Old world monkeys compare to apes in the primate cognition test battery

Abstract

Understanding the evolution of intelligence rests on comparative analyses of brain sizes as well as the assessment of cognitive skills of different species in relation to potential selective pressures such as environmental conditions and social organization. Because of the strong interest in human cognition, much previous work has focused on the comparison of the cognitive skills of human toddlers to those of our closest living relatives, i.e. apes. Such analyses revealed that apes and children have relatively similar competencies in the physical domain, while human children excel in the socio-cognitive domain; in particular in terms of attention sharing, cooperation, and mental state attribution. To develop a full understanding of the evolutionary dynamics of primate intelligence, however, comparative data for monkeys are needed. We tested 18 Old World monkeys (long-tailed macaques and olive baboons) in the so-called Primate Cognition Test Battery (PCTB) (Herrmann et al. 2007, Science). Surprisingly, our tests revealed largely comparable results between Old World monkeys and the Great apes. Single comparisons showed that chimpanzees performed only better than the macaques in experiments on spatial understanding and tool use, but in none of the socio-cognitive tasks. These results question the clear-cut relationship between cognitive performance and brain size and--prima facie--support the view of an accelerated evolution of social intelligence in humans. One limitation, however, is that the initial experiments were devised to tap into human specific skills in the first place, thus potentially underestimating both true nonhuman primate competencies as well as species differences.

Figure 1. Performance of the monkeys in the PCTB.

Shown are the proportions of correct responses of the baboons (white) and macaques (grey) in the 16 tasks of the PCTB grouped into the respective scale. Boxes show the interquartile range from the 25^th to the 75^th percentile. The line across the boxes represents the median. The whiskers indicate the maximum and minimum values excluding outliers (circles) and extreme values (crosses). The dotted lines represent the chance level and baseline, respectively for each task.

Figure 2. Comparison of species.

Shown are the proportions of correct responses on the scale level for the four different primate species. Boxes show the interquartile range from the 25^th to the 75^th percentile. The line across the boxes represents the median. The whiskers indicate the maximum and minimum values excluding outliers (circles) and extreme values (crosses).