Symbolic quantity discrimination and summation in Western lowland gorillas

Some non-human primate species have demonstrated the capacity for quantity discrimination and summation with symbolic representation in the form of tokens. I examined this capacity in seven Western lowland gorillas (Gorilla gorilla gorilla). In Phase I of the experiment, the gorillas were asked to make a choice between two unequal values (e.g. 1 cylinder token = 5 blueberries vs. 1 cube token = 1 blueberry). In Phase II, two subjects were presented with homogeneous choice combinations (e.g. 2 pyramid tokens = 6 blueberries vs. 4 cube tokens = 4 blueberries). Three of the gorillas performed successfully in Phase I while one performed successfully in Phase II, utilizing the strategy of choose the larger sum, under some conditions, over the alternative strategies of choose the larger number of tokens or choose the higher value token. This research demonstrates that gorillas have the capacity to perform symbolic quantity discriminations and summation judgments.