Exploring evolutionary drivers of parental care and family life in burying beetles of the genus Nicrophorus

According to current evolutionary theory, the evolution of parental care is a crucial driver for the evolution of social structures and family life. Parental care, which has evolved independently across numerous animal taxa, can take different forms, such as nest site selection and supporting the offspring’s food intake. However, environmental conditions and intrafamilial interactions can influence the evolution of care. This dissertation aims to shed light on the complex evolution of parental care and family life in the genus Nicrophorus, commonly known as burying beetles. The goal is to clarify the relationship between parental care, offspring dependence, and sibling cooperation. Burying beetles are ideal subjects for studying the evolution of family life because of their remarkable and elaborate parental care behavior. They form temporary family units by reproducing on small vertebrate carcasses, which they bury to supply offspring with food and protection within the carcass nest. The first part of this dissertation encompasses a common-garden experiment, employing a phylogenetically controlled approach to compare multiple Nicrophorus (Coleoptera: Staphylinidae: Silphinae: Nicrophorini) species with related, non-parental Silphini species (Coleoptera: Staphylinidae: Silphinae). Our results demonstrate that parental care in Nicrophorus allows offspring to grow faster than their non-parental relatives. We consider this enhanced growth an evolved outcome of parental provisioning and defense mechanisms. Notably, our results highlight differences in growth rate and offspring dependence on parental care across various burying species, which enhances our understanding about the drivers of parental care. In the second part of this dissertation, I investigated these interspecific differences in offspring dependence on parental care. First, my results confirm the importance of post-hatching care for the survival of the offspring. Second, by examining larval metabolic differences and interspecific variation in larval self-feeding abilities, I revealed the essential role of parental oral secretions in the context of post-hatching care. We found that an easily ingestible paste of carrion alone was not sufficient to ensure larval survival. However, the addition of parental oral secretions to the easily ingested diet of pureed carrion significantly increased the survival of highly dependent N. orbicollis larvae, but not to the level of larval survival with parental care. Burying beetles use these oral secretions not only to directly feed their offspring, but also to preserve the vertebrate carcass and form a specific area within the carcass, in which the offspring aggregate, known as the ‘feeding cavity’. Because the design of the experiment did not allow us to determine whether these parental- derived oral secretions were intended for direct transfer by parental regurgitation or whether they were intended to modify the feeding cavity, we decided to investigate the parental oral secretions more closely in the third part of this study. The results of the third part of my dissertation show that both direct and indirect care (food provisioning and carcass modification, respectively) are important, but that direct care, specifically parental regurgitation, has the greatest effect on larval survival. We propose that the oral secretions transferred from parental beetles to their offspring may contain enzymes or growth hormones, similar to what has been observed in some ant species. In the case of burying beetles, existing studies already showed that parental secretions contain beneficial microbes, potentially assisting the larvae in self-feeding from the carcass. In natural broods of burying beetles, the construction of the feeding cavity is not solely done by the parents. Our second study showed that burying beetle larvae are, to some extent, capable of self-feeding from the carcass. Thus, larvae might be able to manipulate the feeding cavity themselves, thereby increasing their ability to consume food. If this is true, then larvae should develop better if they grow up with siblings (i.e., with increasing brood size) and this effect would be stronger the more independent the offspring are from parental care. We tested these hypotheses and found that larvae benefit from developing with conspecific larvae and suggest that these benefits may be mediated by shared costs associated with digestion, social immunity-related factors, and/or thermal effects. In addition, the social environment was found to play an important role in this context. While all three tested species benefited from developing with conspecific larvae under harsh social environmental conditions (absence of pre-hatching parental care), only the most independent species benefited from developing with conspecific larvae under more favorable conditions, specifically when pre-hatching care was provided. In summary, this dissertation advances our knowledge of the complex facets of family life in burying beetles. I elucidated certain aspects of the relationships among parental care, offspring dependency, sibling interactions, and social environmental conditions. These findings enhance our understanding of the evolution and mechanisms of parental care and sociality in the animal kingdom.