Within group-living animals, individuals appropriately tailor their attitudes and responses to other group members according to the social context and external environment. At the simplest level, the behavioral output can be described as approach and affiliation (positive response) versus agonistic behavior and avoidance (negative response). The neural substrate that works between sensory input and behavioral output, or the integrative circuits underlying decision-making processes, however, is vast and mysterious. To address this issue, we have focused on medaka fish, a model animal used mainly in the field of molecular genetics. Previously, we demonstrated that medaka females recognize familiar males following prior visual exposure, and social familiarity influences female mating receptivity. Medaka females exhibit a positive response (high receptivity) to familiar males, and a negative response (low receptivity) to unfamiliar males. Further, we demonstrated the essential role of a subpopulation of gonadotropin-releasing hormone-producing neurons (GnRH3 neurons) in switching from low to high female receptivity. Recently we established a new methodology for heat-inducible Cre/LoxP recombination in the medaka brain. Using the IR-LEGO system, heat shock induced in a very small area of the developing brains leads to spatially controlled recombination of progeny cells in adult medaka fish, which allows for genetic modulation and/or visualization of neuronal populations of interest. Using this system, medaka may serve as an ideal model animal to understand how the “social brain” works at molecular and neuronal levels.

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Biologging is a powerful technique to measure biological information of free-ranging animals and the ambient environmental conditions they experience. Recent technological developments in micro data-loggers and transmitters have made it possible to simultaneously monitor and record the swimming behavior of various species of animals, as well as ambient physical environmental conditions and internal physiological information such as body temperature, electromyogram (EMG), electrocardiogram (ECG) and electroencephalogram (EEG) data. We introduce a few studies on physiological behavior of fishes that used biologging techniques, and provide an overview of several recent research projects: “Behavioral thermoregulation in Pacific bluefin tuna Thunnus orientalis”, “Migratory behavior of adult chum salmon, Oncorhynchus keta assessed by EMG transmitters”, and “ECG analysis of adult chum salmon during spawning behavior using ECG dataloggers.” We also discuss the directions for future biologging research on fish physiology in natural environments.

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