Elephants have bigger brains than humans, so why aren’t they smarter than we are?
The classic answer has been to play down absolute brain size in favor of brain size relative to body. Sometimes people justify this as “it takes a big brain to control a body that size”. But it really doesn’t. Elephants have the same number of limbs as mice, operating on about the same mechanical principles. Also, dinosaurs had brains the size of walnuts and did fine. Also, the animal with the highest brain-relative-to-body size is a shrew.
The classic answer to that has been to look at a statistic called “encephalization quotient”, which compares an animal’s brain size to its predicted brain size given an equation that fits most animals. Sometimes people use brain weight = constant x (body weight)^0.66, where the constant varies depending on what kind of animal you’re talking about. The encephalization quotient mostly works, but it’s kind of a hack. Also, capuchin monkeys have higher EQ than chimps, but are not as smart. Also, some birds have lower encephalization quotients than small mammals, but are much smarter.
So although EQ usually does a good job predicting intelligence, it’s definitely not perfect, and it doesn’t tell us what intelligence is.
A new AI Impacts report on animal intelligence, partly based on research by Suzana Herculano-Houzel, starts off here. If we knew what made some animals smarter than others, it might help us figure out what intelligence is in a physiological sense, and that might help us predict the growth of intelligence in future AIs.
AII focuses on birds. Some birds are very intelligent: crows can use tools, songbirds seem to have a primitive language, parrots can learn human speech. But birds have tiny brains, whether by absolute standards or EQ. They also have very different brains than mammals: while mammals have a neocortex arranged in a characteristic pattern of layers, birds have a different unlayered structure called the pallium with neurons “organized into nuclei”. So bird intelligence is surprising both because of their small brains, and because it suggests high intelligence can arise in brain structures very different from our own.
To cut to the conclusion: birds have lots of cortical neurons, and number of cortical neurons may be one of the most important biological substrates of intelligence.
It looks like the main driver behind the encephalization quotient results is that bigger animals have bigger neurons. Although elephants have big brains, each of the neurons in those brains is also big, so they don’t have many more neurons than smaller animals. One exception is primates, who have “managed to escape this scaling factor”. In primates, bigger brains translate into more neurons at about a 1:1 rate, which is part of why we’re so smart.
The other exception is birds. Driven by the need to stay light enough to fly, birds have scaled down their neurons to a level unmatched by any other group. Elephants have about 7,000 neurons per mg of brain tissue. Humans have about 25,000. Birds have up to 200,000. That means a small crow can have the same number of neurons as a pretty big monkey.
Does this mean they are equally smart? There is no generalized animal IQ test, so nobody knows for sure. But AII tried to get a rough feeling for this by asking blinded survey participants to rate the intelligence of various animal behavioral repertoires (which, unknown to them, corresponded to the behaviors of either a primate or a bird). They found that participants judged birds to be about as smart as similarly-neuroned primates. In particular, birds with more neurons were rated as smarter than primates with fewer neurons, which is a pretty crushing blow to us monkeys. It also suggests that the different organization of the mammalian cortex and the avian pallium doesn’t matter much.
So does that mean that intelligence is just a function of neuron quantity? That the number of neurons in your brain, plugged into some function, can spit out your IQ?
It…comes pretty surprisingly close to meaning that. Sure, some people point out that elephants have more neurons than humans. But most of those are in the cerebellum, which maybe should’t count. If you focus on cortical neurons, humans have 15 billion and elephants only five billion. A list of animals by cortical neuron count really beautifully matches our intuitive perceptions of which animals are more intelligent, whether we’re talking about primates or birds or whatever. All else being equal, people with larger brain volumes tend to be smarter than people with smaller brains, suggesting that the neuron number/intelligence relationship holds true for us too.
The most sober researchers add a little bit more to this picture. In the best review paper I could find on this subject, Dicke and Roth write:
The best fit between brain traits and degrees of intelligence among mammals is reached by a combination of the number of cortical neurons, neuron packing density, interneuronal distance and axonal conduction velocity—factors that determine general information processing capacity (IPC), as reflected by general intelligence. The highest IPC is found in humans, followed by the great apes, Old World and New World monkeys. The IPC of cetaceans and elephants is much lower because of a thin cortex, low neuron packing density and low axonal conduction velocity. By contrast, corvid and psittacid birds have very small and densely packed pallial neurons and relatively many neurons, which, despite very small brain volumes, might explain their high intelligence. The evolution of a syntactical and grammatical language in humans most probably has served as an additional intelligence amplifier, which may have happened in songbirds and psittacids in a convergent manner.
So although number of neurons is a big deal, quick and efficient neural communication is also important. Within species, there are no doubt other variations based on pure negative mutations or whether you’ve poisoned your brain with alcohol or whatever.
The problem with this elegant new paradigm is pilot whales. Quantitative Relationships In Delphinid Neocortex suggests they have about 37 billion cortical neurons, which is twice as many as humans. Maybe it’s the conduction velocity? Maybe the study got it wrong? Or who knows? Maybe they’re smarter than we are. Has anyone ever tried to get them to create great art or anything like that? No? Okay, paradigm is fine, let’s move on.
What does this mean for AI?
First, it suggests there’s no particular reason to study or try to mimic the columnar structure of the primate cortex; bird brains have a different structure and do just as well, neuron for neuron, as we do.
Second, it means that if we ever get AIs that are “on the intelligence ladder” – doing the same thing as animal brains – we should expect that their abilities may scale linearly-ish with available computing power. Which dectuples every 5-12 years. Great.
(for reference, giraffes have one-tenth of humans’ neuron count)
At this point we have only one hope: pilot whales. We must convince them to leave their watery repose and take their rightful place as guardians of organic life. Only then will we be truly safe.