Urination frequency and behaviour of two neighbouring tilapia males. (A) Frequency of urination (pulses per minute every 5 min; mean ± SEM; n = 16) by males in social isolation (open horizontal bar) followed by contact with a neighbour male (dark horizontal bar). (B) Examples (1–3) of behaviour (submissive: white; not aggressive: light grey; aggressive displays: dark grey; highly aggressive: black) and release of urine pulses (urination), of around 1 s, during 45 min of interaction between two territorial male tilapia (a) and (b). In (1), male (a) increased its urination frequency 25 min after coming into contact with male (b) and initiated aggressive displays which escalated to symmetrical high aggression (circle fight); in turn, male (b) changed from submissive, not urinating, to aggressive displays and its urination frequency increased as the agonistic interaction escalated to high symmetrical aggression. In (2), both males increased their urination frequency within 10–15 min when both initiated aggressive displays which further escalated to high symmetrical aggression; although the two males maintained this level of aggression throughout the observation period, their urination frequency dropped significantly. In (3), we see the only case of high urination frequency observed during non-aggressive behaviours, before aggressive behaviour escalation, during which the frequency of urination was also high; male (a) became submissive within a few minutes after the symmetrical high aggression and stopped urinating, whereas male (b) continued with aggressive displays and stopped urine release as well.