Biological motion stimuli are attractive to medaka fish

Abstract

In many social fish species, visual cues play an important role in inducing shoaling behaviour. The present study is the first to examine whether and how "biological motion" depicting a moving creature by means of only a small number of isolated points induces shoaling behaviour in fish. Medaka (Oryzias latipes) were used because they are known to have high visual acuity and exhibit a strong tendency to form shoals. In experiment 1, we found that the presentation of medaka biological motion resulted in heightened shoaling behaviour when compared with that of non-biological motion (depicted by a small number of points placed at fixed distances that moved at a constant speed in a constant direction). In experiment 2, it was indicated that medaka biological motion was more effective at inducing shoaling behaviour when compared with human biological motion. In experiment 3, it was demonstrated that shoaling behaviour was largely dependent on the smoothness of the biological motion. In experiment 4, we revealed that shoaling behaviour was maximised in normal speed group and decreased in faster- and slower-than-normal speed groups. In experiment 5, it was shown that shoaling behaviour was slightly reduced when a reversed movie was presented. These results suggest that motion information extracted from conspecifics was sufficient to induce shoaling behaviour in medaka and that deviation from normal and familiar motion impeded shoaling behaviour. The naturalness of motion may be responsible for the induction of shoaling behaviour.

Fig. 1

A schematic representation of the method. a An animation sequence depicting biological motion. Six points were automatically placed along the body trunk of a moving fish using computer software. Based on the tracking data, the movements of medaka were expressed as those of six grey dots. b A schematic of the experimental set-up for the behavioural test. Stimuli were presented on a CRT display. The tip of the head of the fish was automatically tracked using computer software. We analysed the time that the head was near the CRT display (an area of 7.5 mm in width from the inner surface of the tank on the display side), the travel distance on the x axis (in a horizontal direction against the display), and the travel distance on the y axis (in a vertical direction against the display)

Fig. 2

The results of experiment 1, in which the effects of biological motion (BM group) were compared with those of non-biological motion (NBM group). a The time during which medaka were close to the display, b the travel distance on the x axis, and c the travel distance on the y axis

Fig. 3

The results of experiment 2, in which the effects of medaka biological motion (M group) were compared with those of human biological motion (H group). a The time during which medaka were close to the display, b the travel distance on the x axis, and c the travel distance on the y axis

Fig. 4

The results of experiment 3, in which the effects of the degradation of motion information were examined. In addition to the smoothly moving biological motion (60FPS group), jerky types of biological motion were presented (15FPS, 10FPS, 5FPS, and 1FPS groups). a The time during which medaka were close to the display, b the travel distance on the x axis, and c the travel distance on the y axis

Fig. 5

The results of experiment 4, in which the speed of biological motion was manipulated. Biological motion was presented at the normal speed (1× group), the two faster-than-normal speeds (2× and 1.5× groups), and the two slower-than-normal speeds (0.5× and 0.25× groups). a The time during which medaka were close to the display, b the travel distance on the x axis, and c the travel distance on the y axis

Fig. 6

The results of experiment 5, in which the effects of the forward playback movie (F group) were compared with those of the reverse playback movie (R group). a The time during which medaka were close to the display, b the travel distance on the x axis, and c the travel distance on the y axis