The social environment and the evolution of morphology in the burying beetle, Nicrophorus vespilloides
Social interactions within species are ubiquitous in nature, and cause animals to exist in a social environment. In this thesis, I investigate how the social environment can influence morphological evolution, using a combination of observation and experiments on burying beetles Nicrophorus vespilloides. This species exhibits a set of social interactions at all life stages that centre around the acquisition and use of small carrion during reproduction. Larvae grow and develop on an edible carrion nest fashioned from a corpse by their parents. Here they interact with siblings and caring parents as they acquire resources from the carrion to grow and develop. Later, as sexually mature adults, they themselves compete with conspecifics of their own sex for carrion to breed upon. I investigate how these various social interactions influence morphology at these different lifestages.
I start by investigating how the social environment influences morphological evolution of offspring, by collecting data from replicate experimental populations that had been evolving for 39 generations in different social environments when I began work. In two populations, parents were able to supply care (‘Full Care’ populations), whereas in two other populations parents were prevented from supplying any post-hatching care (‘No Care’ populations). In Chapter 2, I show that hatchling morphology evolves divergently in these two different social environments. In the Full Care populations, first-hatched larvae have relatively larger head morphology and body size compared with last-hatched larvae. In the No Care populations, by contrast, first- and last-hatched larvae are significantly more uniform in their head morphology and body size throughout the entire brood. In Chapter 3, I show that third instar larval morphology also diverges between the ‘Full Care’ and ‘No Care’ environments. In general, larvae from the Full Care populations have disproportionately larger heads, compared to larvae from the No Care populations. In each case, I suggest that this divergence in the evolution of larval morphology between populations is due to the selection pressures of sibling competition that materialises only in the presence of parents.
Next, I investigate how the social environment contributes to morphological change in adult beetles. In Chapter 4, I discover a new form of sexual dimorphism in burying beetles: male burying beetles exhibit disproportionately larger heads than females. I show that this head morphology is directly linked to biting performance, and that beetles with larger heads exert a greater bite force. I then link this to the burying beetle’s natural history and show that head size predicts the outcome of contests over carrion, in both males and females.
Finally, in Chapter 5, I investigate how head morphology functions in the preparation of the carrion nest. Males typically invest more than females in nest preparation, though females can prepare a nest singlehandedly if widowed. I use an experimental approach in the laboratory to investigate how parental division of labour influences the evolution of adult morphology; taking advantage of ongoing experimental evolution in a different set of experimental populations, in which females were induced to prepare a carrion nest without any help from the male. I show that females evolve a relatively larger head in response to this additional parental responsibility, whereas males evolve smaller head morphology. In Chapter 6, I discuss how these different results together show that key morphological traits do evolve in response to divergent social environments, whilst highlighting the role of constraints in morphological evolution.
Biotechnology and Biological Sciences Research Council (1943273)