I had bought two male chimps from a primate colony in Holland.  They lived next to each other in separate cages for several months before I used one as a [heart] donor.  When we put him to sleep in his cage in preparation for the operation, he chattered and cried incessantly.  We attached no significance to this, but it must have made a great impression on his companion, for when we removed the body to the operating room, the other chimp wept bitterly and was inconsolable for days.  The incident made a deep impression on me.  I vowed never again to experiment with such sensitive creatures.