The urination pattern of the Mozambique tilapia (Oreochromis mossambicus) depends on social context, and the olfactory potency of urine released depends on social rank (males) and reproductive status (females). This strongly suggests that urine mediates chemical communication in this species. The current study tested, firstly, whether urine production rate depends on sex or social status and, secondly, whether differences in urination pattern and volume of urine stored are associated with variation in the morphology of the urinary bladder. Finally, the effect of urination during aggressive male–male interactions was assessed. Urine production in catheterized fish depended neither on sex nor social status (males). Nevertheless, males had larger kidneys than females. Dominant males had heavier urinary bladders than subordinate males or females, mainly due to enlarged muscle fibres, thicker urothelium and a thicker smooth muscle layer. In male pairs wherein urination was prevented by temporary constriction of the genital papillae, social interaction escalated to aggression (mouth-to-mouth fighting) more rapidly and frequently than control pairs. This was accompanied by elevated plasma testosterone and 11-ketotestosterone levels. In control encounters, the male that initiated the aggressive behaviour was usually the winner of the subsequent fight; this did not happen when the males could not urinate. These results suggest that the larger, more muscular bladder of dominant males is an adaptation, facilitating higher urination frequency, post-renal modulation and storage of larger urine volumes for longer. It is likely that urinary pheromones modulate aggression in male–male encounters by providing information on the social rank and/or motivation of the emitter; males are unlikely to invest in costly highly aggressive fights if they judge their opponent to be more dominant. Thus, a morphological explanation for the differing urination patterns of dominant and subordinant males, and females, has been provided, and a possible function for this behaviour in male–male interactions is suggested.

In: Behaviour