The two male Sumatrans were called Bonzo & Adriaan. The two females were Julia and Tineke. They were successful breeders as outlined above and are mentioned in Barbara Harrison's old book on the species-'Orangutan'. Specifically regarding mate preference- one of the males(Adriaan?) would only live with one of the females but the other could be kept with either. I have an old postcard of one of the males but sadly when I first visited Rotterdam this original group had already been virtually wiped out.