The lemur baseline: How lemurs compare to monkeys and apes in the Primate Cognition Test Battery

Abstract

28Primates have relatively larger brains than other mammals even though brain tissue is 29 energetically costly. Comparative studies of variation in cognitive skills allow testing of 30 evolutionary hypotheses addressing socioecological factors driving the evolution of primate 31 brain size. However, data on cognitive abilities for meaningful interspecific comparisons are 32 only available for haplorhine primates (great apes, Old-and New World monkeys) although 33 strepsirrhine primates (lemurs and lorises) serve as the best living models of ancestral primate 34 cognitive skills, linking primates to other mammals. To begin filling this gap, we tested 35 members of three lemur species (Microcebus murinus, Varecia variegata, Lemur catta) with 36 the Primate Cognition Test Battery, a comprehensive set of experiments addressing physical 37 and social cognitive skills that has previously been used in studies of haplorhines. We found no 38 significant differences in cognitive performance among lemur species and, surprisingly, their 39 average performance was not different from that of haplorhines in many aspects. Specifically, 40 lemurs' overall performance was inferior in the physical domain but matched that of 41 haplorhines in the social domain. These results question a clear-cut link between brain size and 42 cognitive skills, suggesting a more domain-specific distribution of cognitive abilities in 43 primates, and indicate more continuity in cognitive abilities across primate lineages than 44 previously thought. 45 46

Validity of Cognitive Tests for Non-human Animals: Pitfalls and Prospects

“…The opposite pattern was found for ruffed lemurs in that shyer subjects who took longer to approach and spent less time in the testing area performed better in the social PCTB tasks than their bolder conspecifics. This was not the case for ring-tailed lemurs whose temperament did not correlate with performance in the physical or social tasks of the PCTB ( Fichtel et al, 2020 ). Olive baboons, particularly males, spent more time next to new objects than long-tailed macaques and showed a shorter approach latency toward new stimuli than long-tailed macaques, particularly females.…”

“…Similarly, older chimpanzees and bonobos performed better in some of the physical tasks (causality scale) but not as well as younger individuals in some social tasks (Theory of Mind scale, Herrmann et al, 2010 ). In the other studies applying the PCTB, no such age-effects on cognitive performance were reported ( Schmitt and Fischer, 2011 ; Fichtel et al, 2020 ). Another interesting finding was that high levels of curiosity appeared to alleviate cognitive decline in marmosets ( Gokcekus, 2020 ).…”

“…However, in Hopkins et al’s (2014) chimpanzee study, a subject’s sex did not affect its performance in the PCTB and neither did it in baboons and long-tailed macaques, although male and female individuals differed in terms of temperament in these two Old World primate species ( Schmitt et al, 2012 ). Similarly, sex did not influence the performance of three lemur species in the PCTB ( Fichtel et al, 2020 ). In marmosets, males were generally more easily emotionally aroused in the test situation than females and less food motivated to participate in the cognitive tasks ( Schubiger et al, 2015 ).…”

“…An individual’s rank in its social group is typically inferred via the number of decided conflicts between individuals gathered via focal observations. An individual’s social rank did not affect its cognitive performance in any of the PTCB tasks in olive baboons, long-tailed macaques ( Schmitt et al, 2012 ) or ring-tailed and ruffed lemurs ( Fichtel et al, 2020 ).…”

“…Importantly, such differences have to be considered when planning and conducting cognitive tests. For instance, nocturnal strepsirrhines such as mouse lemurs were tested under infrared light to adapt testing conditions to their activity period ( Kittler et al, 2018 ; Fichtel et al, 2020 ). In marmosets, most males are dichromats (red-green color blind) whereas most females have trichromatic vision owing to a cone receptor polymorphism ( Pessoa et al, 2005 ; Freitag and Almeida Pessoa, 2012 ).…”