Representational format determines numerical competence in monkeys

A range of animal species possess an evolutionarily ancient system for representing number, which provides the foundation for simple arithmetical operations such as addition and numerical comparisons. Surprisingly, non-human primates tested in ecologically, highly valid quantity discrimination tasks using edible items often show a relatively low performance, suggesting that stimulus salience interferes with rational decision making. Here we show that quantity discrimination was indeed significantly enhanced when monkeys were tested with inedible items compared with food items (84 versus 69% correct). More importantly, when monkeys were tested with food, but rewarded with other food items, the accuracy was equally high (86%). The results indicate that the internal representation of the stimuli, not their physical quality, determined performance. Reward replacement apparently facilitated representation of the food items as signifiers for other foods, which in turn supported a higher acuity in decision making

Female meadow voles, Microtus pennsylvanicus, respond differently to the scent marks of multiple male conspecifics

Many non-human animals are capable of discriminating a group or entity containing more objects from one containing less of the same objects. The capacity for making judgments of numerousness may also allow individuals to discriminate between potential mates. Female meadow voles (Microtus pennsylvanicus) may use judgments of relative numerousness to distinguish between potential suitors by selecting males that signal their interest by depositing more scent marks relative to other males. We used a familiarization-discrimination paradigm in the absence of training to test the hypothesis that female voles will discriminate between the different numerosities of scent marks of two male conspecifics that are similar in features of their phenotype and quality. During the exposure phase, we presented female voles with different ratios of feces scent marks from two males. During the test phase, we presented females with a single, fresh fecal scent mark from each of the two male donors, whose marks they had previously encountered during the exposure phase. In both phases, females spent more time investigating the scent mark(s) of the male that deposited more scent marks than that of the male that deposited fewer scent marks provided the difference in the ratio of scent marks provided by the male donors in the exposure phase was greater than or equal to 2. Our results are consistent with studies on a variety of taxa suggesting that numerosity discriminations are evolutionarily ancient and spontaneously available to non-human animals and humans

Can we measure brain efficiency? An empirical test with common marmosets (Callithrix jacchus)

Various measures of brain size correlate with cognitive performance; however, the fit is not perfect, which bears the question of whether brains also vary in efficiency. Such variation could be expected if a species faces constraints on brain enlargement, for example due to the impossibility of slowing down life history as a consequence of predator pressure, while simultaneously experiencing selective benefits from enhanced cognitive ability related to particular ecological or social conditions. Arguably, this applies to callitrichid monkeys and would lead to the prediction that their relatively small brains are particularly efficient in comparison to their sister taxa, Cebus. This study investigated whether callitrichids' cognitive performance is better than would be expected given their brain size rather than comparing absolute performance between the taxa. As a measure of cognitive performance, we used the reversal learning paradigm, which is reliably and closely associated with brain size across primate taxa, and assessed performance in this paradigm (transfer index) in 14 common marmosets (Callithrix jacchus) as representatives of the callitrichids. These marmosets were found to show higher performance than would be expected for their brain size, and this relative performance was also higher than the relative performance in capuchin monkeys. We outline how these effects may be due to the cooperative breeding system of the callitrichids, particularly the enhancement of behavioural and cognitive propensities associated with shared care and provisioning