Dogs, according to their owners, are more intelligent than cats. Some even have jobs; rounding up sheep, guarding buildings and performing circus tricks. Cat enthusiasts, however, consider the slavish grovelling ways of dogs to be slightly degrading.
Cats, they say, have self-respect and their relationships with us are more subtle. You never really own a cat; it lives with you on its own terms. Working as rodent exterminators, cats display intuition know-how and ingenuity.
Curiosity, it’s said, kills them, but it’s also a sign of intelligence.
The cerebral cortex, a wrinkled overlay region of the brain, is active during thinking, problem-solving and planning.
The more cortical neurons, “grey matter”, in a creature’s brain, the greater its intellectual sophistication is likely to be. Or so it is thought.
Suzana Herculano-Houzel, of Vanderbilt University, has discovered a way to count these neurons. She and her colleagues examined the cerebral cortexes of selected mammals, including vegetarian and meat-eating ones. They also looked at the brains of dogs and a cat.
Comparing the brains of the cat, a golden retriever and a mongrel, the team found that the dogs had about 530m cortical neurons, whereas the cat had only 250m.
Humans have 16bn. This means that “dogs have the biological capability of doing much more complex and flexible things with their lives than cats can”, says Herculano-Houzel, who is an ardent dog-lover.
But does having more neurons in its cervical cortex necessarily mean that an animal is more intelligent, an ability notoriously difficult to define?
Racoons are genetically closer to cats, and their nocturnal life-styles are cat-like, but they have comparable numbers of cortical neurons to dogs.
The research produced another unexpected result. Vegetarian animals are thought to be less intelligent than meat-eating ones; it doesn’t take great ingenuity to munch grass and sit around chewing the cud.
Nor have herbivores developed complex strategies to avoid predators; they rely instead on safety of numbers in herds.
Meat-eaters, on the other hand, must outsmart their prey. Constantly facing new situations, they improvise and innovate, relying on past experience to predict what is likely to happen.
Carnivory, scientists tend to say, is cognitively demanding. The cerebral cortex of a predatory animal, therefore, should contain more neurons than a grazer’s.
Surprisingly, that wasn’t what Herculano-Houzel found. Herbivore brains, the counts showed, have just as many cortical neurons as small to medium-sized carnivores. It seems that “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”.
Successful hunts are intermittent; inevitably there are times when few kills are made. Hunting is energy-demanding and these lean periods put limits on how big a brain can become; the resources aren’t available to expand it indefinitely. Intelligence must deliver results; it has to pay its way.
The neuron-to-total-brain ratios of different carnivores are also intriguing. All of the small to medium-sized hunter species had similar numbers of cortical neurons.
This was not the case, however, with large carnivores. Lions hyenas and brown bears have much larger brains than small hunters, but their cortical neuron counts were lower.
The golden retriever’s brain had more of these nerve cells than the hyena’s or the lion’s.
A bear’s brain is 10 times the size of a domestic cat’s, but it has only the same number of cortical neurons.
D. Alvargenga 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.’ Frontiers in Neuroanatomy. 2017.
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