Parental care in Canthon cyanellus cyanellus maintains brood ball moisture and shapes bacterial communities: implications for offspring survival.

Parental care in Canthon cyanellus cyanellus (Coleoptera: Scarabaeidae) is essential for offspring survival, yet the mechanisms underlying its protective role remain incompletely understood. While microbial transfer from parents to brood balls has been proposed as the primary explanation for the reduced fungal growth and increased larval survival observed under parental care, physicochemical maintenance of the brood balls may also play a critical role. In this study, we evaluated the effect of parental care on the physicochemical properties (gravimetric water content, pH, and hardness) and bacterial community assemblage of brood balls under four levels of relative humidity (RH: 30%, 50%, 65%, and 70%), and assessed larval survival and development under each condition. Our results demonstrate that parental care significantly contributed to maintaining the moisture and structural integrity of brood balls and that these effects were strongly dependent on ambient RH. High RH (≥ 65%) was identified as a critical factor for larval survival and progression to advanced developmental stages, regardless of parental care, whereas at RH ≤ 50% larval mortality reached 100% even in the presence of parents. Parental care also produced a slightly more alkaline pH in brood balls across most treatments. Metataxonomic analysis of the 16S rRNA gene (V3-V4 region) showed that parental care significantly influenced the bacterial assemblage on the brood ball surface, increasing alpha diversity and favoring genera associated with organic matter degradation and antimicrobial activity, such as Nocardioides, Bacillus, and Nannocystis. A bacterial core composed of eight genera-including Acinetobacter, Sphingobacterium, and Micromonospora-was identified across all brood balls. Based on these findings, we propose that parental care in C. c. cyanellus enhances offspring survival primarily by preventing brood ball desiccation, thereby preserving the structural plasticity necessary for larval migration to the food chamber while simultaneously promoting a beneficial bacterial community. These results also suggest that environmental shifts toward drier conditions, such as those driven by climate change or deforestation, could significantly compromise the reproductive success of this species.