Roots of Primate Cognition. The Primate Cognition Test Battery applied to three species of lemurs (Varecia variegata, Lemur catta and Microcebus murinus).

Compared to other mammals, primates have evolved relatively large brains and outstanding cognitive skills. Given that brain tissue is energetically very costly, one major evolutionary question concerns possible selection pressures favouring such large brains. Comparative studies on the cognitive abilities of multiple species are essential for answering this evolutionary puzzle. Systematic studies on the cognitive skills of strepsirrhine primates (lemurs & lorises) were missing until now, although they can serve as living models of the ancestral primate state. In my thesis, I therefore tested lemurs in a systematic battery of experiments covering the physical and social cognitive domain. Chosen for differences in key socioecological traits, subjects were black-and-white ruffed lemurs (Varecia variegata, n=13), ring-tailed lemurs (Lemur catta, n=27) and grey mouse lemurs (Microcebus murinus, n=15). To facilitate comparisons to haplorhine primates (great apes, Old- & New World monkeys), I used the experimental setup of the Primate Cognition Test Battery which has been tested with chimpanzees and orangutans (Herrmann et al., 2007), as well as baboons and macaques (Schmitt et al., 2012). The three lemur species did not differ in performance and had a better understanding of the physical than the social domain. The overall comparison to the four haplorhine primate species revealed that although lemurs performed slightly inferior in the physical domain, they were at level with haplorhines in the social domain. Specifically, lemurs were outperformed by haplorhines in tasks on spatial understanding and tool usage, while lemurs outperformed great apes in the scale theory of mind. However, in several experiments performance might have been influenced by confounding factors such as the lemurs’ limited dexterity, local enhancement or a heterospecific human demonstrator in the social tasks, and hence results need to be handled with care. For example, I could show in a sub-study that performance in socio-cognitive tasks can be influenced by the nature of the stimulus, as well as the level of human socialisation. The overall results of my thesis suggest that in many aspects of the physical and social domain, haplorhine and strepsirrhine primates do not differ substantially from each other, at least in the experiments of the PCTB. However, they differ strongly in their absolute brain sizes and thus, my results question the notion of a clear-cut correlation between brain size and cognitive abilities, as well as assumptions of domain-general cognitive skills in primates. Since my thesis represents the first systematic comparative investigation of the cognitive skills of lemurs it provides important insights into the cognitive evolution of primates