There are several ways in which food items could differ in quality but two obvious characteristics are size and weight. if food item differ in size then larger items might be preferred by discriminating squirrels. The reason for this follow from simple surface-to-volume relationships. A squirrels must cut through the shell of a nut in order to get at the nut meat. if the shell thickness does not increase with out volume. then a squirrel will obtain a larger nut meat for the same amount of effort if it chooses larger nuts. in the most general sense, though, we can only postulate that the weight of the nut increase faster than the shell thickness and that as a consequence a squirrel may invest less energy per calorie gained by choosing larger nuts.