Carrots turned into a maths test at Barcelona Zoo, and two giraffes seemed to pass.

Researchers tested four giraffes and found that two appeared able to mentally combine quantities to pick the container with the most carrots, according to a paper published June 26 in Scientific Reports. All four failed tasks involving subtraction-like reasoning or more complex calculations.

The study adds giraffes to a growing list of animals shown to think in numbers. In recent years, scientists have found numerical cognition in species including fish, crows, bees and wolves.

Based on the zoo experiments, at least some giraffes appear able to combine quantities in their minds. The findings suggest giraffes can “remember quantities they have observed, mentally update that information following changes and make optimal decisions based on it,” study co-author Iker Loidi, a biologist at the University of Barcelona, says in a statement.

Loidi and his colleagues said some giraffes may have the cognitive foundation for more complex arithmetic skills.

The team was inspired by an earlier study at Barcelona Zoo that found captive giraffes could make statistical inferences to improve their chances of getting carrots instead of zucchinis. In those experiments, the animals chose the mix with the higher proportion of carrots. “We could see that they were quite good with numbers,” Loidi tells National Geographic’s Cara Giaimo.

For the new study, researchers showed each giraffe several containers holding different numbers of carrots, then added extra carrots to one of them. The giraffe could then choose one container to open and eat from.

In one example described from a video Loidi recorded, a giraffe named Nuru was shown three opaque containers, two yellow and one green, placed at her eyeline. One yellow box held two carrots, the other held three, and the green container held two.

After Nuru inspected them, Loidi closed the two yellow containers. He then took the two carrots from the green container and added them to the yellow container that already held two carrots, bringing its total to four.

When Nuru was offered the two closed yellow containers, she chose the one with the larger total and got the carrots. Loidi said that to make the correct choice, Nuru had to do something like simple addition, though “It’s more like the perceptive basis for addition,” he tells National Geographic.

Only Nuru and a male giraffe named Njano repeatedly solved the addition problem. The other two chose correctly only when scientists had handled the right container last.

The researchers said it is not clear why Nuru and Njano performed better. Loidi said they may naturally have stronger numerical abilities, or past training at the zoo may have helped them.

When researchers ran similar tests aimed at subtraction-like reasoning, all four giraffes failed. They also failed more complex tests that combined addition-like and subtraction-like operations.

“These results are consistent with what we observe in humans: There are individual differences in numerical problem-solving and, in general, subtraction is more difficult than addition,” Loidi says in the statement.

The report says giraffes seem better at distinguishing quantities than at least eight other species of hoofed mammals, including rhinoceroses and llamas.

Tyrone Lucon Xiccato, a zoologist at the University of Ferrara in Italy who was not involved in the research, tells National Geographic that the finding some giraffes can perform simple addition “is not, by itself, a revolutionary discovery.”

But, he said, each study like this helps build a “more complete picture of how cognitive abilities evolved across animals.”

Loidi said the study also shows that even “a species with a relatively small brain can perform mental operations,” in an audio clip in the statement. “These results challenge the idea that complex cognitive abilities are exclusive to humans and other primates and support the view that sophisticated capacities may have evolved convergently across different animal lineages,” he adds.

Read more from Smithsonian Magazine.