Institute of Biology of Inland Waters, Russian Academy of Science, Borok, Yaroslavskaya obl. 152742, Russia

Abstract

Swimming trajectories of goldfish in a novel environment were studied. The experimental device consisted of circular corridor provided with uniform illumination and no landmarks inside or outside the corridor. In the corridor, fish trajectories can be analysed as a sequence of gaps of various length between turns in two opposite directions. The sequence consisted of restricted searches (series of short gaps between turns inside some area of the corridor) and rangings (longer gaps which divided searches). In their turn, restricted searches can be divided into shorter searches and rangings when analysed visually on shorter scale. This observation suggests a fractal behavioural pattern, and this is proven to be true using quantitative methods. The model is developed, which explains the observed behaviour by the influence of spontaneous nervous activity on the control mechanism of behaviour. This influence causes noise-induced phase transition in the behavioural sequence. The model produces the similar fractal pattern and matches the fish behaviour both qualitatively and quantitatively, including values of the Korcak-Mandelbrot exponent. The observed fractal pattern resulted in essential individual variability of fish behaviour, and possible significance of this variability for an animal adaptive behaviour is discussed.