RAPID BEHAVIORAL CHANGES IN MEDAKA (ORYZIAS LATIPES) CAUSED BY SELECTION FOR COMPETITIVE AND NONCOMPETITIVE GROWTH

Behavior is a major component of growth rate variation in fish, but the genetic relationship between behavior and growth is largely unknown. In particular, it is not known how behavior responds to natural or artificial selection on growth. It has been argued that artificial selection for fast growth in competitive environments might lead to higher levels of overall aggression, and therefore there would be no net gain in assimilation efficiency or growth. The contrary has also been argued. It is not immediately obvious, therefore, whether fish that avoid interacting with conspecifics should grow faster, or whether more aggressive, dominant individuals should grow faster. In the present study, we artificially selected fish on the basis of growth in two environments that differed in the intensity of social interactions. In the high interaction environment (HI), food was provided to excess inside a floating cork ring, which limited access to the food and allowed fish to attempt to monopolize the food supply. In the low interaction environment (LI), an equivalent amount of food was spread over the container's surface. In each social environment, fish were selected within family for fast and for slow growth rate during two generations (G₁ and G₂ ). The response was measured in the third generation (G₃ ) on the directly selected trait (growth) and on a correlated trait (agonistic behavior). The magnitude of the direct response to selection on growth in medaka was directly related to population density, i.e., selection was most effective at high densities. Agonistic behavior was inversely related to growth when individuals were raised and selected in an environment where enforced social interaction took place, so long as food was not limited in quantity.