Medaka (Oryzias latipes) males are famous for their intense, repeatable spawning behavior during breeding season. In a new study, researchers investigated what happens to sperm performance when a male mates multiple times in succession. Previous work from Osaka Metropolitan University had already shown that sperm number drops after repeated spawnings and that fertilization success can sharply decline. Yet, whether sperm quality—specifically how fast sperm swim right after activation—also deteriorates with repeated mating was still unclear.
In medaka, fertilization occurs externally: eggs and sperm are released into water, where both disperse quickly. That means the initial swimming velocity of sperm strongly influences how efficiently sperm can encounter eggs. If velocity declines, even a reduced sperm count could compound reproductive failure.
To test this, the team led by Specially Appointed Assistant Professor Yuki Kondo and Professor Satoshi Awata ran two experiments. Each male was placed in a tank with either one female or fifteen females, allowing mating to occur. After mating was confirmed, females were returned to stock tanks, and males were then used for sperm quality assessments.
Sperm analyses focused on swimming behavior immediately after sperm activation. The results showed a measurable effect on early performance: compared with males that spawned only once, sperm from repeatedly mating males swam about 20% slower during the first 30 seconds after activation. Interestingly, this difference faded by 40 seconds, suggesting that the earliest phase of motility is the most sensitive to sperm depletion.
The findings extend the idea of reproductive trade-offs beyond sperm quantity alone. “Previous studies have mainly focused on sperm quantity, but our findings show that it is also necessary to consider changes in sperm quality,” said Dr. Kondo, emphasizing that velocity changes can alter fertilization dynamics.
In the final dataset, sperm counts differed between groups as well, ranging from 2,538 in the single-mating group (12 males) to 1,265 in the multiple-mating group (10 males). This supports a picture where repeated mating reduces both the amount and the early propulsion ability of sperm.
The study appears in the Journal of Ethology under the title “Qualitative sperm depletion: Successive mating reduces initial sperm velocity in medaka fish” and was published on 16-Apr-2026.
Looking ahead, the researchers say future work must determine how much sperm-count reductions versus velocity declines each contribute to fertilization success. They also aim to uncover the physiological mechanisms behind the early motility drop associated with successive mating, which could reveal general principles of fertility under behavioral stress.
Subject of Research: Animals
Article Title: Qualitative sperm depletion: Successive mating reduces initial sperm velocity in medaka fish
News Publication Date: 16-Apr-2026
Web References: http://dx.doi.org/10.1007/s10164-026-00887-7
References: Journal of Ethology
Image Credits: Osaka Metropolitan University
Keywords: medaka, sperm depletion, sperm velocity, external fertilization, reproductive strategy, mating frequency, qualitative sperm quality
