The researchers in this latest study chose to analyze carnivores due to the wide range of sizes it incorporates – from the powerful polar bear to the little least weasel – and the fact that it includes both wild and domestic animals. The team looked at the brains of eight species: ferrets, mongooses, raccoons, cats, dogs, hyenas, lions, and brown bears.
Before the study, they predicted that these creatures would have more cortical neurons than the herbivores they hunt. However, to their surprise, they found that the neurons-to-brain size ratio in small- and medium-sized carnivores was actually the same as in herbivores, and that in large carnivores like bears, it was lower. Although the team's sample size was small, it appears that prey animals require more intelligence to escape being hunted than we tend to give them credit for.
The lower numbers of cortical neurons in large carnivores is likely due to the energy that the brain requires – energy-wise, the brain is our most expensive organ.
"Meat eating is largely considered a problem-solver in terms of energy, but, in retrospect, it is clear that carnivory must impose a delicate balance between how much brain and body a species can afford," said Herculano-Houzel in the statement.
So, dogs are smarter than cats, but cortical neuron density is not the be-all and end-all of intelligence. “There are multiple ways that nature has found of putting brains together – and we're trying to figure out what difference that makes," explained Herculano-Herzel.