Deer are nocturnal creatures, and they live in groups of up to 100 animals. They are grouped by gender, and males are larger than females. Although deer are solitary animals, they do have a tendency to travel in herds. The stags tend to travel together and are usually found near rivers or lakes. They live in forests and are found in many countries. In North America, the most common deer species are the whitetail and the American bison.