The white tail deer's coat is a reddish-brown in the spring and summer and turns to a grey-brown throughout the fall and winter. The deer can be recognized by the characteristic white underside to its tail. It raises its tail when it is alarmed to warn the predator that it has been detected. The white-tailed deer is highly variable in size, generally following Bergmann's rule that the average size is larger farther away from the Equator.