It's a bad news/good news situation for Fluffy: Cats don't have as many neurons as dogs, suggesting they just aren't as cognitively capable.
On the other hand, they've got as many neurons as brown bears.
Those are the results of a new study that counted neurons in the brains of eight animals in the order Carnivora, a diverse group of mammals whose members' diets usually (though not always) include meat. Researchers thought they might find that hunting gives carnivorans a brain boost over herbivores. Instead, they discovered that the number of neurons in any given carnivoran's brain has more to do with brain size — at least to a point. The biggest animals in this group, such as lions and bears, have a relatively piddling number of neurons.
In fact, the animal in the study that boasts the most neurons isn't the wily hyena or the noble lion, but the domestic dog (specifically, the lovable golden retriever). [10 Things You Didn't Know About Dogs]
"It looks like there's a trade-off," said study leader Suzana Herculano-Houzel, a professor of psychology at Vanderbilt University. "Once a carnivoran reaches a certain large body size, feeding that body starts to become so expensive that it comes at a cost of decreasing numbers of neurons in the cortex."
Neuronal density
Neurons, Herculano-Houzel told Live Science, are expensive cells; they take a lot of energy to nurture and support. The number of neurons in the brain is also the best physical approximation of that brain's capability, she said. But brain size is not a good indication of how many neurons a brain contains.
"If you just compare species by brain size, you get some pretty weird things, like cows and chimpanzees have brains of a similar size," Herculano-Houzel said.
When you compare neuron counts per brain volume, on the other hand, the results fall more in line with what might be expected by looking at behavior and intelligence. Humans, for example, have the most neurons — 16 billion — in their cerebral cortexes, the outer, folded part of the brain where we do most of what we'd term "thinking." [The 5 Smartest Non-Primates on the Planet]
Herculano-Houzel and her colleagues wanted to count the neurons in carnivorans because the group includes animals smaller than ferrets and as large as elephant seals and walruses. They analyzed the brains of ferrets, banded mongooses, raccoons, domestic cats, domestic dogs, hyenas, African lions and brown bears, dissolving the brain tissue in a special detergent that destroys cell walls and leaves free-floating cell nuclei. Neuron nuclei can then be identified and counted based on a particular protein found only in those cells.
The researchers expected that the carnivorans that hunt large prey would have the most neuron-dense brains because they'd need the smarts to outwit their dinner.
"To my surprise, that's not at all what we found," Herculano-Houzel said.
Instead, these meat eaters' brains followed similar patterns as herbivore brains when it came to neuron number per body mass. Lions and hyenas, for example, have between 2.9 billion and 4.7 billion neurons in total, much like their similarly sized prey, blesboks and greater kudus, which boast between 3 billion and 4.9 billion.
In the cerebral cortex, size and neuron number tracked together up to medium-size animals. Ferrets have 39 million neurons in their 0.11 ounce (3.1 grams) cortexes. The slightly larger mongoose has a 0.33-ounce (9.3 grams) cortex and 116 million neurons. Cats have 250 million neurons in their cortexes, which weigh 0.85 ounces (24.2 grams). A small dog of unknown breed had 429 million neurons, while a larger dog, the golden retriever, had 627 million. The hyena had 495 million neurons in its cortex. Bigger isn't better
In larger animals, things got weird. The lion's cortex is twice the size of a dog's, but its number of cortical neurons (545 million) is about the same as that of Fido or Rex. The brown bear, which has a cortex weighing 7.8 ounces (222 grams), has just 251 million neurons, which is about on a par with the domestic house cat. [Here, Kitty, Kitty: 10 Facts for Cat Lovers]
Animals this large have to struggle, Herculano-Houzel said. They must expend large amounts of energy to catch prey, and they don't get to depend on a successful hunt each day. They likely can't support huge numbers of neurons, simply because the metabolic demands are too high. Other factors, like domestication or social behavior, didn't seem to play a role in neuron number.
What's interesting, Herculano-Houzel said, is that primates follow a similar pattern. The largest primates — gorillas and orangutans — don't have the neuron load that humans do, she said. Humans manage to pack a huge cognitive punch into our relatively small cortexes because ancient Homo sapiens learned to cook, Herculano-Houzel said. Cooking enables humans to extract more calories from their food with less energy expended on digestion. It's a subject she's written about before, in her book "The Human Advantage: How Our Brains Became Remarkable" (MIT Press, 2016).
Bears and lions could clearly benefit from the human technique, Herculano-Houzel said, if only they could master the finer arts of gastronomy. But there's a different member of Carnivora that manages to punch above its class, despite its small body size and small cortex: the raccoon. Raccoon brains are about the same size as cat brains, the researchers reported, but raccoon cortexes are packed with a whopping 438 million neurons — nearly as many as a large dog or hyena has. It's not really clear how raccoons pull this off, Herculano-Houzel said, but the numbers are impressive.
"There are so many neurons; to give you an idea, that if you gave me those numbers, I would tell you this is a primate brain," she said.
Cats versus dogs
So what about cats versus dogs, that perennial pet-owner argument? Herculano-Houzel's work suggests that dogs have more cognitive capability than cats. However, she stressed that cognitive capability shouldn't be confused with ability, meaning actual smarts and the opportunity to use them. But neurons aren't everything, she said. They're great for problem solving — if you can metabolically afford them.
Modern cats and dogs probably don't come by their neuron numbers because of any challenges or problem solving they have to face today, Herculano-Houzel said. Instead, their brains are the inheritance of their domestication history. All dogs, even those bred to have smaller bodies, descend from wolf-like ancestors, she said. Those were large animals, with the corresponding large cortexes and large number of neurons.
"It's to be expected that even if we start selecting for smaller bodies, there are going to be variations of this animal that have pretty large brains with a large number of neurons," she said.
Cats, on the other hand, all came from a small-bodied ancestor, probably much like the feral European wildcats that still live in some forests in Europe today. Their neuron number likely reflects that of this wild ancestor.
The results have been accepted for publication in the journal Frontiers in Neuroanatomy.
Editor's Note: This article was updated to change a reference to "whales" that was meant to be "walruses."
Original article on Live Science.
