Spontaneous discrimination of food quantities in the jungle crow, Corvus macrorhynchos

• Jungle crows were given a choice between two food quantities in a spontaneous discrimination task.
• Crows selected the larger of the two quantities even when one of the quantities was more than four.
• Crows' performance was negatively correlated with numerical magnitude.
• These results may suggest use of an object-file mechanism for numerical judgement.

Despite considerable research on numeric judgements in animals, uncertainty remains about both the underlying mechanisms and the role of training. To address these issues, we study quantity discrimination in jungle crows that have previously been shown to select the larger of two quantities following training. In the current study, we examined whether jungle crows are able to discriminate between different quantities of food items without prior training. Using a simultaneous two-alternative test, we studied whether their performance reveals the underlying mechanism being used for discrimination of small and large quantities in other animals. First, jungle crows were tested with a choice between two discrete homogeneous quantities; one smaller and one larger, with ratios of 0.5 (1 versus 2, 2 versus 4 and 4 versus 8), 0.67 (2 versus 3, 4 versus 6 and 8 versus 12) and 0.75 (3 versus 4, 6 versus 8 and 12 versus 16). Then, we conducted a test using a non-numerical cue where the volume of comparison stimuli was equalized. Jungle crows selected the larger of the two quantities in all comparisons, except those when both quantities were large (6 versus 8, 8 versus 12 and 12 versus 16). Furthermore, accuracy of selection of the larger quantity declined with increasing numerical magnitude. These results suggest that in a spontaneous discrimination task, jungle crows use an object-file mechanism to compare quantities, even when the number of items in one of the arrays exceeds four. The crows showed no preference for the smaller or the larger quantity when the volume cue was removed. This lack of bias may suggest an ecological role of selection for a patch with higher amounts of food and not necessarily the exact number of food items for optimal foraging in jungle crows.