The Amboseli playback studies recorded vervet alarms, then broadcast them from hidden speakers with no predator present. Animals responded to each recording with the escape behaviour matched to that predator: up into trees for the leopard call, down from exposed branches and looking skyward for the eagle call, bipedal standing and ground-scanning for the snake call. Because no predator was visible, the response had to come from the call itself.
The result reshaped how animal signals were discussed, since the calls appeared to refer to categories of thing rather than merely to broadcast alarm. Young vervets make errors, calling eagle at harmless birds, and refine their usage over the first years of life, which points to learning within an inherited framework.
Whether the calls are truly referential remains argued. A caller producing a leopard alarm may be encoding its own escape response rather than naming a leopard, and the two accounts are hard to separate by experiment.