Abstract
In behavioural ecology, aggression is comprised of a suite of agonistic behaviours displayed by animals during confrontations, which may or not involve direct fights between groups or individuals. Aggression serves a range of ecological functions, including competing for resources, subjugating prey, self-preservation, and brood defence. Aggressive behaviours are near ubiquitous across animal taxa and have been theorized to play a major role in biological evolution.
Social Hymenoptera (ants, social bees, and social wasps) are renowned for their expressive aggression behaviours, especially in the context of nest defence. Being able to defend their brood against predators is essential to ensure colony survival and reproduction, and aggression has been suggested as a main trait underlying the ecological success of social insects and the evolution of eusociality. Many mechanisms have been suggested to help drive defensive aggression in the social Hymenoptera, including abiotic, social, and molecular factors.
In this thesis, I studied the nest defence behaviour of Vespula vulgaris. This social wasp, an introduced pest species in New Zealand, is an ideal model to study mechanisms underlying aggression behaviours due to its dense populations, quantifiable response to simulated predator attacks, and variation in aggression phenotype.
I start with a systemic review of worldwide predation pressures on social wasp colonies. I describe the taxonomic diversity of predators of wasp individuals and colonies, and how they vary across different social wasp taxa in different areas of the world. Based on my findings, I draw conclusions on how predator-prey interactions are shaped by their behavioural ecology, and make inferences on how these relationships might have developed over the groups’ evolutionary history.
I then investigated how V. vulgaris nest defence varies according to age and experience over time at the colony level in a field setting. I found that colony aggression fluctuates over the colony cycle, but provide no evidence towards the effect of habituation or sensitization over time as colonies experience continuous simulated predator attacks. I relate these findings to shifts in the colonies’ physiological age, interpreted in the context of natural demographic changes happening over the course of the colony’s seasonal cycle. I also provide insights on the absence of abiotic effects on colony-level aggression.
Finally, I investigated individual brain transcriptome differences between nest defenders and foragers, aiming to identify genomic bases for aggressive behaviour variation within and across colonies. I found that while colonies have differing aggressive phenotypes and distinct transcriptomic patterns, very few genes are differentially expressed between behavioural castes within colonies. I briefly discuss the biological significance of these genes of interest, and provide ideas for further investigating the study of aggression at the molecular level.
Overall, this thesis contributes to the advancement of the field of behavioural ecology by providing evidence on different mechanisms that may play different roles at determining consistent behavioural variation in a model study system. Although there are many challenges and limitations for field-based studies, such as sub-optimally standardized experiments and snapshot phenotypical sampling, by investigating ecological and molecular influences on aggression behaviours in V. vulgaris I add to the current knowledge on how behavioural variation has shaped the evolution of eusociality in Hymenoptera.