AIMS:

Social interactions within animal families are ideal for analysing the evolutionary tension between cooperation and conflict, and how indirect genetic effects contribute transitions between them.   We used experimental evolution to investigate how the provision of parental care influences social interactions among siblings. We established experimental populations of burying beetles in the lab from wild populations, in which individuals show substantial variation in the level of care they provide for their larvae. We then manipulated burying beetle families so that some populations routinely experienced parental care at each generation, while others routinely experienced no post-hatching care. After more than 20 generations of experimental evolution, we investigated whether these sustained manipulations of parental care had changed the social interactions among larvae. We also determined how the indirect genetic effects of these sibling interactions contributed to evolutionary change.   We found that in populations with parental care, offspring were competitive and negatively influenced each other's growth (psi was reciprocally negative). Parental care permitted sibling rivalry because it compensated for the negative effects of sibling conflict on larval growth. In populations without parental care, however, the negative consequences of sibling rivalry were fully exposed and this favoured greater levels of larval cooperation. Sibling cooperation caused larvae to positively influence each other's growth (psi was reciprocally positive), and high levels of larval cooperation then evolved very rapidly.   Our experiments show that the extent of cooperation (or conflict) among social partners is contingent on other social interactions. These contingent interactions influence the fitness benefits and costs associated with the focal social behaviour, and can tip the optimal balance to yield conflict or cooperation. Furthermore, the pace of the evolutionary transition to conflict, or to cooperation, is accelerated by indirect genetic effects.