Seeing the forest before the trees–spatial orientation in freshwater stingrays (Potamotrygon motoro) in a hole-board task

Abstract

Freshwater stingrays (Potamotrygon motoro) have been shown to use a variety of spatial learning strategies including directional, landmark and place learning. In the present study, the significance of landmarks and geometric cues was investigated in a hole-board task. The aim was to determine cue preferences and collect additional information on the orientation mechanisms used in elasmobranchs. In four experiments, five juvenile stingrays had to memorize a fixed goal location within either a rectangular or a circular arena in the presence of goal-associated, signaling landmarks, proximal and distal cues. Transfer tests elucidated which cues the rays used or favored to reach the goal position. All rays successfully solved three of four tasks; as expected, different strategies were used in the process. Small alterations in the positioning of signaling landmarks (causing a spatial conflict between the previous feeding location and the new position of the signaling landmark) caused individuals to visit both locations equally often, whereas large alterations caused animals to ignore signaling cues and return to the previous feeding location. In the last and most complex experiment, three of five rays found the feeding location by remembering the positions of both proximate and distal landmarks in addition to memorizing particular swimming paths. Results showed that rays generally placed more importance on the overall environmental or geometric arrangement of the arena than on (individual) landmarks. This seems ecologically feasible, as distinct landmarks (e.g. rocks, pieces of wood, water plants) in the rays' natural environment may be more easily altered, removed or obscured from view than global ones (e.g. a river bend), which tend to be more stable. Overall, these results confirm those of previous studies, in that freshwater stingrays orient visually, learn quickly and can apply various orientation strategies, which are not mutually exclusive.