Parental Social Environment Has no Effect on Offspring Development in the Dung Beetle: A Test of Adult Sex Ratio Effects

The adult sex ratio (ASR) is a key demographic parameter that shapes sexual selection and social interactions. While ASR variation drives profound behavioral plasticity within a generation, it remains unclear whether parental experience of skewed ASRs influences offspring development via transgenerational plasticity (TGP), and if so, which of the two core nongenetic pathways: pre-zygotic germline-mediated information transfer and post-zygotic parental investment, predominates. To disentangle these mechanistically distinct pathways, we conducted a controlled egg-transplantation experiment in the dung beetle Onthophagus taurus. We exposed parental beetles to female-biased, unbiased, and male-biased social environments, first confirming that our ASR manipulations generated the predicted gradients of social stress by quantifying contest and courtship behaviors. We then transplanted eggs from naturally produced brood balls across different ASR treatments into standardized artificial brood balls to strictly isolate germline-mediated TGP effects, while weighing original dried brood ball mass to independently assess parental investment. Our results revealed a striking dissociation between parental behavioral responses and intergenerational outcomes. ASR strongly modulated adult social interactions: male-biased treatments exhibited the highest contest intensity, whereas female-biased treatments displayed significantly higher courtship frequency than both unbiased and male-biased groups. Despite these pronounced parental adjustments, parental ASR experience had no significant effect on any measured offspring developmental trait, including developmental speed, emergence weight, and stage-specific developmental duration. Concurrently, parental investment (dried brood ball mass) did not differ across ASR treatments. These findings demonstrate that ASR-induced social pressures do not propagate to offspring metamorphic development via either core TGP pathway within a single generation, suggesting that offspring development is strongly canalized against parental social fluctuations. Future research should disentangle cryptic non-nutritional transmission pathways (e.g., microbiome inoculation) and employ gametic epigenetic assays to determine if social experiences leave molecular traces under alternative ecological contexts or longer evolutionary timescales.