Animals

Dogs Are Twice as Smart as Cats, Says Brain Comparison Study

Dogs and raccoons are among the most intelligent animals on Earth, according to a new investigation of mammal brains.

Intelligence is hard to define in humans, but much less in other animals. One reason is that all species have evolved to be as “smart” as is necessary for their survival. In fact, some of the most populous animals on the planet have relatively unimpressive brains.

“Mice have tiny brains with few neurons and, if asked, they would certainly say that they're doing very well, thank you,” Suzana Herculano-Houzel of the Vanderbilt Brain Institute told Seeker.

Dogs have average-sized brains, so when Herculano-Houzel and her colleagues recently compared them to those of other mammals, they were shocked by the resulting data. It turns out that dogs have about twice the number of cerebral cortex neurons as cats.

“To put our findings into context: we humans have twice the cortical neurons that gorillas have,” said Herculano-Houzel, whose work on primates is outlined in the book The Human Advantage. “So, our new study provides reason to expect that dogs have the biological capability for much more complex and flexible behavior than cats do.” 

An illustration of carnivore brains. | Suzana Herculano-Houzel, Vanderbilt University

She explained that there is a direct correlation between total numbers of cortical neurons and intelligence, given that neurons are the information-processing units in the brain. She added that the cerebral cortex is the part of the brain that can combine information from different sources and create new associations, recognize patterns, make decisions to act differently based on past experience, and start to make predictions for the future.

“For that reason,” she continued, “whatever species has the most neurons in the cerebral cortex is therefore expected to be capable of more complex and flexible behavior.”

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Dogs didn’t just beat out cats for braininess, according to the researchers — who admit to being dog aficionados. According to the new study, which has been accepted for publication in the journal Frontiers in Neuroanatomy, dogs also possess more cortical neurons than striped hyenas, African lions and brown bears, even though bears have up to three times larger cortices than dogs.

Joe Howell, Vanderbilt University

Dogs are no match for humans in terms of this particular measurement, however. A person has about 16 billion cortical neurons while a dog has approximately half a billion. In other words, we have around 32 times as many of these information-processing units.

Prior research determined that a honeybee has approximately 1/1000 as many neurons as a dog cortex does.

“Yet honeybees certainly can do pretty complex things, like navigate to a particular spot and remember that location,” Herculano-Houzel noted. “However, they do that for a very short time. I have a suspicion that having more neurons is a requirement for living long, complex lives.”

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This complexity could extend to the animal's thoughts as well. Herculano-Houzel, lead author Débora Alvarenga and their team are therefore now considering the possibility that dogs are deeper thinkers than most of us might imagine.

The neuronal composition of carnivore brains is not affected by domestication either, according to the scientists. Genetics then appears to be a key determinant of intelligence.

Genetics could help to explain why dogs have so many cortical neurons. Dogs — from toy poodles to Great Danes — are all thought to have descended from wolves, which are larger animals than the ancestors of today's cats. Despite the varying sizes and shapes of dogs, they can cross-breed and are thought to possess similar levels of intelligence.

Another major factor underlying braininess appears to be energy availability.

“We humans could not afford all the neurons that we have if we still ate raw foods like other primates,” Herculano-Houzel said.

Suzana Herculano-Houzel. | Joe Howell, Vanderbilt University

Non-human great apes, on average, spend over eight hours per day eating to sustain not only their cortical neurons, but also the rest of their large bodies. Since larger body sizes can benefit terrestrial carnivores during hunting, there appears to be a trade-off between nutrition devoted to sustaining body size and cerebral cortex neurons.

Herculano-Houzel thinks that human ancestors partly overcame the problem when they began to process foods, first with stone tools and then later with controlled fire. Meat-eating animals without these advanced skills must exert more energy to obtain their prey, likely putting them at greater risk of metabolic constraints that impose the trade-off between braininess and body size.

“When I found the literature on how costly it is for large carnivores to hunt, my perception of them changed completely,” she said. “It must suck to be a lion.”

Raccoons could have a cerebral edge over lions and many other animals. The scientists discovered that a raccoon has dog-like numbers of neurons in its cat-sized brain. This makes raccoons comparable to primates in respect to neuronal density.

At present, the researchers are trying to estimate the energetic “cost” of various animals’ brains, and how that relates to the number of neurons each has.

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