title: Eye color as an indicator of social rank in the fish Nile tilapia.
abstract: We investigated the association of eye color with the dominant-subordinate relationship in the fish Nile tilapia, Oreochromis niloticus. Eye color pattern was also examined in relation to the intensity of attacks. We paired 20 size-matched fish (intruder: 73.69   11.49 g; resident: 75.42   8.83 g) and evaluated eye color and fights. These fish were isolated in individual aquaria for 10 days and then their eye color was measured 5 min before pairing (basal values). Twenty minutes after pairing, eye color and fights were quantified for 10 min. Clear establishment of social hierarchy was observed in 7 of 10 pairs of fish. Number of attacks ranged from 1 to 168 among pairs. The quartile was calculated for these data and the pairs were then divided into two classes: low-attack (1 to 111 attacks - 2 lower quartiles) or high-attack (112 to 168 attacks - 2 higher quartiles). Dominance decreased the eye-darkening patterns of the fish after pairing, while subordinance increased darkening compared to dominance. Subordinate fish in low-attack confrontations presented a darker eye compared to dominant fish and to the basal condition. We also observed a paler eye pattern in dominants that shared low-attack interactions after pairing compared to the subordinates and within the group. However, we found no differences in the darkening pattern between dominants and subordinates from the high-attack groups. We conclude that eye color is associated with social rank in this species. Moreover, the association between eye color and social rank in the low-attack pairs may function to reduce aggression.
authors.affiliation: Research Center on Animal Welfare - RECAW, Laborat rio de Fisiologia e Comportamento Animal, Departamento de Fisiologia, Instituto de Biociências, Universidade Estadual Paulista, Botucatu, SP, Brasil. volpato@ibb.unesp.br
authors.forename: G L
authors.initials: GL
authors.lastname: Volpato
authors.forename: A C
authors.initials: AC
authors.lastname: Luchiari
authors.forename: C R A
authors.initials: CR
authors.lastname: Duarte
authors.forename: R E
authors.initials: RE
authors.lastname: Barreto
authors.forename: G C
authors.initials: GC
authors.lastname: Ramanzini
mesh_terms: D000374:Aggression / Q000502:physiology; D000818:Animals; D004291:Dominance-Subordination; D005127:Eye Color; D006608:Hierarchy, Social; D017210:Tilapia / Q000033:anatomy & histology* / Q000502:physiology