Home range is not constrained by number of hippocampal neurons across mammals

Numbers of hippocampal neurons vary by over three orders of magnitude across mammalian species. What evolutionary pressures shape this diversity? Given the role of the hippocampus in spatial mapping, the greater spatial navigation demands of larger home ranges may drive selection for more hippocampal neuron. Using data from 342 species, we crossed home range and population density data with cortical and hippocampal neuron counts predicted from clade-specific brain scaling laws to examine whether home range scales universally with estimated hippocampal neuron numbers across mammals. We confirm that home range scales universally with the inverse of population density across species and increases with body mass and metabolic rate. However, home range does not scale universally with hippocampal or cortical neuron numbers. Rather, scaling relationships differ by clade, such that carnivorans and cetartiodactyls traverse home ranges over 1,000-fold larger than primates with equivalent hippocampal neuron numbers. These findings persist across data subsets controlling for study method, duration, and temporal scope. Numbers of hippocampal neurons are thus not limiting to spatial navigation in the wild, calling into question adaptationist explanations for the evolution of more hippocampal neurons based on a supposed need for increased spatial processing capacity. We propose that home range is determined primarily by population density, mediated by field metabolic rate and diet. The diversity in hippocampal neuron numbers across mammals, in turn, arises as a byproduct of clade-specific scaling of numbers of cortical neurons which we suggest is contingent on energetic opportunity, not on navigational or other cognitive demands.