Sentence:  This is a “<WSD>placer</WSD>” sheep, as it is called. The prerequisites to this condition are that the young sheep must be still nursing, but must have begun to nibble grass. It must be the young of a mother that has been somewhat isolated, away from the corral and away from the herd, by herself out on the prairie. Now, when the mother dies, the lamb remains close to the mother′s body.... 
Meaning of the word marked with <WSD> tags:  A lamb whose mother has died and which has transferred its attachment to an object, such as a bush or rock, in the locality.