Hank: We’re always hearing about how smart certain animals are, from octopuses that solve puzzles to birds that use tools. And who among us hasn’t wondered whether our cats are secretly much smarter than our dogs? But animal intelligence is actually really tricky to define -- and even harder to measure.

Like are we talking about the ability to innovate? To do arithmetic? To remember hiding places? Or are we talking being able to learn from, or maybe deceive, fellow creatures? Even if scientists can agree on what intelligence actually is, they still need to design tests that measure these abilities objectively. And that is not easy.

For one thing, food is usually used to motivate animals in the experiments. But if your subjects aren’t hungry, they won’t always cooperate. And if they’re not all equally hungry, then your data are kinda useless. But a few specific tests have become very popular over the years, and have been used on a variety of species.

In one, called the pointing test, an animal is conditioned to expect food to be hidden in a certain place. Once they find the food, the hidden treat is moved, and a human points to the new spot.

If the animal can take the hint -- and go over to the new location -- that’s supposed to indicate the ability to understand what another animal is thinking. After all, if you can learn new information from another animal -- like, the food isn’t where you think it is, it’s over there -- then it seems like you must at least be a little smart.

Another common experiment is the mirror test. It’s supposed to measure an animal’s ability to recognize itself. This ability, some scientists say, is an indicator of self-awareness -- suggesting that the creatures can conceive of things like individuality and identity.

So in the mirror test, researchers put a mark on an animal’s forehead and put a mirror in front of it. If the animal is highly self-aware -- the thinking goes -- it will realize that it’s looking at itself in the mirror, and show that by touching or rubbing the strange mark on its face. Dogs, as you might guess, ace the pointing test.

But they fail at the mirror test miserably, and sometimes adorably. They usually just want to play with, or bark, at their reflections. But does that really mean that dogs aren’t aware of themselves?

Some scientists say no. In fact, many argue that both of these tests are flawed. After all, how relevant are pointing and mirrors to animals that don’t have hands or fingers or that don’t rely on vision? These studies also usually have really small sample sizes -- like 11 elephants in one pointing test -- which makes it hard to compare one whole species to another.

So, in 2014, an international team of scientists decided to tackle the question of animal intelligence more systematically. They picked a single mental ability that they thought they could evaluate most fairly, and then performed the same tests on more than 500 animals across 36 species. They chose to test for self control.

That might not seem like intelligence, but being able to suppress a powerful but counterproductive urge is an important part of making decisions, whether you’re a bird or a human or an aardvark. To measure self control, the researchers used two tests.

In one, a piece of food was moved from under one box to under another. Animals were tested on whether they’d resist the urge to keep looking under the first box – kind of like a pointing test, minus the pointing. In the second test, food was placed in the middle of a clear plastic tube. The animals then had to go around to the side to get the food, instead of just trying to reach through the tube.

Both tasks tested animals’ ability to go against their instincts to get a reward. And the animals you generally think of as smart came out on top. Apes outperformed birds, for instance, and dogs were somewhere in the middle.

So no shockers there. But looking at the big picture, there were a couple of surprises. For one thing, the scores of different species correlated with their absolute brain size.

In the past, scientists had thought that the smartest animals would be the ones that had big brains relative to the size of their bodies. But here, all that seemed to matter was the overall size. And among the primates that the researchers tested, there was no link between how well they did, and how large the groups are that their species usually lives in.

This was important, because it contradicts the idea that primate intelligence evolved to help us deal with complex social situations. Instead, the primates with the best self control turned out to be the ones that had the most varied diets. This suggested that their smarts help them keep track of a wide range of foods in the wild, like where different fruit trees grow and when they’re in season.

Of course, the study, as ambitious as it is, isn’t the final word on animal intelligence. But it does offer some clues as to why we and other animals might have evolved certain abilities. Bigger brains mean more neurons, and having more of them might have allowed animals to evolve specialized networks. Even if we haven’t solved all of the mysteries of animal intelligence, we do know one thing -- our sophisticated brains aren’t as unique as we might like to think.

If you’d like to learn more about your brain and how it works, then you should totally check out our new channel, SciShow Psych -- it’s all about the workings of the human mind. And of course don’t forget to go to youtube.com/scishow and subscribe here, that helps us out, and also, you get science, all the time.