Sheep, IMO, are the stupidest creatures on the planet.
Cattle will follow each other off cliffs, in a stampede. Many herd animals will, in fact.
Sheep, on the other hand, have been known to race each other off the cliff. Of all the other races I know of, only humans duplicate this behavioral pattern.
And Ria's right. The whole thing of lemmings-off-a-cliff was forced by a Disney film crew.