Fighting behavior: Understanding the mechanisms of group-size-dependent aggression

Aggressive behavior is a common behavior in animals. In most cases, an animal’s behavior toward an opponent is a violent attack. Male crickets (Gryllus bimaculatus) exhibit intensively aggressive behavior toward other males, most often culminating in fighting. The detection of conspecific male cuticular substances initiates aggressive behavior in male crickets. After a fight, a loser no longer exhibits aggressiveness in a second bout or in separate encounters with another male; rather the defeated male exhibits avoidance behavior. Aggressive behavior in crickets provides an excellent model system to understand neuronal mechanisms underlying real-time control of sophisticated behavior and social adaptability of animals. Animals alter their behavior in order to respond to the demands of changing social environments. Society and crowding conditions are dynamic environments. In this chapter, we focus on how crickets determine their behavior depending on their social interactions, focusing on behavioral and physiological aspects. Whether the nitric oxide (NO) system and octopaminergic (OAergic) system in the central nervous system of crickets could mediate aggressive behavior of the crickets is discussed. Based on these results, a neurophysiological model is designed to elucidate the mechanisms of social adaptability. This model demonstrates that a multiple feedback structure, composed of a feedback loop in the nervous systems and individual interactions with other crickets, may be a key to aggression influenced by group size (group-size-dependent aggressive behavior).