Parental and offspring origins independently influence larval growth in body-size-divergent populations of a burying beetle

Phenotypic divergence among populations can arise not only from intrinsic differences among individuals but also from variation in the social environments shaping development. In species with parental care, offspring growth depends on both parental and offspring traits, yet how these contributions are associated with geographic body-size divergence remains unclear. In natural populations of the burying beetle Nicrophorus quadripunctatus , adults from Tokyo are larger than those from Hokkaido. To investigate the basis of this geographic body-size divergence, we examined descendants of both populations reared under common laboratory conditions. Without manipulating brood size, Tokyo-origin adults had approximately 12% greater pronotum width than Hokkaido-origin adults. Among size-selected mothers, Tokyo females produced eggs approximately 5% larger in volume at a given maternal body size, whereas egg number and hatchling head width showed no population differences after adjusting for their respective predictors. The number of larvae retained for rearing was also similar. We then reciprocally cross-fostered one larva from each population together with an unrelated mother of either origin. Among broods in which both larvae survived, Tokyo-origin larvae were approximately 10% heavier than Hokkaido-origin larvae raised by the same foster mother. Larvae also attained greater mass with Tokyo foster mothers, independently of their own origin, with no significant interaction. The relationship between larval mass at 120 h and adult pronotum width also differed between populations. These findings support an inherited component to geographic body-size divergence and reveal population differences in maternal investment, caregiver-associated effects, offspring-associated growth, and subsequent developmental relationships.