A “head-like” component of a terrestrial robot promotes anxiety-like and defensive behaviors

Recognizing threats is a vital ability in social interactions across the animal kingdom. Yet, the role of specific perceptual elements, especially the head-body silhouette in aversive situations, remains relatively unexplored. In our study, we investigated the modulation of defensive behavior in rats facing a four-wheeled robot designed to simulate a natural predator. The robot featured an inflatable top allowing instant changes in appearance. In Experiment 1, rats encountered the head-inflatable robot (HEAD) in two sessions – a training session, where the rats were sequentially chased by the robot in both head-deflated (HEAD-Off) and head-inflated (HEAD-On) states, and a test session with a stationary HEAD-On or HEAD-Off robot 3 weeks later to assess long-term behavioral changes. The rats displayed reduced velocity and exploration in the center area during the HEAD-On phase of the training session. During the test session, the rats maintained a greater distance from the stationary HEAD-On robot than from the HEAD-Off robot, indicating sustained alertness based on the memory of the previous threat encounter. In Experiment 2, an identical procedure with the body-inflatable robot (BODY) was conducted. No significant differences emerged between BODY-On and BODY-Off conditions, except for a slight reduction in movement velocity during the BODY-On phase in the training session. Considering the substantial difference in behavioral reactions to HEAD-On versus BODY-On robots, we concluded that the emergence of head-like component in a chasing robot produced heightened vigilance and alertness. Since the two types of robots adopted a minimal design and differ only by the position of the inflatable top portion, our findings highlight the significant impact of a clearly recognizable head-like component in a threat encounter. The head-body silhouette provides a key perceptual framework for designing a social robot, with implications for both animal-robot and human–robot interactions.