Artificial selection on brain size leads to matching changes in overall number of neurons

Abstract

Neurons are the basic computational units of the brain, but brain size is the predominant surrogate measure of brain functional capacity in comparative and cognitive neuroscience. This approach is based on the assumption that larger brains harbor higher numbers of neurons and their connections, and therefore have a higher information‐processing capacity. However, recent studies have shown that brain mass may be less strongly correlated with neuron counts than previously thought. Till now, no experimental test has been conducted to examine the relationship between evolutionary changes in brain size and the number of brain neurons. Here, we provide such a test by comparing neuron number in artificial selection lines of female guppies (Poecilia reticulata) with >15% difference in relative brain mass and numerous previously demonstrated cognitive differences. Using the isotropic fractionator, we demonstrate that large‐brained females have a higher overall number of neurons than small‐brained females, but similar neuronal densities. Importantly, this difference holds also for the telencephalon, a key region for cognition. Our study provides the first direct experimental evidence that selection for brain mass leads to matching changes in number of neurons and shows that brain size evolution is intimately linked to the evolution of neuron number and cognition.

Brain size does not predict learning strategies in a serial reversal learning test

“…To test the hypothesis that relative brain size predicts the ability to adopt an efficient learning strategy, we used female guppies, Poecilia reticulata W. Peters 1859, artificially selected for small and large relative brain size and with known differences in neuron numbers ( Marhounová et al, 2019 ), in a serial reversal learning assay. Previous experiments with brain size-selected lines have shown that large-brained guppies outperform small-brained guppies in a number of cognitively demanding tasks ( Herczeg et al, 2019 ).…”

Yes, correct context is indeed the key: An answer to Haave‐Audet et al. 2019

“…Taken together, this means that selection for larger brains in these selection lines changed the relative (and absolute, since the lines usually do not differ in body size) amount of neurons. This in turn provides experimental evidence that the overall number of neurons is strongly associated with overall brain size, at least in populations under directional selection for these traits, and we propose this is the causal factor behind the observed differences in cognitive ability in these brain size selection lines (Marhounova et al, 2019).…”

“…Gonda, Herczeg, & Merilӓ, 2009;Gonda, Herczeg, & Merilӓ, 2011;Herczeg, Vӓlimӓki, Gonda, & Merilӓ, 2014;Kolm, Gonzalez-Voyer, Brelin, & Winberg, 2009;Kotrschal, Deacon, Magurran, & Kolm, 2017;Li et al, 2017;Noreikiene et al, 2015). We recently tested this using the isotropic fractionator method and found that neuron number differences matched closely the differences in brain size (Marhounova, Kotrschal, Kverkova, Kolm, & Nemec, 2019). Moreover, the differences were consistent across different brain regions.…”

“…Given the recent methodological advances and discoveries of how neuron number and neuron densities vary across taxa and potentially are linked to brain size and/or cognitive abilities (Herculano‐Houzel & Lent, ; Olkowicz et al, ; Herculano‐Houzel, Messeder, Fonseca‐Azevedo, & Pantoja, ), we were very interested in how neuron number and neuron densities would (or would not) vary across the large‐ and small‐brained guppy selection lines. We recently tested this using the isotropic fractionator method and found that neuron number differences matched closely the differences in brain size (Marhounova, Kotrschal, Kverkova, Kolm, & Nemec, ). Moreover, the differences were consistent across different brain regions.…”

Brain size affects responsiveness in mating behaviour to variation in predation pressure and sex ratio

“…A recent artificial selection experiment on relative brain size in this species (Kotrschal et al, ) provides the opportunity to study how brain size and cognitive ability might affect mating behaviour and decisions under different social and physical environments. These selection lines differ by up to 13.6% in relative brain size with a matching increase in neuron numbers (Marhounová et al, ). The selection lines likewise differ in several aspects of cognitive ability (Buechel, Boussard, Kotrschal, Bijl, & Kolm, ; Kotrschal, Corral‐López, Amcoff, & Kolm, ; Kotrschal et al, ), and in a range of physiological, behavioural and life‐history traits (Kotrschal et al, ; Kotrschal et al, ; van der Bijl et al, ; Kotrschal, Corral‐López, Zajitschek, et al, ).…”