As day length declines in the fall, thick-necked bucks start rubbing trees, focusing on does, and fighting other bucks.
This is the culmination of the deer rut and in Montana, the animals will start to release various pheromones as they primarily use scent for communication.
Publications on mammalian behavior suggest that each reproductively active deer exudes a unique odor that communicates its identity as well as its sexual status.
The tarsal gland is on the inside of the back knee. Early in the rut, it turns dark brown in color.
Bucks urinate on the gland which reacts with bacteria that live on their fur which then steadily darkens with time. By the time rut is in full swing, the glands are almost black in color.
Scent is then released onto the surrounding vegetation as the buck moves through their territory.
Females also urinate over their tarsal gland, and when it eventually turns black, they are in estrus, meaning that they are ready to mate.
The forehead gland -- located between the base of the antlers and the eyes -- is easily identified in adult bucks because the hair in this region is particularly dark and brown.
Prior to the rut, the bucks rub trees with the base of their antlers exposing the moist undersurface of the trees to the secretions from the forehead gland as well as the preorbital gland which is located on the inside corner of the eye.
By doing this, the bucks are advertising their territory and dominance and the most dominant bucks have the most active forehead glands.
The length of day is the main environmental cue that kicks deer rut activity into gear.