Fear is a critical brain function that enables us to learn to avoid danger via reinforcement learning (RL). While many researchers have argued that fear has evolved to escape predators, how varying predatory pressures have shaped fear and other rewards, including positive social rewards for collective grouping, remains an open question. In this study, we investigate the relationship between predatory pressure and fear using an evolutionary simulation of RL agents with evolving rewards. In our simulation, prey and predator RL agents co-evolve their reward functions, including visual rewards for observing prey and predators. While fear-like negative visual rewards for predators often evolved in prey, we also observed cases in which positive rewards for both predators and prey evolved, the latter serving as a social reward for collective grouping. A comparison between different environmental conditions revealed that stronger predator hunting capability promoted stronger fear reward, while less food supply promoted more negative social reward. Moreover, fear did not evolve in response to static pitfalls with non-lethal damage, suggesting that actively hunting predators played an important role in its evolution. These results highlight the special role of predators in the diverse evolution of fear and social rewards.

翻译：恐惧是一种关键的大脑功能，使我们能够通过强化学习（RL）学会规避危险。尽管许多研究者认为恐惧是为了逃避捕食者而演化出来的，但不同的捕食压力如何塑造了恐惧及其他奖励（包括促进集体群聚的积极社会奖励）仍是一个未解决的问题。在本研究中，我们通过具有演化奖励机制的强化学习智能体进化模拟，探究了捕食压力与恐惧之间的关系。在我们的模拟中，猎物与捕食者强化学习智能体共同演化其奖励函数，包括观察猎物和捕食者时产生的视觉奖励。虽然猎物中常演化出对捕食者类似恐惧的负面视觉奖励，但我们也观察到捕食者和猎物均演化出正面奖励的情况，后者作为一种促进集体群聚的社会奖励。不同环境条件的比较表明，捕食者更强的狩猎能力会促进更强的恐惧奖励演化，而食物供给的减少则会促进更负面的社会奖励演化。此外，面对仅造成非致命伤害的静态陷阱时，恐惧并未演化出来，这表明主动狩猎的捕食者在恐惧演化过程中扮演了重要角色。这些结果突显了捕食者在恐惧与社会奖励多样化演化中的特殊作用。