According to the first study to count the number of cortical neurons in the brains of a number of carnivores, dogs have about 530 million cortical neurons while cats have about 250 million neurons; for comparison, humans have 16 billion cortical neurons. The findings appear in the journal Frontiers in Neuroanatomy.
According to Alvarenga et al, dogs are brainier than cats. Image credit: Michal Jarmoluk.
“In this study, we were interested in comparing different species of carnivorans to see how the numbers of neurons in their brains relate to the size of their brains, including a few favorite species including cats and dogs, lions and brown bears,” said senior author Dr. Suzana Herculano-Houzel, of Vanderbilt University.
“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.”
Dr. Herculano-Houzel and colleagues picked carnivorans to study because of their diversity and large range of brain sizes as well as the fact that they include both domesticated and wild species.
The researchers analyzed the brains of one or two specimens from each of eight carnivoran species: ferret, mongoose, raccoon, cat, dog, striped hyena, lion and brown bear.
“We expected that our measurements would confirm the intuitive hypothesis that the brains of carnivores should have more cortical neurons than the herbivores they prey upon,” they explained.
“That is because hunting is more demanding, cognitively speaking, compared to the herbivore’s primary strategy of finding safety in sheer numbers.”
However, that proved not to be the case — the team determined that the ratio of neurons to brain size in small- and medium-sized carnivores was about the same as that of herbivores, suggesting that there is just as much evolutionary pressure on the herbivores to develop the brain power to escape from predators as there is on carnivores to catch them.
In fact, for the largest carnivorans the neuron-to-brain-size ratio is actually lower.
The authors found that the brain of a golden retriever has more cortical neurons than a striped hyena, African lion and even brown bear, even though the latter species have up to 3 times larger cortices than dogs.
The bear is an extreme example. Its brain is 10 times larger than a cat’s, but has about the same number of neurons.
The scientists also found that raccoons have dog-like numbers of neurons in their cat-sized brain, which makes them comparable to primates in neuronal density.
“Raccoons are not your typical carnivoran,” Dr. Herculano-Houzel said.
“They have a fairly small brain but they have as many neurons as you would expect to find in a primate — and that’s a lot of neurons.”
The study’s findings also challenge the prevailing view that domesticated animals have smaller brains than their wild cousins.
The ratios of brain size to body weight of the domestic species the researchers analyzed — ferret, cat and dog — did not scale in a significantly different manner from those of their wild relatives — mongoose, raccoon, hyena, lion and brown bear.
_____
Débora J. Alvarenga et al. Dogs have the most neurons, though not the largest brain: Trade-off between body mass and number of neurons in the cerebral cortex of large carnivoran species. Front. Neuroanat, published online November 29, 2017; doi: 10.3389/fnana.2017.00118
