These tracks in the snow show where a bird has been caught by a predator, probably a grey wolf (Canis lupus).  Sudden stresses such as this swift capture produce short term responses in brain neural circuits and hormonal shifts in the body.  Longer term stresses produce lasting changes in the autonomic nervous system, altering the sensitivity of the emotion-associated neural circuits. Vertebrates from zebrafish to apes, when living under prolonged fear stress, show symptoms which appear equivalent to helplessness and depression in humans.   (Image: Wikimedia commons) 