Visual discrimination and resolution in freshwater stingrays (Potamotrygon motoro).

Potamotrygon motoro has been shown to use vision to orient in a laboratory setting and has been successfully trained in cognitive behavioral studies using visual stimuli. This study explores P. motoro's visual discrimination abilities in the context of two-alternative forced-choice experiments, with a focus on shape and contrast, stimulus orientation, and visual resolution. Results support that stingrays are able to discriminate stimulus-presence and -absence, overall stimulus contrasts, two forms, horizontal from vertical stimulus orientations, and different colors that also vary in brightness. Stingrays tested in visual resolution experiments demonstrated a range of visual acuities from < 0.13 to 0.23 cpd under the given experimental conditions. Additionally, this report includes the first evidence for memory retention in this species.

Serial reversal learning in freshwater stingrays (Potamotrygon motoro).

Serial reversal learning is considered a reliable approach for the testing of behavioral flexibility, and animals that inhabit fluctuating habitats and different environments are expected to possess behavioral and cognitive flexibility. The ocellate river stingray (Potamotrygon motoro) is one such species. Comprising the first serial reversal learning experiment among elasmobranchs, this study trained seven juvenile P. motoro in a visual two-alternative forced-choice task, in which a food-rewarded stimulus and an unrewarded alternative stimulus were presented in pseudo-random order on either side of a barrier. In the session after a stingray reached the learning criterion (LC), food was associated with the alternative stimulus, and this reward association continued to be switched whenever LC was achieved. Overall, five stingrays reversed successfully at least once. All of them required more sessions (mean = 67.8 sessions) for the first reversal than they needed during training (mean = 22 sessions). One stingray demonstrated progressive improvement across four reversal phases, thus showing that Potamotrygon motoro can inhibit a previously learned association faster with experience, probably as it develops strategies for rule identification. One individual became slower to reach the (LC) across three reversal phases, which may indicate a dominant influence of proactive interference. Another stingray demonstrated large fluctuations across three reversals. The fourth individual completed two reversals and required approximately the same number of sessions for each. One stingray had just completed one reversal by the end of this study. Since stingrays took longer than a related species to reverse learning, future studies could look at the effects of changing stimulus type or training regime on the performance of P. motoro.