• http://persona.is/index.php?action=articles&method=display&aid=65&pid=12

• http://www.health.com/health/gallery/0,,20515167_3,00.html

• https://www.youtube.com/watch?v=2PJxRH-EmaE

• https://www.youtube.com/watch?v=bsBUU-3uBzw

• https://en.wikipedia.org/wiki/Pit_of_despair

• http://pets.webmd.com/dogs/features/depression-in-dogs

• http://www.catchannel.com/cat-depression.aspx

• http://birds.about.com/od/birdhealth/tp/5-Signs-That-Your-Bird-Is-Depressed.htm

• https://en.wikipedia.org/wiki/Major_depressive_disorder

• http://www.webmd.com/depression/guide/depression-types

• http://www.webmd.com/depression/guide/depression-types?page=2#2

• ​https://en.wikipedia.org/wiki/Dysthymia