Rutting season has evolved to ensure that young deer are born in spring when there is more new grass available for lactating mothers. This means the timing of the rut is different, depending on the gestation period of each species. But the trigger is the same: shortening daylight hours in autumn. Special cells in the eye that aren’t used for vision called retinal light-sensitive ganglion cells measure the length of daylight hours. This is relayed to the suprachiasmatic nucleus located in the brain and, when the autumn days get short enough, it prompts the release of the appropriate hormones in males so they start competing for mating rights.