Video playback versus live stimuli to assess quantity discrimination in angelfish (Pterophyllum scalare)

Abstract

Video playback is a widely used technique for presentation of visual stimuli in animal behavior research. In the analysis of behavioral responses to social cues, presentation of video recordings of live conspecifics represents a consistently reproducible stimulus. However, video-recordings do not interact with the experimental subject, and thus this stimulus may be inferior in the social context. Here, we evaluated how angelfish (Pterophyllum scalare) respond to a video playback of conspecifics versus a live shoal of conspecifics. Using binary choice tests, subjects were presented different stimuli. Time spent close to one versus the other stimulus was considered an index of preference. We found angelfish to prefer a live shoal of conspecifics to an empty tank, and also the video playback of a shoal of conspecifics to a blank screen, although the level of preference in the latter was lower than in the former. These results indicate that video-playback of live conspecifics may be appropriate in angelfish, thus allowing manipulation of specific cues that angelfish may use in quantity discrimination. However, when we directly contrasted a live and a video recorded shoal, both having the same number of members, experimental fish preferred the live shoal. When the choice consisted of a live shoal of four conspecifics versus a video playback of a shoal of nine conspecifics no clear preference emerged. These results imply that video-playback has disadvantages in quantity discrimination studies with angelfish. Exploring procedural and/or technological parameters will verify the suitability of video-recording-based stimulus presentation for future use in angelfish.

Keywords: Video playback; angelfish; quantity discrimination; shoaling; social stimuli.

Fig. 1

Experimental set up for dichotomous test, consisting of two monitors and a ~ 70-l tank. Videos were displayed on the two side monitors. In some conditions, the test aquarium was flanked by two small aquaria (only one is shown on the right of the figure) with live stimuli, or at one side by an aquarium with live stimulus and at the other side by a monitor. Dashed line with arrows indicates that monitors (where the video playbacks were displayed) and stimulus aquaria with live shoals were exchanged in some of the experimental conditions (see text). Monitors were connected to a computer

Fig. 2

Proportion of time (preference index) spent by test fish in the preference zone close to the stimuli. Box plots show median (horizontal line in the boxes), 25% and 75% quartiles (boxes), and the lowest and highest values within the range of 1.5 times the respective quartiles (whiskers). Open circle shows outlier. Blades represent the mean proportion value. L = Live shoal, V = Video playbacked shoal, 0 = No fish (empty tank), B = Blank screen. Values above 0.5 indicate a preference for the numerically larger shoal or the live shoal. A significant departure from the null hypothesis of no preference is indicated by asterisks, ***p < .001, **p < .005