Attraction of Ovulating Females by Hypertrophied Male Kidney Secretions During the Breeding Season in the Freshwater Goby, Rhinogobius flumineus

In nest-breeding fish species, olfactory signals can provide an effective means for males to attract distant females to nests without leaving them. Previous studies have shown that substances produced in the male kidney may be involved in female chemoattraction in typical nest-breeding fishes, including members of Gasterosteidae and Cottidae. However, this function, particularly in female attraction, has not been investigated in Gobiidae. In this study, we used the freshwater goby Rhinogobius flumineus to examine changes in male kidney size and urine accumulation in the bladder during the breeding season, as well as the ability of male kidney extracts to attract females. The kidneys of males approximately double in size between April (just before the breeding season) and May (during the breeding season). When two males and one nesting site were placed in the tank during the breeding season, only the male that occupied the nesting site accumulated urine in its bladder. In choice experiments using a Y-maze, ovulating females showed a tendency to enter the channel containing kidney extract from breeding-season males first more frequently, and spent significantly more time in the channel than the channels containing the control or female kidney extract. These results suggest that, in male R. flumineus, kidneys enlargement during the breeding season may be associated with the production of olfactory signals that attract ovulating conspecific females to the nest, which are subsequently released in urine.