An African grey parrot spontaneously showed he understood the concept of zero
Alex, a parrot trained for decades by researcher Irene Pepperberg, had learned to name quantities and objects but was never taught the word 'zero.' During a test, shown a tray with nothing matching the question, he spontaneously answered 'none' — a label he'd only ever used before to describe an absence of similarity between two objects. It made him the first bird shown to grasp something like the concept of zero.