A study in Royal Society Open Science has shown that male medaka (Oryzias latipes; also known as Japanese rice fish) can mate up to 19 times a day, but their fertilisation rate diminishes significantly after the tenth session. 

These findings, reported by researchers from Osaka Metropolitan University, offer new insights into the reproductive behaviour and biological cost of gamete production of the male medaka.

They discovered that on average, the male medaka released more than half of their daily sperm supply during the first three mating sessions, for which near-perfect fertilisation rates were achieved. However, after the 10th mating, the fertilisation dropped significantly, and there was even no confirmed fertilisation in some of the later cases. 

Although the male medaka can mate multiple times a day (19 times on average), the female produces eggs only once a day and releases all of them during a single mating session. This means many eggs are “wasted” when the females mates with males who have already depleted their sperm reserves.

“This is the first study to quantitatively show the clear daily mating capacity of male medaka, as well as the volume of sperm released during each mating, fertilisation rate, and the behaviour of males and females during this process,” said Professor Satoshi Awata. 

“Our research provides important insights into the relationship between the cost of gamete production and sexual selection,” he added.