"I believe the absolute number of neurons an animal has, especially in the cerebral cortex, determines the richness of their internal mental state and their ability to predict what is about to happen in their environment based on past experience", said Suzana Herculano-Houzel, associate professor of psychology and biological sciences at Vanderbilt, who oversaw the study with a collection of global researchers. A team of researchers from six different universities in the U.S., Brazil, Denmark, and South Africa contributed to the research.
One of the key questions the study hoped to answer was whether carnivores have more brain neurons than the herbivorous species they prey upon, with the researchers hypothesising that they should because hunting is cognitively more demanding. Herculano-Houzel herself admits that, while the study was objective, she herself does have a bit of a bias.
"You take the brain and turn it into a soup", she said, matter-of-factly, as the first step to finding these neurons.
"In 2005, my lab developed a very simple, fast and cheap method to count cells in brains and brain parts", Herculano-Houzel said.
"Neurons are the basic information processing units", said Herculano-Houzel. The more neurons the brain contains, the faster that processor is and the brain has the capacity for more complex behaviors, pattern recognition and the ability to make decisions based on past experiences or future perceptions.
"As far as dogs and cats go, the study found that dogs have about 530 million cortical neurons while cats have about 250 million". Humans, by comparison, have around 16 billion. "And this implies that dogs have more cognitive capabilities than cats". Though Herculano-Houzel notes they also have higher-than-typical neuron counts in their cerebellums, the part of the brain that controls motor skills.
The researchers analyzed the brains of other animals like lions and hyenas as well, comparing the number of neurons in their brains compared to the size of their brains.
The study also found that brain size has little relationship to intelligence.