Bachelor of Science (honours).pdf

Bachelor of Science (honours)

Abstract

To avoid the many predators that are after them, animals need to make quick and accurate decisions. However, many small animals such as invertebrates have limited visual information, due to lack of depth perception and low spatial resolution. Thus, these animals rely on simple visual cues for decision making in predator avoidance. Most invertebrates rely on size dependent cues, such as angular size, for their escape responses. However, angular size confuses predator size and distance and provides no information about the speed of the predator. A recent study has found fiddler crabs are unique among arthropods in timing their escape behaviours based on angular expansion speed of predatory stimulus rather than angular size. However, the mechanisms they use to measure angular expansion speed remain unknown. This study I exposed fiddler crabs to computer-generated stimuli in order to identify which aspect of the visual scene the crabs use to measure the expansion speed. I measure the timing of their escape in response to stimuli that varied in shape (full objects vs edges only), direction of expansion (horizontally vs vertically), or approach trajectory relative to the crab (direct loom vs. near-miss trajectory). Contrary to previous studies, the crabs did not differentiate between horizontally or vertically expanding objects and responded earlier not later to high approach speeds. This study shows that fiddler crabs use edges of approaching objects to measure angular expansion speed. I also found that edge speed of approaching stimuli affects the previously found angular expansion speed threshold allowing the crabs to improve the accuracy of their escape responses based on more speed dependent visual cues, and not just angular expansion speed as previously found.