Zero is a difficult idea. Even young children learn it later than other numbers. But a 2018 study showed that honeybees may understand it.

Scientists showed bees cards with one to six black shapes. The bees learned to choose the card with fewer shapes and got sugar water as a reward. Then the scientists showed them a new, empty card. The bees chose the empty card as "less than one."

Humans took a long time to develop the idea of zero. Treating "nothing" as a number is very abstract. So a 2018 study caught many people by surprise. It suggests that honeybees, whose brains contain fewer than a million nerve cells, may understand it.

Researchers in Australia trained free-flying bees to choose cards showing fewer black shapes. The shapes changed in size, form and position, so the bees had to focus on number.

Then the bees saw an empty card for the first time. When it was paired with a card showing one shape, they chose the empty card as the smaller amount. That means the bees put zero below one. The next year, the same team taught bees to add and subtract one using colors.

Researchers at RMIT University in Australia trained free-flying honeybees to pick whichever card showed fewer black elements, using between one and six shapes, and rewarded correct choices with sugar water. The shapes varied in form, size and position, so the bees could not rely on simple cues such as total area. After training, the bees were shown something they had never seen, a blank card.

Offered a choice between this empty set and a card with one element, they chose the empty card as the lesser quantity, in effect placing zero below one. Critics have raised objections. Low-level visual cues may not have been fully controlled. Putting an empty set in order is not the same as understanding the symbol 0. And the bees took many training trials to learn the task, so how they might use such an ability in nature is an open question.

Even so, the 2018 study, published in Science, drew wide attention. Zero is a fairly recent human invention, and even children grasp it as a number later than other quantities. The authors compared the bees' performance with that of gray parrots, monkeys and preschool children. A honeybee has fewer than a million neurons.