Some shield bugs in the family Acanthosomatidae lay egg masses on leaves, with female parents shielding them until they hatch. A research group from Naruto University of Education and other institutions has revealed that the eggs have evolved to become elongated in shape to efficiently protect them from natural enemies during this process. The peripheral areas of egg masses have a high risk of predation, and elongated shapes are easier to defend than perfect spheres.
Research on egg shape has attracted attention in recent years, but studies examining the relationship with post-laying parental care have been limited to birds. The research group, led by Associate Professor Shinichi Kudo, who studies behavioral ecology and evolutionary ecology at the Graduate School of Education at Naruto University of Education, researched the hypothesis that, since egg shape has important implications for reproductive behavior and ecology not only in birds but also in fish and reptiles, similar changes related to these factors might be occurring in shield bugs as well. Kudo particularly hypothesized that elongated eggs would be advantageous for reducing the surface area of egg masses while maintaining egg number and size, and conducted fieldwork based on this hypothesis.
Provided by Naruto University of Education
The study focused on shield bugs, many of which inhabit forests, with some species having the unusual characteristic of females caring for their eggs. These eggs are easily targeted by small arthropods such as ants and spiders. Since shield bugs inhabit all of Japan, the researchers spent 30 years searching for shield bugs from Hokkaido to Okinawa and continued to examine the aspect ratio of their eggs, which is the ratio of egg length to width.
Provided by Naruto University of Education
The results showed that among shield bugs, species where females protect egg masses by embracing them had more elongated eggs compared with species that do not provide such protection. Furthermore, using the phylogenetic tree of the family Acanthosomatidae, the researchers revealed the evolutionary process through which eggs became increasingly elongated after the evolution of maternal protection.
Provided by Naruto University of Education
Kudo's research team had previously revealed that offspring protection in the family Acanthosomatidae evolved in association with "small eggs" and "egg masses containing numerous eggs." These results differ significantly from conventional theories regarding the evolution of parental care. Kudo spoke about his future prospects: "Insects that provide parental care for eggs are few in number. I want to clarify the general principles of how and under what conditions parental care evolved through insect research."
The research was conducted in collaboration with SOKENDAI (The Graduate University for Advanced Studies) and Hokkaido University, with funding from the Japan Society for the Promotion of Science's Grants-in-Aid for Scientific Research program, the Fujiwara Natural History Foundation, and Taiwan's National Science and Technology Council. The results were published online in the British scientific journal Biological Journal of the Linnean Society on May 27 and announced by Naruto University of Education on May 30.
Original article was provided by the Science Portal and has been translated by Science Japan.