Positional Intraclutch Variation in Egg Shape Associated with Maternal Guarding in a Shield Bug

Despite recent advancements in the understanding of egg shape adaptation, evidence for adaptive intraclutch variation in egg shape is still lacking. In subsocial true bugs, females lay eggs in a single layer and guard them from predators by covering the clutch with their bodies. Previous studies showed that females of some species, including Sastragala esakii, lay heavier eggs at the clutch center compared with those at the periphery, suggesting greater maternal investment in more protected central eggs. In this study, we investigated whether egg shape exhibits positional intraclutch variation in S. esakii and found that central eggs were more elongated while maintaining a width comparable to that of peripheral eggs. This positional variation in egg shape may allow females to compactly pack the clutch within the limited protective space beneath their bodies, minimizing the number of vulnerable peripheral eggs while allocating more resources to central eggs.