Nighttime Light Shrinks Insect Sperm — Nighttime Brightness, Unexperienced by Organisms in Evolution, Affects Reproduction —
A research team at Chiba University discovered that artificial light at night (streetlights, building lights, etc.) affects insect sperm, causing it to shrink. This effect is particularly pronounced with blue light. Urban-dwelling individuals show less impact, suggesting they may have acquired resistance to artificial light. This is a significant achievement that sheds new light on the impact of urbanization on living organisms.
📋 Article Processing Timeline
- 📰 Published: April 30, 2026 at 23:00
- 🔍 Collected: April 30, 2026 at 14:31
- 🤖 AI Analyzed: April 30, 2026 at 14:48 (16 min after Collected)
Mr. Ryushin Takamoto, a doctoral student in the Graduate School of Advanced Science and Engineering, Chiba University, and Professor Yuma Takahashi of the Graduate School of Science, Chiba University, have revealed that artificial light at night (such as streetlights and building lights) affects insect sperm.
In recent years, with advancing urbanization, artificial light has been increasing worldwide. Organisms living in and around cities are also exposed to "bright nights" due to artificial light, and this nighttime artificial light is pointed out as one of the causes of insect decline. This study found that weak nighttime light causes male Drosophila to produce smaller sperm, and that individuals living in urban areas are less affected by this. These results suggest that urban nighttime light environments can affect reproductive function, and furthermore, that evolution towards mitigating these effects may be occurring in urban populations. This is an important achievement that sheds new light on the impact of urbanization on living organisms.
The results of this research were published on April 22, 2026, in the international entomological journal Entomological Science.
(Paper here: 10.1111/ens.70018)
Figure: Comparison of sperm head size in Drosophila suzukii reared under various nighttime light environments with conditions without nighttime light.
■ Background of the Research
In recent years, urbanization has progressed globally, and bright environments continue to spread even at night. Such artificial light at night (ALAN) is an environment that organisms have not experienced in the course of evolution and is thought to affect the behavior and survival of many organisms. This effect is also pointed out as one of the causes of recent insect decline (Reference 1). Previous research has shown that artificial light at night can disrupt insect circadian rhythms and alter developmental timing, thereby affecting individual growth and behavior (Reference 2). On the other hand, the impact on reproductive function, especially on sperm production by males, has been largely unexplored. Sperm are important cells that transmit genetic information to the next generation, and their shape and size are deeply involved in fertilization success rates and reproductive efficiency. Therefore, changes in sperm are a critical issue that can affect the maintenance of populations. Against this background, elucidating the effects of nighttime light on sperm was sought.
■ Key Findings of the Research
1. Illumination with nighttime light from egg to adult: Drosophila suzukii were reared under conditions where weak LED light (white, blue, green, or red) was irradiated at night, and the sperm head size (Note 1) of the obtained male adults was compared.
2. Artificial light at night shrinks sperm: It was revealed that artificial light at night affects sperm head size, with a particularly significant reduction occurring with blue light (Figure). Furthermore, effects were confirmed even with red light, which is invisible to flies, suggesting the possibility that the effect of light occurs through pathways other than vision.
3. Urban individuals (Note 2) show less impact: It was confirmed that urban individuals showed smaller changes in sperm size due to artificial light at night (Figure). This result suggests that urban individuals have acquired resistance to artificial light.
■ Future Outlook (Researcher's Comment)
By focusing not only on light intensity but also on differences in wavelength, this research has the potential to lead to biologically friendly lighting design and improvement of urban environments, offering a new perspective that connects environmental issues with evolutionary problems. In the future, by clarifying how changes in sperm size actually affect fertilization success and reproductive efficiency, it is expected that we will be able to deeply understand the impact of urban environments on living organisms.
■ Terminology
Note 1) Sperm head size: The size of the tip part of the sperm. It is an important structure containing genetic information and is considered a crucial trait involved in fertilization success.
Note 2) Urban individuals (suburban individuals): In this study, strains created with individuals (females) captured in urban (or suburban) environments as founders were designated as urban (or suburban) strains and maintained under constant laboratory conditions. Individuals from these strains are referred to as urban individuals (or suburban individuals). Differences observed between urban and suburban strains are due to genetic differences (as they are reared under constant conditions, there are no differences attributable to the rearing environment), providing evidence of evolution in urban environments.
■ Paper Information
Title: The effects of spectrally distinct nighttime dim light on sperm production in Drosophila
Authors: Ryushin Takamoto and Yuma Takahashi
Journal: Entomological Science
DOI: 10.1111/ens.70018
■ Reference 1)
Press release published on August 27, 2025, "Light pollution disrupts insect circadian clocks and shortens lifespan."
Keywords:
In recent years, with advancing urbanization, artificial light has been increasing worldwide. Organisms living in and around cities are also exposed to "bright nights" due to artificial light, and this nighttime artificial light is pointed out as one of the causes of insect decline. This study found that weak nighttime light causes male Drosophila to produce smaller sperm, and that individuals living in urban areas are less affected by this. These results suggest that urban nighttime light environments can affect reproductive function, and furthermore, that evolution towards mitigating these effects may be occurring in urban populations. This is an important achievement that sheds new light on the impact of urbanization on living organisms.
The results of this research were published on April 22, 2026, in the international entomological journal Entomological Science.
(Paper here: 10.1111/ens.70018)
Figure: Comparison of sperm head size in Drosophila suzukii reared under various nighttime light environments with conditions without nighttime light.
■ Background of the Research
In recent years, urbanization has progressed globally, and bright environments continue to spread even at night. Such artificial light at night (ALAN) is an environment that organisms have not experienced in the course of evolution and is thought to affect the behavior and survival of many organisms. This effect is also pointed out as one of the causes of recent insect decline (Reference 1). Previous research has shown that artificial light at night can disrupt insect circadian rhythms and alter developmental timing, thereby affecting individual growth and behavior (Reference 2). On the other hand, the impact on reproductive function, especially on sperm production by males, has been largely unexplored. Sperm are important cells that transmit genetic information to the next generation, and their shape and size are deeply involved in fertilization success rates and reproductive efficiency. Therefore, changes in sperm are a critical issue that can affect the maintenance of populations. Against this background, elucidating the effects of nighttime light on sperm was sought.
■ Key Findings of the Research
1. Illumination with nighttime light from egg to adult: Drosophila suzukii were reared under conditions where weak LED light (white, blue, green, or red) was irradiated at night, and the sperm head size (Note 1) of the obtained male adults was compared.
2. Artificial light at night shrinks sperm: It was revealed that artificial light at night affects sperm head size, with a particularly significant reduction occurring with blue light (Figure). Furthermore, effects were confirmed even with red light, which is invisible to flies, suggesting the possibility that the effect of light occurs through pathways other than vision.
3. Urban individuals (Note 2) show less impact: It was confirmed that urban individuals showed smaller changes in sperm size due to artificial light at night (Figure). This result suggests that urban individuals have acquired resistance to artificial light.
■ Future Outlook (Researcher's Comment)
By focusing not only on light intensity but also on differences in wavelength, this research has the potential to lead to biologically friendly lighting design and improvement of urban environments, offering a new perspective that connects environmental issues with evolutionary problems. In the future, by clarifying how changes in sperm size actually affect fertilization success and reproductive efficiency, it is expected that we will be able to deeply understand the impact of urban environments on living organisms.
■ Terminology
Note 1) Sperm head size: The size of the tip part of the sperm. It is an important structure containing genetic information and is considered a crucial trait involved in fertilization success.
Note 2) Urban individuals (suburban individuals): In this study, strains created with individuals (females) captured in urban (or suburban) environments as founders were designated as urban (or suburban) strains and maintained under constant laboratory conditions. Individuals from these strains are referred to as urban individuals (or suburban individuals). Differences observed between urban and suburban strains are due to genetic differences (as they are reared under constant conditions, there are no differences attributable to the rearing environment), providing evidence of evolution in urban environments.
■ Paper Information
Title: The effects of spectrally distinct nighttime dim light on sperm production in Drosophila
Authors: Ryushin Takamoto and Yuma Takahashi
Journal: Entomological Science
DOI: 10.1111/ens.70018
■ Reference 1)
Press release published on August 27, 2025, "Light pollution disrupts insect circadian clocks and shortens lifespan."
Keywords: