Dog wins tally of nerve cells in the outer wrinkles of the brain

If more nerve cells mean more smarts, then dogs beat cats, paws down, a new study finds. That harsh reality may shock some friends of felines. However, scientists say the real surprises came from the brains of less popular carnivores. Raccoon brains are packed with nerve cells, for instance. But brown bear brains are sorely lacking.

The researchers tallied the numbers of nerve cells, or neurons, in eight species. The ferret, banded mongoose, raccoon, cat, dog, hyena, lion and brown bear are all carnivores. Comparing how many neurons each hosted in their brains gave the scientists a better understanding of how different-sized brains are built. This neural tally appears in a December 12 Frontiers in Neuroanatomy paper. Ultimately, such data may also help reveal how brain features relate to intelligence.

For now, the multispecies nerve-cell count raises more questions than it answers, says Sarah Benson-Amram. She’s a zoologist at the University of Wyoming in Laramie. “It shows us that there’s a lot more out there that we need to study to really be able to understand the evolution of brain size and how it relates to cognition,” she says.

Suzana Herculano-Houzel is a neuroscientist at Vanderbilt University in Nashville, Tenn. She and her colleagues gathered brains from the different species. For each animal, they then whipped up batches of “brain soup.” This was brain tissue dissolved in a detergent. To this they added a molecule in this slurry that attaches only to neurons. This let the researchers count the neurons in each bit of brainy real estate.

Counting neurons

The cerebral (Seh-REE-brul) cortex is the wrinkly outer layer of the brain that’s involved in thinking, learning and remembering. Based on work with other mammals’ brains, the team knew that there was a fairly predictable link between the size of a carnivore’s cerebral cortex and how many neurons were in it. (Primates are the exception. This group includes monkeys, lemurs, chimps, gorillas and humans. Primate brains tend to pack in lots of neurons without growing bigger.)

In the new study, the expected relationship between brain size and neuron number generally held up. But for some larger carnivores — with correspondingly larger cortices — the neuron count seemed really low. For instance, lions and bears both had fewer neurons than a golden retriever. This canine had 623 million neurons packed into its doggy cortex. (For scale, the human cortex holds roughly 16.3 billion neurons.)

The brown bear’s was surprisingly low. Its cortex is about 10 times bigger than a cat’s. Yet the bear’s cortex contained roughly the same number of neurons as a cat, some 250 million. “It’s just flat out missing 80 percent of the neurons that you would expect,” Herculano-Houzel notes. She suspects that there’s a limit to how much food a big predator like a bear can catch and eat, especially one that hibernates. That caloric limit might also cap the number of its neurons. After all, those cells burn a lot of energy.

Another surprise — but in the opposite direction — was the raccoon. Its cat-sized brain hosts a doglike number of neurons. This fits with the nocturnal mammal’s reputation as a clever problem-solver. Benson-Amram cautions, however, that it’s not clear how brain-neuron numbers relate to potential intelligence. Raccoons are very dexterous, she says. It’s possible that a beefed-up brain region that handles touch, which is part of the cortex, could account for the coon’s high neuron count.

Herculano-Houzel expected large predators such as lions to have lots of neurons. “We went into this study with the expectation that being a predator would require smarts,” she says. But in many cases, a predator didn’t seem to have more neurons than its prey. A lion, for instance, has about 545 million neurons in its cerebral cortex. But the blesbok antelope, one of its prey, has about 571 million, the researchers previously found. And that antelope’s high neuron count was packed into a cortex that was somewhat smaller than the lion’s.

It would be nice to know how neuron count relates to smarts. By counting neurons, “we’ve figured out one side of that equation,” Herculano-Houzel says. Those counts now need to be linked to animals’ thinking abilities.   

Some studies have found correlations between brain size, neuron number and problem-solving skills across various species. This includes one by Benson-Amram. But finding ways to measure intelligence across different species is challenging, she adds. “I find it to be a really fun puzzle. But it’s a big challenge to think, ‘Are we asking the right questions?’”