Dr. Alexander Walton, a Postdoctoral Researcher working collaboratively at Iowa State University and Cornell University, discusses with us his recent paper, “Resource limitation, intragroup aggression, and brain neuropeptide expression in a social wasp.”
Nourishment can affect behaviour in many ways, including social behaviours. In this study, my co-author and I explored the link between a social animal’s nutritional environment and how cooperative or aggressive they are to the other members of their social group. We investigated how nutrition regulates these behaviours in the paper wasp Polistes fuscatus. Like other well-known social insects (such as honeybees and ants), Polistine paper wasps form ‘feminine monarchies’ – where individuals live together on a shared nest with a reproductive division of labour (i.e., the queen lays her eggs and the other wasps provision, defend, and maintain the nest). However, the social structure of the paper wasp colony can be somewhat tenuous, as individuals may have the opportunity to move up in rank and even usurp the queen to maximise their own personal reproduction. Thus, paper wasps often exhibit constant overt aggression between colony members, and the degree of an individual’s behavioural dominance is directly related to their reproductive capacity (the size of her ovaries and her potential to develop eggs). As such, the group cohesion of a paper wasp nest is in a constant flux – a balance between cooperation and aggression. In this study we show that nutrition is a key regulator of this balance.
As social animals ourselves, we are familiar with a link between nourishment and behaviour. When we are hungry, we often become a lot more aggressive! We know from experience that a lack of nourishment can put a strain on the fragile cohesiveness of our social and familial groups, and lead to breakdown of cooperation. These personal, and seemingly universal, observations of ourselves, friends, and families, are backed up by theoretical evidence that nutritional limitation leads to increased aggression as individuals compete to secure dwindling resources. However, our findings in paper wasps were just the opposite! Nutritional restriction led to a decrease in aggression among nestmates. Moreover, this not a totally unique finding. Rather, these results add to a growing body of evidence that in social animals, resource limitation does not hinder, but promotes cooperation and cohesion. We suggest that this may be because limited nutrition can prevent an individual from investing in their own reproduction (via shunting resources to their own ovarian development or fat stores), and so they adopt a behavioural strategy that increases group reproduction (via increased cooperation and limited intra-colony aggression). This is likely especially true in family groups (such as a nest of paper wasps), where investment in the group has the potential to increase an individual’s inclusive fitness.
I first observed this relationship between social behaviours and nutritional restriction in honeybee workers (as reported in Functional Ecology: Walton, Bakken, Dolezal & Toth (2018), “Hungry for the queen: Honeybee nutritional environment affects worker pheromone response in a life stage-dependent manner”). When I challenged developing honeybee worker larvae with a brief period of starvation, they grew to be adults that behaved more cooperatively. However, when I challenged adults with nutritional restriction they behaved less cooperatively. So, the particular life stage that workers experienced nutritional restriction was key to how cooperatively they behaved, as the challenge had opposite effects on larvae and adults. My collaborators and I proposed that this difference in how life stages responded to nutritional restriction was regulated by the plasticity of an individual’s reproductive development. As larvae, honeybee ovaries are still developing, and ovarian size (as measured by the number of ovarian filaments known as ovarioles) was directly linked to nutritional input at this stage. Adults, however, have ovaries of a fixed size. No matter how much or how little nourishment they get at this life stage, they can’t invest those resources in their ovarian size. So, the best strategy is to invest those resources back into the colony. These investment strategies manifest as cooperative (or the lack of cooperative) behaviours. With these findings from honeybees, we were inspired to test the generality of this nourishment/cooperation relationship. Thus, we selected paper wasp workers, which evolved sociality independently of honeybees and (unlike honeybees), maintain reproductive flexibility as adults! Interestingly, the negative correlation between nourishment and cooperation we observed in honeybee larvae held true for paper wasp adults, corroborating our conclusion that reproductive plasticity is a key component to the regulation of social cohesion by nutritional restriction.
I am currently a postdoc working collaboratively at Iowa State University and Cornell University to develop functional genomics tools to study the behavioural biology of paper wasps. I have worked with (and plan to continue, indefinitely, to work with) ants, bees, and wasps to investigate the function and maintenance of individual behavioural differences in social insect colonies, the causes and consequences of phenotypic plasticity, and the evolution of cooperation.