Cuttlefish have an intuitive understanding of quantity are able to discern between close numbers like four and five. Here's how scientists made the finding:
From an article on the finding at Discover:
The researchers hatched cuttlefish eggs and waited until the animals were a month old. Then they started testing the young cephalopods. In each test, a cuttlefish waited on one end of a tank, and researchers lowered a two-chambered box with clear walls into the other end. A partition jutted out from the center of the box toward the cuttlefish. As it swam toward the apparatus, the cuttlefish would have to make a choice between one chamber and the other.
Image: Neil Liddle