According to Ronald K. Siegel, a pharmacologist who has studied intoxication in animals, it is common for animals deliberately to experiment with plant toxins; when an intoxicant is found, the animal will return to the source repeatedly, sometimes with disastrous consequences.Besides all the sex and violence, yet another reason not to allow your children to watch Animal Planet.
Cattle will develop a taste for locoweed that can prove fatal; bighorn sheep will grind their teeth to useless nubs scraping a hallucinogenic lichen off ledge rock. Siegel suggests that some of these adventurous animals serves as our Virgils in the garden of psychoactive plants. Goats, who will try a little bit of anything, probably deserve credit for the discovery of coffee: Abyssinian herders in the tenth century observed that their animals would become particularly frisky after nibbling the shrub's bright red berries. Pigeons spacing out on cannabis seeds (a favorite food of many birds) may have tipped off the ancient Chinese (or Aryans or Scythians) to that plant's special properties. Peruvian legend has it that the puma discovered quinine: Indians observed that sick cats were often restored to health after eating the bark of the cinchona tree. Tukano Indians in the Amazon noticed that jaguars, not ordinarily herbivorous, would eat the bark of the yaje vine and hallucinate; the Indians who followed their lead say the yaje vine gives them "jaguar eyes".
via Botany of Desire