In a weird and wacky finding, scientists have discovered the limit to how many times a strange fish can have sex in a single day.
These fish, named medaka or Oryzias latipes, were found to be able to mate 19 times per day on average, with some males managing up to 27 matings, according to a paper in the online journal Royal Society Open Science.
Mating more would be expected to increase a male's reproductive success—allowing him to have more offspring. But producing and releasing more sperm takes a lot of energy. The new study reveals that medaka males release less and less sperm with each successive mating, eventually hitting a daily limit.
"Medaka are among the fish that spawn, where fertilization occurs after the eggs and sperm are released in water. As these gametes are difficult to collect, the number of sperm released and the fertilization rate during successive matings had remained a mystery," said study co-author Yuki Kondo, a researcher at Japan's Osaka Metropolitan University, in a statement.
"Our research group previously developed an accurate method for measuring the sperm count of medaka, which is why we were able to successfully conduct this experiment," Kondo said.
The researchers found that the medaka males mated an average of 19 times per day, with some only achieving four and others up to 27 times. They released their sperm into the water, where they can fertilize the eggs of the female, also released into the water but only once a day.
After the first few matings, the amount of sperm released dropped significantly, declining to 50 percent of the original content after the first three matings and then to only 0.5 to 6.3 percent by the final mating of the day. As a result, the rates of egg fertilization also dropped after the first 10 matings.
Additionally, the effort males put into courtship and the duration of the mating also declined with the number of times they had already mated that day.
Therefore, if the females mate with a male that has mated several times previously that day, she "wastes" her single opportunity to release eggs because the fertilization rate will be much lower.
"Despite the reduced sperm availability, females did not adjust their clutch size as a counterstrategy," the researchers wrote in the paper. "These findings imply that sperm is a limited resource for both sexes, potentially leading to sexual conflicts."
Sexual conflict occurs when the reproductive interests of males and females within a species diverge. This is because strategies that maximize reproductive success for one sex may reduce the fitness or reproductive success of the other, leading to evolutionary "arms races" between the sexes.
One strange example of this is traumatic insemination in bedbugs, where males bypass the female's reproductive tract and inject sperm directly into her abdomen. This benefits the male because it ensures fertilization, but it can harm the female, reduce her lifespan and lower her future reproductive output.
Previous studies have found that males of other species also have a limited number of times they can mate. The mite Tetranychus urticae mates an average of 13 times a day, and the moth Ephestia kuehniella mates five times in its lifetime.
Additionally, further research also found that males of other species released less and less sperm with subsequent matings.
"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, fertilization rate, and the behavior of males and females during this process," said study co-author Satoshi Awata, a biology professor at Osaka Metropolitan University, in the statement.
"Our research provides important insights into the relationship between the cost of gamete production and sexual selection," Awata said.
Reference
Yuki, K., Masanori, K., and Satoshi, A. (2025). Male medaka continue to mate with females despite sperm depletion R. Soc. Open Sci.12241668. https://doi.org/10.1098/rsos.241668
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