Nesting behavior is associated with increased urinary volume in the urinary bladder during the reproductive period in small-egged Kajika, Cottus pollux SE
We found that male small-egged Kajika, Cottus pollux SE sensu Yokoyama and Goto (2005), retained a large volume of urine (approximately, 1 % body weight) in their urinary bladder while nesting under spawning substrate. This increase in urine volume occurred three days after the males were transferred to the nesting tank. Serum 11-ketotestosterone (11-KT) concentrations of males also increased approximately twofold after the males were transferred to the nesting tank. While this increase in the urinary volume was observed only during the reproductive period (late December), the increase in serum 11-KT concentration from the basal levels associated with nesting occurred from the spermatogenic period (from October to mid-December) to the reproductive period (late December). These findings suggest that the urine of nesting males plays a role in reproduction, whereas the increase in serum 11-KT levels may be associated with social status/dominance. The increased urine due to nesting contained an approximately sixfold level of 11-KT compared to the initial control values, suggesting that the urine of nesting male Kajika may have functioned as social signals to females and/or to other males.