Different land-based predators are responsible for different numbers of vervet monkey deaths. Vervet monkeys avoid land-based predators by climbing trees but avoid predation from the air by diving into foliage. Certain land-based predators feed only on vervet monkeys, whereas every predator that attacks vervet monkeys from the air feeds on many different animals. No predators that pose a danger to vervet monkeys can attack both from land and from the air.
Vervet monkeys use different alarm calls to warn each other of nearby predators, depending on whether the danger comes from land or from the air.
Which one of the following, if true, contributes most to an explanation of the behavior of vervet monkeys described above?