QUESTION: Why do deer move more at night? Simple question, not so simple answer. - F. Fairchild
ANSWER: While A simple question should generate a simple answer, right? Wrong! I've often pondered the same question myself, and I'm still not completely satisfied with what I've turned up.
First, let's be precise. Behaviorally, deer are considered crepuscular - meaning they are most active at twilight (dawn and dusk) as opposed to nocturnal - most active at night. Regardless, based on what we know from the science of ecology, there must be some selective advantage to a particular behavior. In this case, something about being crepuscular improves a deer's chances for survival. But what?
Part of the answer might lie in the advantages most often cited for nocturnal behavior.
1) It's easier to hide from predators.
Prey species must move in order to find food, which presents a certain level of risk. Moving during daylight presents the greatest risk because that is when they are most visible and most easily detected by predators. They're harder to see at night, but that advantage is reduced by their increased activity. And predators have adapted by also becoming nocturnal. However, if a prey species such as deer were to move their peak activity period away from the predator's peak (night) and into the fringes (twilight), they might gain a slight edge.
2) There's less competition for food.
I'm not buying it. If you look at the whitetail's direct competitors, most are also crepuscular, meaning there would actually be more competition. This one might work for other species, but not for deer. Besides, food usually isn't a limiting factor. When it is ... surprise! ... deer move more during the day.
3). It's cooler at night.
In addition to predation, daytime movement could also present risk from overheating or dehydration, depending on climate and environment. That might not be a major concern for deer in general, but even the little things are sometimes important.
Natural selection is an ongoing process that constantly tests relationships between predator and prey to ensure only the fittest survive to pass along their genes. Even the most subtle differences can tip the balance one way or the other, albeit temporarily. Any advantage seldom lasts, as predators adapt to prey, and vice versa. It seems, at least for now, that living in the shadows between day and night offers whitetails their greatest chance at survival.