecting them by ultraviolet light they reflect, they should respond by turning whatever the analogue of white is in the UV spectrum.

To explain the UV vision of some prey animals as an adaptation to the fact that their predators are otherwise difficult to see, you'd need an additional assumption along these lines:

1. UV vision is a recent development in the prey animal. Eventually the predator will develop UV camouflage, but this just hasn't happened yet.

2. UV camouflage is inherently difficult. (Might be true! This is the generally accepted theory for why insects, reptiles, and birds can be blue, but mammals can't -- mammals are more recent.) This is the same theory as (1), but allowing a longer timescale.

3. Detecting this predator is more important to this prey animal than ambushing this prey animal is to this predator. (For example, maybe reindeer are primarily threatened by polar bears, but polar bears mostly eat seals.)