When I got home on Friday a doe was standing in the corner of my yard, by the edge of the woods. I've said before that they're getting much too tame. She just stood and looked at my car, and didn't move for a few seconds even after I got out. She finally bolted and to my astonishment six more patches of white exploded in the woods as her companions took off in various directions. I hadn't noticed any of them until they turned tail.
A couple of minutes later, they had reconvened and I saw them walking single file through the woods as they often do. So, I got to thinking. Why is a creature which is only prey and a threat to nothing but vegetation, which is otherwise well camouflaged, equipped with a conspicuous advertisement of its presence? For you city slickers, the white-tailed deer has a brown coat that fades into the background of fallen leaves and shrubbery. When there's a group of deer in the grove below my living room window, it can take several minutes before I manage to count them all, assuming I ever do.
But, the underside of their tails, which they usually carry raised so as to show it off, is a big patch of almost incandescent white. Now, this evolved before people were shooting at them with firearms, but as many a tragic story shows, hunters will fire when they see something white in the woods. I don't expect that people were shooting at them with arrows long enough to affect the evolution of their wardrobe either.
So, this signal evolved while they were being hunted by cougars and wolves. I suppose they were perfectly capable of spotting their prey even without the illuminated posterior, but still, what good does it do them? My guess is that the white tail helps them spot each other. It keeps the group together, helps the fawns and mothers keep track of each other, helps the followers stay behind the leader. The value of maintaining social cohesion outweighs a slightly greater chance of winding up as lunch.
Does anyone have a better idea?