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Quantification of neuron numbers in over a hundred avian species helps unravel the relationship between brain size and intelligence

Image: Corvids and psittacines (pictured above) are more innovative in their behaviour than pigeons and pheasants (below). They also have larger brains relative to their size, resulting from the accumulation of more neurons in the pallium, and a longer development time after hatching.
A greater accumulation of neurons in the pallium (the brain area involved in cognition) is related to one of the main forms of intelligence the ability to innovate and helps explain why intelligence requires bigger brains.
The work appears in the latest issue of Nature Ecology and Evolution, and is the result of an international collaboration between researchers from the CSIC and the CREAF, Mcgill University, Charles University, University of Montana and the Institute for Research on Biodiversity (IRBio) of the University of Barcelona.
14th July 2022.  What makes one animal more intelligent than another? Ever since Darwin, scientists have suspected that the intelligence of an animal has something to do with the size of its brain. The encephalisation theory, for example, argues that the "extra" brain tissue of a larger brain allows more neurons to be devoted to cognitive tasks. However, until now there was no scientific evidence to support this theory, partly owing to the difficulties of quantifying neuron numbers for a large number of species. The development of a new method for counting neurons by neurobiologist Suzana Herculano-Houzel and collaborators —the isotropic fractionator— now makes this possible.
The work involves the participation of the researcher Joan Garcia-Porta, member of the Evolutionary Genomics and Bioinformatics Research Group -integrated in the Bioinformatics Barcelona (BIB) platform- of the Department of Genetics, Microbiology and Statistics of the Faculty of Biology and the Institute for Research on Biodiversity (IRBio) of the UB. The study is co-directed by the researcher Daniel Sol, member of the CSIC and CREAF, and has been possible thanks to the development of a new method for counting neurons - the isotropic fractionator - by the team of the neurobiologist Suzana Herculano-Houzel, an equipment that makes it possible to count neurons of many species in a short time and carefully.
To conduct the study, the researchers started by estimating the number of neurons in the pallial telencephalon, the area of the bird's brain involved in sensory, associative and pre-motor tasks (equivalent to the neocortex in mammals). The neurons of 81 individuals of 46 species were measured in the laboratory of one of the corresponding authors, Pavel Němec, a neurobiologist at Charles University in Prague (Czech Republic) and one of the few world experts in the isotropic fractionator technique. In addition, neurons in the cerebellum (involved in voluntary movements) and in the brainstem (not directly involved in cognitive functions) were also quantified as a control. Added to the 65 species measured in previous studies, the total number of species analysed in the study was 111, which represents the largest sample of neuron numbers ever used in a single study.
Neuron numbers were then compared with data on innovation propensity extracted from a dataset including 4,400 observations of novel feeding behaviours of birds in their natural habitats. The innovation dataset is the result of two decades of work by Louis Lefebvre, a psychologist at McGill University (Montreal, Québéc, Canada) and also co-leader of the study. The feeding innovations were published in ornithological journals between 1960 and 2020, and include jackdaws cracking nut shells by throwing them on the pavement, sparrows blocking supermarket door sensors to steal food and blue tits opening milk bottles left on doorsteps by delivery men.
A new evolutionary model
The analyses of these data showed that the number of neurons in the pallium, and not those in the cerebellum or brainstem, is a good predictor of cognitive performance: the more neurons a species has in the pallium, the higher its capacity for innovation. In turn, the accumulation of neurons in the pallium causes the brain to grow more than expected by their body size, given that larger animals need larger brains to regulate body functions. Corvids and parrots, the two avian groups that stand out for their intelligence, have the highest number of neurons in the pallium and the largest brains relative to their body size.
Interestingly, corvids and parrots also have the longest maturation time. "We believe this is not a mere coincidence”," says Daniel Sol. "According to some eco-evo models, pallial neurons arise later in the development. Extending the latest stages of development can be the way through which evolution has facilitated the accumulation of neurons in the pallium in lineages where learning is crucial for survival and reproduction“.
In contrast, he notes, "in groups such as pheasants and pigeons, the post hatching developmental stage is shorter, which does not allow them to accumulate as many neurons in the pallium. This may explain why they have relatively small brains and are not very innovative in their behaviour".
Additionally, both corvids and parrots have longer lifespans. "Living longer not only reduces development constrains, but also increases the value of problem-solving through innovation because the time you spend learning a new behaviour only pays off if the behaviour provides long-term benefits. Moreover, a long life means that the animal is more likely to be exposed to changes during their lives".
Brain size: absolute or relative terms
The new study therefore shows that when it comes to intelligence, it is not as relevant the total number of neurons in the brain as the number of neurons allocated in the brain areas involved in cognition. By confirming this, the study helps settle the long-lasting controversy over whether brain size is more important in absolute or relative terms.
"Elephants have a larger brain than humans in absolute terms," says Lefebvre. "Does this mean they are smarter than humans? Not necessarily, if what matters most is relative brain size. The human brain contains more neurons in the pallium, which makes it larger in proportion to our size than elephants”.
According to Louis Lefevbre, "Our results help unify neuroanatomical measures at multiple levels, reconciling conflicting views on the biological significance of brain expansion". According to Daniel Sol, "The results also highlight the value of the life-history perspective in advancing our understanding of the evolutionary basis of the connections between brain and cognition.
Neuron numbers link innovativeness with both absolute and relative brain size in birds
Daniel Sol, Seweryn Olkowicz, Ferran Sayol, Martin Kocourek, Yicheng Zhang, Lucie Marhounová, Christin Osadnik, Eva Corssmit, Joan Garcia-Porta, Thomas E. Martin, Louis Lefebvre and Pavel Němec. Nature Ecology and Evolution. https://doi.org/10.1038/s41559-022-01815-x