After the announcement of a neuroscience experiment on carnivore brains headlines like these leaped onto the internet:
Milo and I, as well as Hoss the Cat, share Grumpy Cat’s assessment of these headlines.
You see, the research paper that triggered this brouhaha about dogs being smarter than cats wasn’t about dogs being smarter than cats. This research was conducted in Suzana Herculano-Houzel’s lab at Vanderbilt University. These scientists took the brains of a ferret, banded mongoose, cat, raccoon, hyena, lion, brown bear, and two dogs, dissolved parts of those brains in a detergent solution, and used that solution to determine the number of neurons the brains contained. Researchers then compared the number of neurons in some parts of a creature’s brain to the size of the creature’s brain and the size of the creature itself. They did not do any intelligence testing of any sort. All of this ‘dogs are smarter than cats’ talk is based on the assumption that the number and density of neurons are correlated with intelligence (in this case it’s good to be dense).
This is a very cool experiment, and the researchers found some surprising results, which by the way, had nothing to do with comparing dog and cat intelligence. Here are some of the things they found:
- A lot of people tend to assume that predators are smarter than prey animals. From this assumption, a person might predict that predator species will have more cortical neurons than prey species. If there’s a correlation between neuronal density and intelligence, the results of this study contradict this prediction. This study found that predators had about the same neuronal density as their prey.
- Scientists expected to find a correlation between how big an animal is and how many neurons it has —bigger animals have bigger brains. This study found that the correlation generally held for small and medium-sized animals but not for big animals. Animals larger than a Golden Retriever had fewer neurons than these scientists expected. Perhaps, these researchers speculate, because neurons are so energetically costly. Think of neurons as race cars in that they take expensive fuel and burn it really fast. It might be the case that large animals cannot afford to have lots and lots of neurons because they lack sufficient fuel.
- There is a general belief that domestication makes animals stupider. These scientists did not find decreased numbers of cortical neurons in the domestic species they looked at. So again, if there’s a correlation between neuronal density and intelligence, this study contradicts the idea that domestication results in decreased intelligence.
When scientists find unexpected results or results that contradict things lots of people assume are true, is when science gets exciting and important. These three findings are the best parts of the study, but you would never know it from the headlines racing around the internet.
To be fair, these scientists did find about twice as many cortical neurons in the brains of a Golden Retriever and a small mixed-breed dog than they did in a cat. This is indeed a striking difference. They also found that bears had way fewer and raccoons had way more cortical neurons than expected for their body size. These findings about the bear and the raccoon are way more scientifically interesting than the cat-dog business.
Even though the headlines miss these exciting bits of the research, I am concerned, cranky even, other reasons.
The study did not measure intelligence. It measured the number and density of neurons in one or two individuals of several species. There is a big difference between the number of neurons a species has and how smart or brainy or intelligent that species is. There are many questions left to answer before we start casting aspersions on the intelligence of cats, or on making any inferences about intelligence at all. I would like to know if there are differences in how well those different neurons work, how big they are, and how they are connected to one another. And even if I had the answer to those questions, I would still feel uncomfortable using that data to draw conclusions about the intelligence of a species. Many news stories about this experiment contain buried disclaimers warning us about making assumptions about intelligence. However, they make the very assumption they warn against throughout the story and rely on it to create catchy headlines to interest people in this research in the first place.
Intelligence is not a thing, it is many things. Or more accurately many different kinds of capacities or skills. Some creatures are good at living in social groups, some are good at making tools, some are good at solving puzzles, and some have great spatial memory. There are many different ways to be intelligent. If you are going to talk about intelligence, you have to be very clear about what you mean.
The reporting of this research bugs me not only because the reasoning is sloppy or because Hoss the Cat is getting an unfair deal, although both of those things are true. It bugs me because it has the potential to hurt people.
There are lots of interactions between human and nonhuman research in areas like medicine, public health, psychology, and neuroscience. Research papers about human neurobiology, psychology, and intelligence draw on animal studies all the time. Sloppy reasoning about cats and dogs can lead to and support sloppy reasoning about people. Sloppy reasoning about people has been used and is still used in some circles, to justify sexist and racist conclusions about humans.
So, let’s be more careful. One way to do this is to distinguish between what the data clearly says and what we interpret the data to mean. Once we do that we can start figuring out where we are making assumptions and deciding whether those assumptions are justified. In the case of the density of neurons in cat’s and dog’s brains, the data tells us that, in the sample of two dogs and one cat, the dogs had more neurons in some parts of their brains than the cat did. It does not measure intelligence. The media stories about this research assume a connection between numbers of neurons and intelligence. It might turn out to be true that the number of neurons in some parts of the brain is, for example, related to a species’ ability to solve spatial problems. But this study does not give us data that supports this assumption.
In fact, exciting things can happen when we take the same data and pair it with a different hypothesis and a different set of assumptions. Let’s start by considering one of the interesting conclusions this experiment suggests. It appears to disprove the long-held hypothesis that predator animals are smarter than prey animals. Here’s the argument against that hypothesis:
- If predators are smarter than prey, [our hypothesis]
- and the number of neurons is correlated with smartness, [an assumption]
- then we would expect predator species to have more neurons than prey species. [a prediction]
- In the experiment, we did not find that predators have more neurons than prey species. [the data]
- So, our hypothesis about predators being smarter than prey is false.
But, look what happens when we flip things around and use the data to test the assumption I made in the above description of the experiment:
- If the number of neurons is correlated with smartness, [a new hypothesis]
- and predators are smarter than prey, [a different assumption]
- then we would expect predator species to have more neurons than prey species. [the same prediction]
- In the experiment, we did not find that predators have more neurons than prey species. [the same data]
- So, our hypothesis about the number of neurons being correlated with smartness is false.
The point is that an experiment and a set of data can be objective and accurate, but can still be used to prove many different things depending on the assumptions that go along with it. We can use the same data to disprove the assumption, OR to disprove the hypothesis. Pay attention to the assumptions and look for independent ways to test them!
And, give your cat a break, she still might be a super genius.
As a philosophy professor (my day job) I spend a lot of time thinking about the 
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e ground vigorously enough to suck dirt and leaves right up into his nose. He takes his sniffing seriously and his days revolve around finding things to smell, and then smelling them.
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