Many situations require human and non-human animals to engage in quantity judgements to deal with essential problems such as food intake maximisation, estimation of distances, or the assessment of the number of opponents in conflict situations (Hanus & Call, 2007; Krebs, Kacelnik & Taylor, 1978). This study aims to compare cognitive abilities of different primate species in a choice test, simulating relative quantity judgements in foraging behaviour. Foraging theory predicts efficient food intake maximisation, but little is known about the accuracy of how primates mentally represent and combine continuous (e.g. size) and discrete (e.g. numerosity) magnitude features to discriminate total quantities. Two sets of food items (patches) of different compositions are presented as a relative quantity discrimination task. Different patches vary in total food quantity, number of items and item size, or a combination of all. Experiments are conducted at the Blair Drummond Safari Park with small populations of untrained captive chimpanzees (Pan troglodytes; n=4), macaques (Macaca sylvanus; n=5) and lemurs (Lemur catta; n=5), and with a group of undergraduate students (n=100) from the University of Stirling. Maximisation performance is expected to reveal (a) numerical combination skills with discrete and continuous magnitude features, (b) composition preferences in equal-quantity patches, and (c) errors in direct payoff maximisation. The latter may indicate either a foraging efficiency adaptation (functional) or a cognitive interference effect (proximate) towards a predominant magnitude feature.