Over eons, deer have evolved to increase their testosterone production based upon photoperiod. The shortening of days triggers chemical changes in the brain of male deer which in turn cause the production of testosterone. This helps prepare bucks for the mating season. The peak of rut in North Dakota is around November 6 for white-tailed deer and November 18  for mule deer. This timing allows fawns to be born in early June when food is most nutritious for their nursing mothers.