Multilevel selection leads to divergent coadaptation of care-giving parents during pre-hatching parental care

Parental care often involves coordination between the mothers and fathers. Their coordination is partly due to flexible "negotiation rules" that allow mothers and fathers to respond adaptively to each other in real time, but could also be due to divergent environmental conditions that cause different negotiation rules to evolve in different populations through coadaptation. We evolved experimental populations of burying beetles Nicrophorus vespilloides in different environments and tested for evidence that the sexes had coadapted to these divergent environments. In two populations, parents supplied pre- and post-hatching care (Full Care environment), while in two other populations, parents supplied only pre-hatching care (No Care environment). We have previously shown that parents in the No Care populations rapidly evolved superior pre-hatching care. Here we show that this is due to the coadaptation of the traits expressed by males and females while they convert carrion into an edible nest for larvae. After 15 generations of experimental evolution, we created heterotypic pairs (No Care females with Full Care males, and No Care males with Full Care females) and found they were less effective at making a carrion nest than homotypic No Care pairs-with negative consequences for brood performance. We suggest that the coadaptation of the sexes is the result of selection acting on pairs in different ways in the No Care versus Full Care environments. We discuss how social co-adaptations within cooperating pairs or teams of individuals could act as post-mating barriers to gene flow. ### Competing Interest Statement The authors have declared no competing interest.