The early evolution of family life

Family living is a key life history characteristic of many species throughout the animal kingdom and is a fundamental early step in the 'major evolutionary transition' to eusociality. To date, most research on family living has focussed on (1) the role of parent-offspring interactions in the maintenance of family groups, and (2), studied such interactions in highly complex social systems that are often found in birds, mammals and eusocial insects. As a consequence, there is currently an incomplete understanding of the processes responsible for the evolutionary emergence and maintenance of simple family groups that form the platform for the evolution of more complex social systems. My thesis uses a family-living lizard, Liopholis whitii, that displays long-term, stable male-female pair bonds and prolonged parent-offspring associations to provide insights into this knowledge gap. Specifically, I quantify the extent of conflict between different dyads within families (i.e., compare levels of sexual, parent-offspring and sibling conflict), and experimentally show how female mating patterns and food availability influence the extent of conflict between different family dyads and the consequences for family structure. I show (1) that conflict is high between monogamous pairs, fathers and offspring, and between siblings, but virtually non-existent between mothers and offspring, (2) that conflict is increased between monogamous pairs when females undertake extra-pair matings, and (3) that there is no effect of food availability or sibling relatedness on sibling conflict. Combined with previous research in this species that demonstrates high paternal aggression towards extra-pair offspring, these results suggest that genetic monogamy, but not food availability, is crucial to stabilising family life in L. whitii, by reducing conflict between monogamous pairs and between fathers and offspring. I complemented these experiments on L. whitii with an investigation of how genetic relatedness and food availability shape sibling conflict in another facultatively family living species, the burying beetle Nicrophorus vespillloides. In contrast to the lizards, I found no effect of genetic relatedness, but an effect of food availability, on the level of begging between siblings. Finally, I investigated how parental effects mediate conflict between family members in a more indirect and complex manner. Specifically, I used a meta-analytical and comparative approach across bird species to examine the consequences of a parentally controlled trait, asynchronous hatching of offspring, for the fitness of different family members. I find evidence that hatching asynchrony benefits parents by reducing their parental effort, but at a cost to last hatched offspring. Overall, my thesis highlights that examining all dyadic relationships within simple family groups and incorporating parental effects is crucial for research that aims to build a detailed and holistic picture of the early evolution of family life, and subsequent evolution of more complex sociality.