Final answer:
The unconditioned stimulus for the vervet monkey's behavior in the scenario is the sight of the hawk circling, which naturally elicits a defensive response due to the predatory threat hawks pose to the monkeys.
Step-by-step explanation:
In the scenario described, the unconditioned stimulus for the vervet monkey's behavior would be 4) the sight of the hawk circling. An unconditioned stimulus (UCS) is something that elicits a natural, reflexive response without prior learning. When a vervet monkey sees a hawk circling, it naturally triggers a response for the monkey to exhibit a defensive behavior, such as crying out to warn others in the group. This is because hawks are predators, and the sight of one poses a direct threat to the monkey's safety.
Regarding other aspects of vervet monkeys behavior, their responses to specific calls indicate they have a complex communication system. For example, when vervet monkeys hear a "leopard" call, they run into the trees for safety, as leopards are ground predators. In contrast, when they hear an "eagle" call, monkeys look upwards and seek cover, as eagles are airborne threats. If they hear a "snake" call, the monkeys stand on two legs to scan the ground since snakes are not easily spotted from above. These varied responses to different types of calls suggest that vervet monkeys use vocalizations in a structured way that could be analogous to basic elements of language. They demonstrate remarkable behavior adaptations for their survival, based on their ability to communicate threats effectively through specific calls.