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 trials. 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.