UK researchers found a link between big brains and low population densities They used statistical models to test different scenarios for hundreds of species The link is less clear in rodents and marsupials - and does not apply to humans Mammals that have big brains tend to be less abundant and at higher risk of extinction than mammals with smaller brains, a new study claims. UK researchers used statistical models to test different scenarios for hundreds of different mammal species, ranging from monkeys to foxes and tigers. ADVERTISEMENT Their findings suggest species with larger relative brain sizes have higher energetic requirements, resulting in lower population densities.