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Primate brain size does not predict their intelligence

A research team has systematically investigated the cognitive abilities of lemurs, which have relatively small brains compared to other primates. Conducting systematic tests with identical methods revealed that cognitive abilities of lemurs hardly differ from those of monkeys and great apes. Instead, this study revealed that the relationship between brain size and cognitive abilities cannot be generalized and it provides new insights into the evolution of primates.

Chimpanzees, gorillas and orangutans are our closest relatives, and like us they have relatively large brains and they are very intelligent. But do animals with larger brains really perform better in cognitive tests? A research team from the German Primate Center (DPZ) — Leibniz Institute for Primate Research in Göttingen has for the first time systematically investigated the cognitive abilities of lemurs, which have relatively small brains compared to other primates.

Conducting systematic tests with identical methods revealed that cognitive abilities of lemurs hardly differ from those of monkeys and great apes. Instead, this study revealed that the relationship between brain size and cognitive abilities cannot be generalized and it provides new insights into the evolution of cognitive abilities in primates.


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Humans and non-human primates are among the most intelligent living beings. Their brain size may underly their intelligence as primates have relatively large brains in relation to their body size.

For example, it is assumed that larger brains enable faster learning and better memory capacities. Within primates, however, species can differ up to 200-fold in brain size. A team of researchers from the German Primate Center (DPZ) has now investigated whether the cognitive performances of lemurs with their relatively small brains differ from those of other primates.

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Using a comprehensive standardized test series of cognitive experiments, the so-called “Primate Cognition Test Battery” (PCTB), small children, great apes as well as baboons and macaques have already been tested for their cognitive abilities in the physical and social domain.

Cognitive skills in the physical domain include the understanding of spatial, numerical and causal relationships between inanimate objects, while cognitive skills in the social domain deal with intentional actions, perceptions and the understanding of the knowledge of other living beings. Initial studies have shown that children possess a better social intelligence than non-human primates. In the physical domain, however, the species hardly differed even though they show great variation in their relative brain sizes.


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For the first time, researchers of the “Behavioral Ecology and Sociobiology Unit” of the DPZ have now tested three lemur species with the PCTB. Lemurs are the most basal living primates and represent the evolutionary link between primates and other mammals, which is why they serve as a living model of primates’ origin of cognitive abilities.

The study examined ring-tailed lemurs, black-and-white ruffed lemurs and grey mouse lemurs, which differ in their social system, diet and brain size, not only among each other, but also compared to the previously tested Old World monkeys and great apes.


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The results of the new study show that despite their smaller brains lemurs’ average cognitive performance in the tests of the PCTB was not fundamentally different from the performances of the other primate species.

This is even true for mouse lemurs, which have brains about 200 times smaller than those of chimpanzees and orangutans. Only in tests examining spatial reasoning primate species with larger brains performed better. However, no systematic differences in species performances were neither found for the understanding of causal and numerical relationships nor in tests of the social domain.

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Neither diet, nor social system or brain size could explain the results from the PCTB experiments. “With our study we show that cognitive abilities cannot be generalized, but that species instead differ in domain-specific cognitive skills,” says Claudia Fichtel, one of the two first authors of the study funded by the German Research Foundation. “Accordingly, the relationship between brain size and cognitive abilities cannot be generalized.”

The study represents the first systematic and comparative investigation of cognitive abilities in lemurs and provides important insights into the evolution of cognitive abilities in primates. However, the research team also emphasizes that further comparative studies in a variety of other species are essential to answer the many questions about the relationship between brain size, diet, social life and cognition.

Source:

Deutsches Primatenzentrum (DPZ)/German Primate Center

Journal Reference:

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

Abstract:

Primates have relatively larger brains than other mammals even though brain tissue is energetically costly. Comparative studies of variation in cognitive skills allow testing of evolutionary hypotheses addressing socioecological factors driving the evolution of primate brain size.

However, data on cognitive abilities for meaningful interspecific comparisons are only available for haplorhine primates (great apes, Old- and New World monkeys) although strepsirrhine primates (lemurs and lorises) serve as the best living models of ancestral primate cognitive skills, linking primates to other mammals. To begin filling this gap, we tested members of three lemur species (Microcebus murinus, Varecia variegata, Lemur catta) with the Primate Cognition Test Battery, a comprehensive set of experiments addressing physical and social cognitive skills that has previously been used in studies of haplorhines.


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We found no significant differences in cognitive performance among lemur species and, surprisingly, their average performance was not different from that of haplorhines in many aspects. Specifically, lemurs’ overall performance was inferior in the physical domain but matched that of haplorhines in the social domain.

These results question a clear-cut link between brain size and cognitive skills, suggesting a more domain-specific distribution of cognitive abilities in primates, and indicate more continuity in cognitive abilities across primate lineages than previously thought.

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