A research team led by Project Assistant Professor Yuo Kondo of the Osaka Municipal University Graduate School has comprehensively clarified the limit to the number of times a male medaka (Oryzias latipes) can ejaculate per day, and the effects of successive spawning behavior on the number of sperm ejaculated, fertilization rate, and the behavior of both males and females.
Knowing how many times a male can ejaculate per day is important for understanding how living things reproduce. However, there has been no comprehensive research on the limit to the number of times a male can ejaculate, the effect of consecutive spawning behavior on sperm counts and fertilization rates, or changes in male and female behavior. In particular, it has been considered difficult to grasp the number of eggs and sperm in fish, because they release eggs and sperm into the water for external fertilization.
The research team prepared several tanks, each containing a female killifish, and placed males in different tanks to observe spawning behavior. As a result, it was found that males can spawn an average of 1 times a day, and consume more than 19% of the total number of sperm released in a day in the first three spawning behaviors. In addition, the fertilization rate was almost 1% for the first few spawning behaviors, but it dropped significantly after the 50th spawning, and sometimes the eggs were completely unfertilized.
Furthermore, we found that females do not adjust the number of eggs laid even when spawning with sperm-depleted males, resulting in successive spawning events in which males can fertilize eggs with a small amount of sperm, but many eggs are wasted by females.
This study provides a perspective on the limited availability of sperm, which has been overlooked in understanding animal reproduction, and gives important implications on the relationship between the costs of producing sperm and eggs and sexual selection and sexual conflict between males and females, which is one of the most important themes in behavioral ecology.
Paper information:[Royal Society Open Science] Male medaka continues to mate with females despite sperm depletion