Perception of biological motion in point-light displays by jumping spiders

Over the last 50 years, point-light displays have been successfully used to explore how animals respond to dynamic visual stimuli--specifically, differentiation of the biological from the non-biological. These stimuli are designed to preserve movement patterns while minimizing static detail, with single dots representing each of the main joints of a moving animal. Imposed by their internal skeleton, vertebrate movements follow a specific semi-rigid dynamic pattern, termed "biological-motion", which can be used to distinguish animate from inanimate objects. Although biological motion detection has not been studied in invertebrates, rigid exoskeletons force many species to also follow semi-rigid movement principles. Due to their highly developed visual system and complex visual behaviors, we investigated the capability of jumping spiders to discriminate biological from non-biological motion using point-light display stimuli. By constraining spiders so that they could rotate but not move directionally, we simultaneously presented two point-light display stimuli with specific dynamic traits and registered their preference by observing which pattern they turned towards. Jumping spiders clearly demonstrated the ability to discriminate between stimuli. However, spiders showed no preference when both stimuli presented patterns with semi-rigid movements, results that are directly comparable to responses in vertebrate systems. This represents the first demonstration of biological motion recognition in an invertebrate, posing crucial questions about the evolutionary history of this ability and complex visual processing in non-vertebrate systems.