To be a father is costly: survival costs of paternal care in Appasus japonicus (Hemiptera Belostomatidae)

Parental care is widespread, but exclusive paternal care is rare in the animal kingdom. To understand the evolution of paternal care, it is important to evaluate the costs and benefits of this behaviour. In the family Belostomatidae (Hemiptera), males care for their eggs until they hatch. In this family, it has been revealed that egg-caring behaviour increases offspring survival and mating opportunities of egg-caring males through female mate choice, while it decreases the swimming and feeding abilities of the males as well as longevity. The behaviour would also decrease the survival rate due to predation. Although few previous studies have conducted field surveys, survival costs were detected in one study but not in another. In this study, we focused on the influence of field survey periods on the detection of egg-caring costs during the entire breeding season in the giant water bug, Appasus japonicus. Using a multi-state modelling approach, we found that the estimated survival rate was lower in egg-caring males compared to non-caring males and females. Furthermore, when we performed similar analyses during different periods (early and peak breeding season, peak breeding season, and peak and late breeding season) separately, the difference in survival rates was not clearly observed in the analyses of the former two periods, although it was still detected in the later period. Our findings suggest that paternal care in A. japonicus could incur survival costs and indicate the importance of survey timing and duration in assessments of parental costs.