Research News

Jul 25, 2023

  • Science

Male medaka fish cleverly regulates sperm volume by recognizing rival males behavior!

A study conducted by Specially Appointed Assistant Professor Yuki Kondo, Professor Satoshi Awata, and Specially Appointed Professor Masanori Kohda of the Graduate School of Science, Osaka Metropolitan University, and Professor Yasunori Koya of the Faculty and Graduate School of Education, Gifu University, has elucidated how the sperm volume of male medaka fish found in southern Japan, adjusts in response to the presence and behavior of rival males.


                Spawning of the southern medaka fish

In this study, three experimental conditions were set up to observe the mating behavior of the fish in detail, and the amount of sperm produced during such behavior (see figure below).

              The female, focal male and rival male are represented in white, dark grey and pale grey, respectively

Condition 1: Direct interaction with rival male inside the same compartment.
Condition 2: Visual interaction with rival male (rival male in a different compartment).
Condition 3: No rival male.

In condition 1, two patterns of participation of the rival male in spawning were observed. The first pattern was simultaneous sperm release, in which the rival male joined the spawning pair and all three released eggs and sperm simultaneously. The second pattern was post-spawn sperm release, in which the rival male released his sperm toward the female on whose abdomen the eggs hung after she had finished spawning. In the first case of simultaneous sperm release, two other patterns were observed when the rival male interfered with the female’s spawning before releasing eggs and sperm. The first was a successful disruption by the rival male that resulted in the paired male and female being separated. While the second was an unsuccessful disruption in which the rival male did not separate the pair.

Studies of the sperm volume of the males in each pattern revealed that the focal male did not change his sperm volume when the rival male was a visual cue or even when the males were in direct contact with each other. It only changed when there was simultaneous sperm release after the rival male successfully disrupted.

Previously, male fish, including the medaka, were thought to visually detect the presence of rivals and allocate their sperm volume, but the results of this study show that the medaka is more flexible than expected in allocating sperm volume depending on the situation. Future studies should investigate whether males allocate the amount of sperm released differently depending on the situations. Because the medaka is a potential model for studying sperm allocation during external fertilization, it is expected that the physiological, genetic, and cognitive mechanisms involved in sperm allocation during fertilization strategies can be elucidated in the future.

The results of this research were published online June 10, 2023 in the international journal Animal Behavior.


This study was funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI (22K20666 to Y. Kondo.,20K20630 to M. K. and 17K19518 to S. A.) and an OCU Strategic

Paper Information

Journal Animal Behaviour
Title Sperm allocation in relation to male–male aggression and courtship in an externally fertilizing fish, the medaka

Yuki Kondo, Masanori Kohda, Yasunori Koya, Satoshi Awata


Reference: Laboratory of Animal Sociology, Osaka Metropolitan University Web site (Japanese)

Research contact

Graduate School of Science, Osaka Metropolitan University
Professor Satoshi Awata
TEL: 06-6605-2607
E-mail: sa-awata [at]

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Media contact

Public Relations Division, Osaka Metropolitan University
TEL: 06-6605-3411
E-mail: koho-list[at]

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