Math isn't just a human thing. All kinds of animals, from African grey parrots to chimpanzees, are thought to have some kind of mathematical ability, but it can be hard to test. Now, a new study finds that certain crows have a way with numbers — one that resembles that of human toddlers.

Counting is a skill that children develop in stages. If you present a young toddler with three blocks, the child may well look at them and say, "One, one, one." Each word acts as a placeholder to refer to one of the blocks, summing to three.

This is an early form of counting, says Diana Liao, a neuroscientist at the University of Tübingen in Germany. The child is keeping track of a quantity of things (in this case, three) by making the same sound several times. This stage appears to be a steppingstone to more sophisticated, adult-like counting.

"A couple months later, when you ask, 'How many blocks are there on the table?' the toddler would just respond with, 'Three,'" says Liao.

The "one, one, one" form of counting requires a toddler to be able to both tally each object with a sound and control the number of sounds they're making. Liao wanted to know whether this second element — of producing a specific number of vocalizations — is something that other animals can do, too. Her critter of choice to test this question: the carrion crow.

"Crows are great," she says. "They're super smart. They're a lot of fun to work with."

And in a new paper in the journal Science, Liao, animal physiologist Andreas Nieder of the University of Tübingen and their colleagues demonstrate that the birds were indeed able to control the number of calls they made. "We show that crows have the capacity to count vocally," says Liao, "which mirrors this important developmental stage in toddlers."

These findings could shed light on the behavior of other birds in the wild, says Chris Templeton, a biologist at Western Washington University who wasn't involved in the research. "Maybe these crows are able to really intentionally produce vocalizations, and they have this idea of what their vocalizations mean," he explains.

In a natural setting, he says, this combination of intention and meaning might allow an animal to communicate something specific to other individuals of their own or a different species. In previous work, Templeton found that the more dangerous a predator is, the more "dee" sections a chickadee produces in its calls. Perhaps the more scared a bird is, the longer its calls are. But this latest research suggests to him the possibility that perhaps the chickadees are intentionally adding more "dee" sections to signal something to their neighbors about the level of danger in the environment.

Counting crows

Liao enrolled three male crows from the university's aviary in the study. She really had to work to design an experiment that the crows couldn't outsmart. First, she trained the birds to produce a different number of calls — one, two, three or four — in response to four arbitrary visual cues (a blue number 1, an orange number 2, a green number 3 and a pink number 4) and to four arbitrary sounds (a guitar chord, a drumroll, a cash register noise and a tonal sweep).

If they responded correctly, they got a food reward — either a bird pellet or a live mealworm. If not, they got a timeout. At first, the crows weren't sure what to do. "They call[ed] louder. They call[ed] longer. They flap[ped] their wings," says Liao.

But it didn't take long for the crows to catch on and start making the correct number of sounds for each cue — or so Liao thought.

"I was like, 'Wow, these crows are brilliant,'" she says. "And then I realized they were actually adopting the simpler strategy of just 'vocalizing until I get a reward.'"

So Liao pivoted. She trained them to call a certain number of times, stop and then peck a screen to report their final answer. That stopped them from cheating. And this time, the crows pretty much nailed it.

But even when they made a mistake, it was revealing. Their wrong answers still tended to hover around the right number. "It's easier to confuse three and four," says Liao, "as opposed to, for example, one and four."

Before the crows responded to the signal, they took a bit of time to react. In fact, the greater the number of calls they had to make, the longer the reaction time. Liao interprets that as a sign the crows were perhaps planning their answer before they began to call.

Then she looked at the acoustic features of the very first call that the crows would make. This analysis revealed that "you are able to predict the number of subsequent vocalizations from the first vocalization alone, which also supports that they might be … planning," says Liao.

These results allowed her and her colleagues to conclude that crows can control the number of calls they produce.

Who's the smartest of them all?

Templeton praises the study as exciting. But he points out that humans aren't necessarily the benchmark of animal intelligence.

"Animals are smart in a whole bunch of different ways. And those may or may not be the same things that we do," he says. "The way that they should be smart really depends on their environment and what they're experiencing in life and what they have evolved to be able to overcome."

Still, the fact that crows and human toddlers can both count this way — that they share this ability despite rather different brain architectures — is striking, says Liao.

"It's one of the mysteries," she says, "like, how are some species much more flexible in being able to control their vocalizations and some species are not?"

That flexibility is scattered across the tree of life. The first step, it seems, is to count the number of branches where it's found.

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