Unilateral Chasing and Courtship-Like Display as Dominance Signals for Hierarchy Maintenance in Male Medaka Fish

Abstract

Aggressive behavior, exhibiting a unidirectional behavioral event across various animal species, is instrumental in fortifying the existing hierarchical order and maintaining social stability within the group. In the present study, we investigated the types of aggressive behavior exhibiting unidirectional properties that function as dominance signals among three male medaka fish (Oryzias latipes). We first examined the directionality of attack/bite and chase behaviors, revealing that males often exhibited bidirectional physical attacks, while demonstrating predominantly unidirectional chasing behavior. This directionality was maintained throughout the hierarchy formation. Upon observing that male medaka fish exhibit courtship-like displays during male-male interactions, we subsequently investigated the timing and directionality of attack/bite, chasing, and courtship-like displays. Males exhibited unidirectional courtship-like displays, with the directionality matching that of chasing behavior. In addition, the number of attack/bite behaviors significantly decreased after the first chasing as well as the first courtship-like displays. Taken together, our findings strongly suggest that chasing and courtship-like displays may function as dominance signals, reinforcing established dominance-subordinate relationships. Furthermore, when the three males were separated for half a day and then reunited the following day, the number of physical attacks decreased significantly, while the directionality of chasing behavior and courtship-like displays remained unchanged. This finding suggests that once a dominance-subordinate relationship is established, it can be maintained through chasing behavior and courtship-like displays. These observations provide insight into how unilateral pseudo-sexual behavior between males can influence hierarchy formation in social animals.