A two-choice test based on spontaneous preferences was utilized in order to asses the ability to visually discriminate between different sizes of food items among and between the gibbons.
First there was a habituation period where the gibbons got used to me, the study setup and the food in cube shapes.
After that the learning period ensued where the gibbons got presented with two food pieces and learned that the one piece they did not choose would be removed from them.
Lastly the testing period itself looks very much alike the learning period with the exception that more different sizes of food items were introduced and each choice made by the gibbons were recorded. By systematically varying the size of the food items I determined the minimum difference in size for which the gibbons still displayed a spontaneous preference.