Male Medaka Fish Can Mate up to 27 Times Daily, Study Reveals

Research shows sperm depletion impacts fertilization rates and highlights reproductive trade-offs in Japanese ricefish.

Male medaka fish, commonly found in Japan, can mate an average of 19 times per day, with some reaching a maximum of 27 matings in controlled experiments.
Sperm output decreases significantly after the first three matings, dropping to as low as 0.5–6.3% of the initial amount by the final mating of the day.
Fertilization rates are nearly 100% during early matings but decline sharply after the 10th mating, leaving many female-released eggs unfertilized.
The study highlights potential sexual conflict, as females do not adjust egg production despite reduced sperm availability in males with multiple prior matings.
These findings provide insights into the energy costs of reproduction, sexual selection, and evolutionary pressures in species with external fertilization.