If an animal is smart enough, should we treat it like a human? An abstract question, but one that found its way into a courtroom recently. A case bidding for consideration by the New York State Court of Appeals sought to extend the legal concept of habeas corpus — which allows a person to petition a court for freedom from unlawful imprisonment — to cover two privately-owned chimpanzees. The case for giving the chimps a human right like freedom from unlawful incarceration is based on their similarity to humans — they can think, feel and plan, argue the people bringing the case on behalf of the chimpanzees, so shouldn’t they have some guarantees of liberty? The court declined to hear the case, but one judge did say that some highly intelligent animals probably should be treated more like people and less like property.

It’s just one judge, but you hear this kind of thing a lot from animal rights activists. The Nonhuman Rights Project, the nonprofit behind the habeas corpus lawsuit, has a stated goal of securing increased, human-like rights for great apes, elephants, dolphins and whales — highly intelligent, charismatic mammals.

So, does a chimpanzee deserve more rights than, say, a pigeon? The logic that leads to “yes” is clear enough, but putting it into practice would be tough, scientists say. Because when it comes to measuring intelligence, we’re actually a little dumb.

One of the problems: Animals don’t stack up the way you’d expect. “[Pigeons have] knocked our socks off in our own lab and other people’s labs in terms of what they can do,” said Edward Wasserman, a professor of experimental psychology at the University of Iowa. “Pigeons can blow the doors off monkeys in some tasks.” Experts who study animal intelligence across species say we can’t rank animals by their smarts — scientists don’t even try anymore — which means there’s no objective way to determine which animals would deserve more human-like rights.

A little more than 100 years ago, scientists started to amass the data necessary to prove that animals had real, independent minds. But everyone still assumed that you could, with the right set of tests, line animals up in a great chain of relative intelligence. Chimpanzees somewhere near the top. Earthworms somewhere near the bottom. Offer respect accordingly.

For most of the 20th century, Wasserman told me, the field of comparative psychology was dedicated to building the blueprints for this ladder. Researchers developed various tests meant to be the universal test of animal intelligence … and then were disappointed when they invariably proved less universal than hoped.

“Learning sets” are a great example of this. First developed by Harry Harlow — the psychologist probably best known for traumatizing baby monkeys with surrogate mothers made of wire — learning sets were, essentially, a test of how well a subject could learn to learn. Scientists might give an animal the choice of two doors to open, one of which had food hidden behind it. Then they’d do the same test, over and over, always with the food behind the same door. The animals t that figured out the game fastest were deemed to learn the best. Humans did better at this than chimps, Wasserman said, and chimps did better than rhesus monkeys, which did better than bush babies (a tiny primate native to Africa). It looked like some kind of cross-species hierarchy of IQ was emerging out of the data. “But then people studied blue jays and I’ll be damned if the blue jays didn’t look better than half the mammals tested,” he said.

By 1969, scientists began to abandon the idea that organizing animal intelligence into a hierarchy even made sense. “I don’t know any comparative psychologist who even likes the word ‘smart’ or ‘intelligent’ in talking about animals,” said Kristin Andrews, a philosophy professor and cognitive scientist at York University who studies animal minds and ethics. “They just aren’t helpful.”

Scientists know animals are capable of demonstrating an array of cognitive skills, and there are some skills that some animals are better at than others. But the problem is that a hierarchy assumes all animals (including humans) evolved for the same environment. And that just isn’t true. Animals are smart in the ways they need to be smart, Andrews said. And because environment and needs differ by animal, trying to rank them is futile. If a polar bear has different skills than an octopus, does that mean one is smarter than other — or does it just mean the ocean is different from an iceberg?

Instead, comparative psychologists now use a range of tests to understand different kinds of abilities. And, on some of these tests, animals do better than people. Birds and mice, for instance, both do better than people on tests of how quickly an individual can navigate a maze and remember the locations of objects, said Louis Matzel, a psychology professor at Rutgers. That doesn’t mean those mice are smarter than men. It just means that’s a skill their species had more need of.

Another barrier to sorting through the whole animal kingdom’s brainpower: There are a ton of animals. Some animals have been put through whole batteries of tests, while others have scarcely been studied, Wasserman said. Dolphins really are phenomenally intelligent, he told me, able to switch quickly between skills and demonstrate a range of abilities. But, he said, we don’t have enough data to know whether this ability is rare. “We’ve been studying task-switching in pigeons, and they’re pretty good, too,” he said.

All of this complicates the idea of tying animal rights to intelligence. At the very least, it means you can’t decide to bestow personhood on only some animals. There’s no scientifically valid way to determine should be in or out of the club.

Meanwhile, Andrews said, the legal side of the animal rights world isn’t really having discussions about the ladder of intelligence — or lack thereof — with the scientists who study the way animals think. As a result, there’s a risk of creating a new legal framework built on pseudoscience. She’d like research to build legal and ethical structures around how animal brains work, addressing the different needs and behaviors of different species. For instance, she said, we already know that pigs need to be challenged and entertained when they’re held in captivity. If that doesn’t happen, they get stressed out. And that’s something even the meat industry is interested in because stressed-out pigs don’t taste as good.

Wasserman echoed that perspective. It’s useless to extend human rights to animals, he told me, because that’s inevitably going to be based on a hierarchy model of intelligence. Instead, we should be giving animals animal rights — starting with the right to a wild habitat. The biggest threat to any species is habitat destruction, he said. So he suggested we start there, and give each species the right to its native place to live, instead of trying to attach human rights to something that is not human. “How about we accord other species the respect they deserve?” Wasserman said.