Brain structural plasticity is heterogeneous in mammals, due to adaptation to ecological pressures. Whether the genesis of new neurons (stem cell-driven adult neurogenesis) is reduced from mice to humans, we recently demonstrated that a population of prenatally generated (non-dividing) “immature” neurons (INs) retaining the expression of immaturity markers (e.g., doublecortin; DCX), is highly abundant in the cerebral cortex of gyrencephalic species. Presence of DCX-positive (DCX+) cells has been reported also in subcortical regions of some mammals, yet the results regarding their origin are conflicting. Here, we analyzed claustrum and amygdala of eight mammalian species widely differing for brains size, gyrencephaly and cortical IN amount, in search for IN densities.A total of 76 brains at different ages were collected. Serial coronal sections (40μm thick) covering both subcortical regions were selected at 480μm interval from each other, then processed for immunohistochemistry (DCX and Ki67 antigen) and positive neurons and nuclei were counted on Neurolucida software.We found DCX+ cells in both subcortical regions of all species, with remarkable abundance in primates, and substantial maintenance through ages in gyrencephalic species. Several aspects converge indicating the “immature” (non-newborn) nature of these cells: their different distribution, the complete absence of coexpression between DCX and Ki67 antigen, the latter being mostly associated with oligodendrocyte precursors.In conclusion, we show that a long-lasting reservoir of cells in arrested maturation does exist in brain subcortical regions involved in socio-emotional development, with a clear prevalence in primates.