One of the species studied by Bouret et al., the Japanese macaque in its wild habitat on Koshima Island. Image credit: Lucie Rigaill (CC BY 4.0)
Primates – such as lemurs, monkeys and humans – can perform a diverse range of cognitive skills, from memory to processing speech. But how did this diversity of cognitive skills evolve?
To answer this question, scientists often compare the brain sizes of different species and measure how this relates to their social behaviors or ecology in the wild. But using the whole brain as a measure of global cognitive capacities seems crude in the light of modern cognitive neuroscience studies, which have shown that specific parts of the brain are responsible for certain cognitive skills.
It is unclear how relevant the findings of cognitive neuroscience studies, which test animals in a laboratory, are to real life situations and evolution. To address this gap, Bouret et al. integrated methods from both behavioral ecology and cognitive neuroscience to examine the size of primate brain regions.
The team studied brain images from 16 primate species, focusing on two regions that have been linked to specific cognitive functions in laboratory experiments. They examined the frontal pole, which is involved in metacognition (the ability to be aware of and assess one’s own thoughts), and the dorsolateral prefrontal cortex, which is involved in working memory (the ability to temporarily store information to solve a problem).
Bouret et al. then compared the size of these regions to socio-ecological factors: how much each species performs complex social interactions and how hard it is to find food in the wild, by measuring their population density and daily travel distance, respectively. This revealed that the volume of the frontal pole is larger in species that experience more complex social interactions and in species that struggle to find food – two tasks thought to require metacognition. The dorsolateral prefrontal cortex, however, is only larger in species that have difficulty foraging, which might require a strong working memory to plan travel routes.
These findings suggest that laboratory experiments linking cognitive skills to specific parts of the brain are reliable enough to predict the size of these regions across wild primates. The work of Bouret et al. also helps bridge the gap between cognitive neuroscience and behavioral ecology, and demonstrates how these two disciplines can be combined to investigate the evolution of cognition.
