Males are worse mothers: Comparing care patterns in a facultatively caring beetle

During biparental pre- and post-hatching care, parents take on energy-consuming tasks for the offspring's benefit and further reduce their own individual costs by specializing in different care aspects. But how is biparental care evolutionarily stable when biparental care is facultative, that is, when offspring survival does not obligately rely on post-hatching care? We examine this phenomenon in the carrion-breeding beetle Nicrophorus vespilloides, whose facultative biparental care involves microbiome control of the carcass nursery through continued application of antimicrobial exudates, shielding offspring from adverse environmental conditions. While evidence suggests synergistic effects of biparental care in Nicrophorus, any adaptive benefits in terms of social immunity are unknown in this genus. We presented Nicrophorus adults with a microbial challenge while manipulating parental care patterns during the period of post-hatching care, investigating consequences in parent and offspring performance. We found that microbial environment and parental care pattern influence larval development and survival. Additionally, we show for the first time that both factors affect personal immunity response in Nicrophorus offspring, responding to challenging conditions. Simultaneously, we show that biparentally caring beetles lose more weight during post-hatching care than uniparentally caring beetles, indicating higher investment and/or higher competition with mates or offspring. We present new evidence that burying beetle offspring adjust their personal immunity based on their microbial and social environment, and that biparental care may allow parents to sustain parental care under challenging conditions, raising further questions about the interplay of care patterns and the microbial environment on immune-regulatory and developmental processes in offspring.