Traces of phylogeny and ecology in hippocampal neuron numbers

It is not known how selective pressures shape the numbers of interconnected neurons in defined neural circuits during the phylogeny of mammals. Consequently, models of function are without phylogenetic bounds, and species differences in neuronal makeup cannot be linked to ecological factors that generate selective pressures. Based on data from 65 species belonging to 11 orders, we here provide an analysis of five interconnected neuron populations in the circuitry of the hippocampus, the forebrain region encoding episodic memories. Related species tend to share traits in the hippocampal makeup, with distinct differences between clades. Phylogenetic signals result in the clustering of related species according to relative neuron numbers, but signal strengths allow the clusters to overlap. Tree-based methods show that neuron numbers can be explained by a selective mechanism that constrains them close to an across-species mean. Neuron numbers concerned with hippocampal input are more constrained than those providing output. An ancestral state estimate is provided, and species close to this phenotype are identified. Of the ecological factors tested, food, in terms of diet breadth, leaves its trace in many neuron numbers and strongly so in hippocampal input populations. Home range effects are more selective and relate to neuron ratios rather than neuron numbers. Phylogenetic constraints and ecologically guided relations seem necessary for the appropriate function of hippocampal input across a wide range of species.