Most animals reproduce sexually, meaning they need both males and females to create offspring. However, some species can reproduce without males through a process called parthenogenesis—where females give birth to young without fertilization. Even in these species, males sometimes appear. But do these rare males still serve a reproductive purpose?
A new study led by researchers from Japan suggests they do not. The team, including scientists from the National Institute for Basic Biology, Kobe University, and Fukushima University, focused on a species of stick insect called Ramulus mikado, found in Japan.
This species mainly reproduces without males, but on rare occasions, male stick insects appear. The researchers wanted to find out whether these males could still reproduce or if they had lost this ability over time.
Males Try to Mate But Do Not Pass on Genes
The researchers discovered that the rare males of R. mikado behaved like normal male insects in some ways. They actively sought out females and engaged in mating. However, when the scientists examined their offspring, they found something surprising—none of the male’s genes were passed down.
Further analysis showed that these males do not produce functional sperm. This means that although they mate, they cannot fertilize eggs or restore sexual reproduction in the species. In addition, female reproductive organs associated with sexual reproduction are shrinking over time, reinforcing the idea that this species has fully adapted to reproducing without males.
Finding Rare Males Was a Challenge
One of the biggest challenges in the study was locating male R. mikado in the wild. The researchers had been collecting stick insects from across Japan for years but had never encountered a male.
“In fact, we have never personally found a male R. mikado in nature,” said Professor Kenji Suetsugu from Kobe University. “To solve this problem, we worked with museums and citizen scientists and were able to collect seven males over four years.”
Once they had these rare males, the team observed their behavior and studied their body structures under a microscope.
Why Did Male Stick Insects Lose Their Ability to Reproduce?
One of the most surprising findings was that the male stick insects had completely lost their reproductive function. In most species, even when males are rare, they usually retain the ability to produce offspring. However, in R. mikado, something different happened—male reproductive traits disappeared altogether.
“Typically, it takes a very long time for male sexual traits to disappear,” said Associate Professor Shingo Kaneko from Fukushima University. “Yet in this species, males have lost their reproductive function entirely. This suggests that R. mikado has been reproducing asexually for so long that even random genetic mutations have removed male reproductive abilities.”
An Evolutionary Mystery
This discovery raises an interesting question: If asexual reproduction is considered less stable in the long run due to a lack of genetic diversity, how has R. mikado survived for so long?
Dr. Tomonari Nozaki from the National Institute for Basic Biology explained, “Our previous studies suggest that R. mikado has survived for hundreds of thousands of years through parthenogenesis.
This is unusual because asexual species are often considered to have a short evolutionary lifespan. Understanding how this species has persisted for so long remains an exciting puzzle for future research.”
This study sheds new light on how some species adapt over time. While most animals rely on sexual reproduction, R. mikado has found a way to thrive without it. The disappearance of male reproductive abilities suggests that this species has fully committed to an asexual future. However, the mystery of its long-term survival remains unsolved.
The research findings can be found in Ecology.
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