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 379 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 scaling with hippocampal or cortical neuron numbers differs by clade, such that bats, carnivorans and cetartiodactyls traverse home ranges over 1,000-fold larger than other mammals 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. Significance StatementWhy do some animals have so many more brain neurons than others? One explanation is evolution through selection for more neurons as needed for instance to navigate their environments to find food. However, this study finds that bats, carnivorans and whales traverse distances more than 1,000-fold larger than other mammals with similar numbers of neurons in the hippocampus, the brain structure required for spatial navigation. The authors question the traditional need-based explanation for brain diversity in favor of an opportunity-based account of the evolution of numbers of brain neurons.