In most parts of the North, winter weather conditions put serious stress on whitetails. With the ground covered by a foot or more of snow in some areas, deer are forced to eat woody browse, old dead leaves or even tree bark and lichens. It's not the best food stuff in the world but whitetails have been doing it for thousands of years and somehow are able to manage fairly well on it (at least in winter). Their digestive system is tuned to winter forage and can make the most of these low-quality foods. They may lose 20-30% of their body weight over the winter but it is how they have learned to survive.