AN ANALYSIS OF AGGRESSIVE BEHAVIOR, GROWTH, AND COMPETITION FOR FOOD AND SPACE IN MEDAKA (ORYZIAS LATIPES (PISCES, CYPRINODONTIDAE))

Abstract

The role and consequences of aggressive behavior in competition for food and space were studied among laboratory populations of juvenile medaka. Growth rate, used to measure the success of an individual fish in different competitive situations, was followed for 648 fish in populations of 1, 2, 4, 8, or 16 fish in 1-, 4-, or 8-liter baskets.Aggressive behavior was initiated by internal state of "hunger" and the presence of food stimuli and smaller medaka. Aggression was at a low level when food was supplied in excess and large fish had no competitive advantage over small fish. Reducing the amount of space from 1 to 1/16 liter per fish did not alter the growth consequences of competition as long as the accumulation of waste products was prevented and food was supplied in excess.When food supply was limited, large medaka were socially dominant, chased small fish away from food, and grew faster than small fish. If food was spatially localized the social hierarchical societies changed into territorial societies in which the dominant defended the food area, but aggression only dispersed the subordinates throughout the habitat when food was evenly distributed. The advantage of social dominance was less if population size was large, if food was evenly distributed, and visual isolation between competitors was great. The advantage was high if food was contagiously distributed and visual isolation between competitors was great.