# Mixed Signals and Interspecies Variation in the Plasticity of Adult Mammal Brains

## TL;DR

This paper explores how brain plasticity varies across mammalian species, highlighting evolutionary trade-offs and the complexity of interpreting findings due to interspecies differences.

## Contribution

The paper introduces the concept of evolutionary trade-offs in brain plasticity and highlights the role of immature neurons in explaining conflicting results.

## Key findings

- Adult neurogenesis decreases in large-brained mammals compared to small-brained species.
- Immature neurons are more prevalent in the neocortex and amygdala of larger-brained mammals.
- Evolutionary pressures have shaped distinct plastic processes across species.

## Abstract

Increased understanding of brain structural plasticity in mammals has led to increasing conceptual complexity and conflicting results in the field.The growing recognition of interspecies variation in neuroplasticity, from mice to humans, and the overlapping of cell markers in different cell populations are the main elements of confusion.Some cell types, cell markers, and biological processes markedly differ among species due to the non-linear sculpting that took place during evolution, including trade-offs in plasticity.The discovery of non-dividing, immature (late-maturing) neurons has increased the heterogeneity in brain plasticity and can explain some controversial findings.Understanding how brain structural plasticity has adapted to diverse mammalian neuroanatomies is essential for meaningful translational research.

Increased understanding of brain structural plasticity in mammals has led to increasing conceptual complexity and conflicting results in the field.

The growing recognition of interspecies variation in neuroplasticity, from mice to humans, and the overlapping of cell markers in different cell populations are the main elements of confusion.

Some cell types, cell markers, and biological processes markedly differ among species due to the non-linear sculpting that took place during evolution, including trade-offs in plasticity.

The discovery of non-dividing, immature (late-maturing) neurons has increased the heterogeneity in brain plasticity and can explain some controversial findings.

Understanding how brain structural plasticity has adapted to diverse mammalian neuroanatomies is essential for meaningful translational research.

Despite the growing interest in brain structural plasticity and the substantial body of knowledge that has accumulated over recent decades, some issues remain poorly defined, leading to confusion in the interpretation of results. In addition to stem cell-driven neurogenesis in adult neurogenic niches (adult neurogenesis), neuronal precursors in a state of arrested maturation have also been described, representing a form of neurogenesis without division based on so-called “immature” or late-maturing neurons. These processes occur in different brain regions yet share certain molecular markers and temporal windows. Recent advances in comparative neuroplasticity have further complicated our understanding. Studies reveal a reduction in adult neurogenesis in the olfactory bulb and hippocampus of large-brained, gyrencephalic mammals compared with small-brained species such as rodents. Conversely, a higher prevalence of immature neurons has been reported in the neocortex and amygdala of larger-brained mammals. It is becoming evident that evolutionary trade-offs took place in distinct plastic processes, resulting in the predominance of certain forms in particular species, while others coexist and share overlapping markers. Regardless of the approach employed (neuroanatomical, immunocytochemical, phylogenetic, or transcriptional), current evidence indicates substantial heterogeneity in cell types with different origins and fates across diverse mammalian species. These patterns appear to be sculpted by evolutionary pressures yet unified by shared transient maturational states.