Discrimination of movement and visual transfer abilities in cichlids (Pseudotropheus zebra)
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Fish rival birds and mammals in many of their cognitive skills, and have been shown to successfully discriminate between a range of stationary and moving objects. The present study tested the ability of Pseudotropheus zebra to recognize unique movement patterns shown in the form of a single moving dot, point displays (PDs), point-light displays (PLDs), and videos of moving organisms in two alternative forced-choice experiments. Cichlids successfully distinguished between (1) different directions of movement, (2) a biological vs. a random movement, (3) a biological (human walking) vs. a scrambled motion pattern, and (4) two biological motion patterns (human walking to the left vs. to the right). Following training in (3), it was tested if the walking human was also correctly identified when either the positive, the alternative, or both stimuli were altered or presented inverted; following training in (4), stimuli were presented inverted or moving backwards. With the exception of the presentation of inverted and backwards-moving stimuli, fish excelled at these tasks. Furthermore, cichlids successfully discriminated between videos of different organisms such as eel vs. trout, human vs. dog, eagle vs. bat, and dolphin vs. shark. Following each training, a series of transfer tests elucidated whether P. zebra could also recognize these organisms when shown in transfer test trials (a) from a different perspective (front or sideways), (b) enlarged or downsized, or (c) as PDs. With few exceptions, all individuals learned all tasks and significantly often chose the previously reinforced (but altered) training stimulus over the alternative one during transfer tests. This indicates that cichlids have the ability to recognize a familiar organism under new conditions, for example, based on its movement alone, which may be helpful in recognizing approaching predators early on.
Fish can solve a variety of learning and memory tasks including visual discrimination of objects. Here it was tested if cichlids (Pseudotropheus zebra) can distinguish between pairs of videos featuring individually moving dots, dot patterns and moving organisms. Overall, fish were very successful and solved most of the presented tasks. They differentiated between movement directions and movement types, recognized stimuli in transfer tests under altered conditions, and identified familiar organisms when shown resized, from new perspectives or in form of point displays. To be able to recognize movement and to identify organisms based on their specific movement patterns (alone) in the wild is essential to individual survival, as predators, conspecifics and/or prey are usually mobile. Additionally, predators are often encountered at night or when light levels are low, making it harder to successfully distinguish general body features such as coloration and adding significance to being able to quickly recognize motion cues. It should be equally advantageous to be able to recognize organisms such as predators from unfamiliar perspectives. Stimuli used in this study were not of any ecological relevance; they were chosen purely for their characteristic type of movement and just served as vectors to determine transfer abilities of gained knowledge onto new situations. Nonetheless, having excelled at tasks using these stimuli, it seems quite likely that cichlids would perform even better under ecologically significant conditions.
KeywordsCognition Operant conditioning Learning Point-light display Fish Memory
We would like to thank Timo Thuenken for help with the statistical analysis, Sven Kappmeyer for help generating the videos and point displays used in E6 and E7, Adrian Klein for designing the stimuli used in experiments E1 and E2, and Slawa Braun for help with animal caretaking, maintenance, and repairs. We specifically thank N.F. Troje for allowing us to use the Program “Biomotionlab” and for helpful comments on the manuscript. We would also like to thank two anonymous reviewers for their constructive criticism of the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.
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