Social relation in a small experimental population of Odontobutis obscurus (Temminck et Schlegel) as related to individual growth and food intake.

Social relations among even-sized male individuals of Odontobutis obscurus, a carnivorous eleotrid goby, were observed in a small experimental population with measurements of individual body size and food intake. 1. Nipping, chasing, threatening, fighting and territorial defence were always observed. The dominance order of an incomplete peck-right type was recognized all the time. The dominant and the second-rank fish widely surpassed other subordinate individuals in the frequency of their attacking behaviour. 2. The dominant fish occupied and defended the whole bottom area of the aquarium (ca. 0.36 m) as his territory throughout the experimental period except only 1 day, though he was frequently challenged by the second-rank fish. Introducing nest-holes in the aquarium did not cause either increase of territory number or decrease of territory size. The holes were used by the dominant fish as the center of his territory. 3. All individuals increased their body weight in the initial period, but the growth of two subordinate fish became stagnant in the middle stage and later on, resulting in a wider difference between the growth rates of dominant and subordinate groups. 4. The second-rank fish showed the greatest food (live goldfish) intake and the highest gross efficiency of food/growth conversion, and his body weight surpassed that of the dominant fish in the later half of the experimental period. Thus the social ranking did not agree with the body-size ranking. 5. Both kinds of ranking did not change during the following periods of starvation and limited feeding. 6. O. obscurus showed predatory behaviour even at midnight in darkness whenever fed with gold fish, which was followed by a short period of aggressive behaviour.