A research group comprising Associate Professor Shin-ya Ohba of Nagasaki University, Assistant Professor Tomoya Suzuki of Hiroshima Shudo University (affiliated with Shinshu University at the time of the study), and Professor Koji Tojo of Shinshu University has revealed a previously unknown and remarkable aspect of parental care behavior in the giant water bug, an insect known for its “paternal parenting.”

The giant water bug is a unique insect species in which males carry eggs on their backs and independently care for them until hatching. Male-only parental care is rare across the animal kingdom and is particularly uncommon among insects. Previous research by Associate Professor Ohba revealed that females are more likely to select males engaged in egg care as mates, suggesting that, in giant water bugs, dedicated paternal care enhances male reproductive attractiveness.

Research Highlight

---

• The egg mass carried on the back of a male giant water bug contains eggs laid by multiple females.
• Male giant water bugs also care for eggs that were fertilized by other males.
• Some males do not care for their own eggs; instead, they are left to be tended by other males while still successfully passing on their genes.

Research Abstract

---

Reproductive strategies are crucial for organisms because they directly affect the organisms’ fitness. “Parental care” is one of the strategies adopted by organisms to improve their fitness. However, even in the case of parental care, females often bear a large burden in raising offspring, and it is extremely rare for males to care for offspring alone (“paternal care”), especially among invertebrates. Offspring care results in fewer re-mating opportunities, resulting in a greater reproductive cost for males. Under such conditions, paternal care has evolved in males of several animal taxa. However, there is a lack of clarity on how paternal care behavior has evolved and how it is maintained. In this study, we allowed males and females of Appasus japonicus, a typical paternal care-exhibiting insect, commonly known as a giant water bug, to mate freely in the laboratory. We also used sensitive molecular markers that we developed for the giant water bug to conduct paternity testing trails. The results of these trials showed unexpectedly low actual paternity rates. Moreover, most males cared for other male’s eggs, often from multiple males, and “cuckoldry” was evident. These discoveries constitute a significant finding regarding the evolution of reproductive strategies in insects because the paternity rate of belostomatid insects had been considered to be high. We provide new insights that overturn the established theory of the evolution of paternal care in insects.

KEY WORDS: Belostomatidae, cuckoldry, maternity, mating trial, SSR marker

Journal Information

---

Journal: Ecology and Evolution

Title: Reproductive strategies in paternal care and remarkably low paternity level in a giant water bug

Authors: Tomoya Suzuki^1-2, Shin-ya Ohba³, Koji Tojo^1,4

¹Faculty of Science, Shinshu University, Matsumoto, Nagano, Japan

²Faculty of Human Environmental Studies, Hiroshima Shudo University, Hiroshima, Japan

³Biological Laboratory, Faculty of Education, Nagasaki University, Bunkyo, Nagasaki, Japan

⁴ Institute of Mountain Science, Shinshu University, Matsumoto, Nagano, Japan

Correspondence Authors: 

Tomoya Suzuki (ORCID: 0000-0002-0315-7500)

Koji Tojo (ORCID: 0000-0002-9362-604X)

DOI: 10.1002/ece3.71316

Publication Date: 23 April 2025

For details on this research, please refer to the journal below.

https://onlinelibrary.wiley.com/doi/10.1002/ece3.71316