This is a wow, from 538 Significant Digits:
The number 0
Bees understand nothing. That is, they understand the concept of the number zero, a concept it typically takes human children years to grasp and for which even the ancient Romans had no numeral. Using drops of sugar as incentives, much like one would with the aforementioned young children, and sheets of paper printed with different numbers of objects, bee researchers taught bees to fly to the sheets with fewer objects. Eventually, the researchers introduced a sheet with zero objects. The bees then flew to that one, too. [Vox]