I recently found myself listening to another episode in the eternal debate about whether dogs are smarter than cats. One of the supporters of the "dogs are smarter" camp had already put forward the evidence that dogs are more trainable and can solve more complex problems than cats.
One of the supporters of the "cats are smarter" camp responded by saying that you can't conclude that dogs are smarter based only on the findings that they learn to obey commands better. She suggested that such an argument fails to take into account the fact that cats are more independent and often choose not to comply. "Dogs allow themselves to become slaves to humans, while cats maintain their freedom to disobey and to not do what is desired or expected of them."
The dog supporter countered with, "I can't agree with that. In the laboratory, you usually have a hungry dog or a hungry cat placed in a situation where learning the proper response or solving the problem will gain them the food that they want and need. I can't imagine that a hungry cat would 'choose' not to respond correctly because of their independent nature. A much more logical conclusion would be that cats are not responding correctly because they haven't been able to solve the problem or are unable to figure out which behavior will gain the food reward for them."
"It makes perfect sense to me," replied the cat proponent. "Cats can solve the same problems that dogs can, and cats certainly understand the behaviors that they are being told to learn. The problem that researchers have is that cats just refuse to bend to a human's will. Even faced with the possibility of starvation cats often choose to avoid slavery. They are much like the American patriot Patrick Henry, who said, 'Give me liberty or give me death!' So what you consider to be a sign of lower intelligence is really just evidence of feline independence."
This debate went on for a while longer and since it was apparent that the participants would never reach an agreement, the conversation eventually switched to less complex and less contentious matters—like politics.
However, science marches on, and it now seems that we have a bit more evidence to insert into discussions like this. Furthermore, since this new evidence is physiological in nature, it seems to circumvent any issues of "feline independence." This most recent study involved an international team of researchers and was done in the lab headed by Suzana Herculano-Houzel at Vanderbilt University. The lead author was Débora Jardim-Messeder, who worked on this as part of her master's degree.
The senior researcher, Herculano-Houzel, had developed a new procedure to quickly and efficiently count the number of cells in brains and brain parts. It involves a device called an isotropic fractionator. The trick is that you take the brain, or a section of the brain that you are interested in, and using a methodology that involves a special detergent, you turn the brain tissue into a kind of soup. In that soup, you can clearly distinguish the nuclei of the cells. Since every cell has one nucleus, counting the nuclei is equivalent to counting the cells. When you view this soup under a fluorescence microscope (which floods the samples with specific wavelengths of light) the nuclei glow, and their color indicates whether each nucleus comes from a neuron (one of the information processing cells) or from some other type of cell in the brain.
Using this methodology, the brains of a number of animals have been studied—not just dogs and cats, but also raccoons, bears, lions, elephants, a number of primates, and even humans. You might ask "Why not just measure the overall size of the brain?" But the total size of the brain is not a good measure of the number of neurons in the cerebral cortex. The cerebral cortex is the crinkly and convoluted outer shell of the brain that sits on top of the brain's other parts. Herculano-Houzel elaborates: "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."
The reason why the density of the neurons in the cerebral cortex is not predicted by brain size alone is that in some larger brains, the increased bulk results from the fact that individual neurons have grown larger, rather than increased in number. Additional mass comes from the fact that these larger neurons have more connective material (the fibers which make up the dendrites and axons). Perhaps most importantly, the largest increase in the size of bigger brains comes because the mass of the non-cortical, lower structures of the brain has grown bigger. The cells which make up these lower structures are not the same kind of cells as the cortical neurons. Since neurons are the brain’s “information-processing units,” Herculano-Houzel explains, “whatever species has the most neurons in the cerebral cortex is therefore expected to be capable of more complex and adaptive behavior.”
The relative number of neurons researchers find in the cerebral cortex of various animals seems to make sense in terms of what we know about animal intelligence. For example, humans have by far the largest number of cortical neurons—as many as 16 billion per person. If we look at our closest primate cousins, orangutans and gorillas each have about 8 to 9 billion neurons, while chimpanzees have about 6 to 7 billion neurons. One of the most intelligent non-primate animals that the research team studied was the elephant, which has 5.6 billion neurons.
So how do cats and dogs differ in terms of their neuronal count? Cats come in with a tally of around 250 million neurons in their cerebral cortex. This, surprisingly, turns out to be almost the same number of neurons found in a much larger animal, the bear. Raccoons, with a brain size equivalent to that of a cat, have a much higher neuronal density (around 400 million neurons). Herculano-Houzel says, "The large numbers of neurons in the small raccoon brains jives very well with how crafty/smart/resourceful these creatures are believed to be."
Two dogs were studied. The smaller dog, a 15-pound mixed breed, was shown to have approximately twice as many cortical neurons as a cat, with a total of 430 million. A Golden retriever (which is ranked as the fourth smartest dog breed based on its working and obedience intelligence) was determined to have roughly 620 million cortical neurons. In comparison, a female lion assessed by the researchers had only 500 million neurons. "It is fair to say, then, that dogs have about twice as many neurons as cats in their cerebral cortex," Herculano-Houzel says, "and this implies that dogs have more cognitive capabilities than cats."
Herculano-Houzel admits that, while the study was objective, she herself does have a bit of a bias. “I’m 100 percent a dog person,” she says, “but, with that disclaimer, our findings mean to me that dogs have the biological capability of doing much more complex and flexible things with their lives than cats can.”
Despite these findings, it is unlikely that the debate over the relative intelligence of dogs and cats will go away. After all, these latest results are based on neuron count estimates rather than on measures of actual intelligent behavior. Simply having more neurons doesn't mean that an individual uses all of that available capacity fully and efficiently. It is important to coordinate these results with studies that directly test the actual behavioral performance of cats and dogs in complex situations. However, as Herculano-Houzel concludes, "At the least, we now have some biology that people can factor into their discussions about who’s smarter, cats or dogs."
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Débora Jardim-Messeder, Kelly Lambert, Stephen Noctor, Fernanda Marques Pestana, Maria Eduarda de Castro Leal, Mads F. Bertelsen, Abdulaziz N. Alagaili, Osama B. Mohammad, Paul R. Manger and Suzana Herculano-Houzel (2017). 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 of Neuroanatomy, doi: 10.3389/fnana.2017.00118