Nutrional ecology in social insects

Abstract

Most living organisms must regulate their nutrient intake to survive and reproduce. This regulation is challenging because animals must manage the fluctuating demands of their own metabolism within the context of nutritionally heterogeneous environments. For social insects, the survival of the group relies on the efforts of only a small number of individual foragers. These foragers do not possess a direct knowledge of the colony’s nutritional state, yet they are able to accurately regulate their intake to meet the varying needs of their nestmates. To further our knowledge of the nutritional ecology of social insects, we need to understand the rules that foragers follow in order to maintain the collective nutrition of the group. Most advances in the field of collective nutrition come from the development of the Nutritional Geometric Framework (NGF). The NGF is a modelling platform that allows the integration of: the animal’s nutritional state, the optimal state it could reach, the foods available and the consequences of eating those foods. The present study combines the use of modelling and experiments implementing the NGF to explore how social insects utilise collective nutrition to fight pathogens and how specialist feeders meet their nutritional needs. Solitary species have been shown to alter their intake of nutrients to fight infections, but how would such a response be achievable on a collective scale? We adapted an existing individual based model of nutrition to investigate the impact of collective nutrient balancing on pathogen spread in a social insect colony. In our model, foragers not only altered their food collection according to their own infection status but also to the status of nestmates, and this social immunity strategy was highly beneficial to the colony when immune responses were short lived. Impaired foraging in infected workers favoured colony resilience when pathogen transmission rate was low (by reducing contact between colony members), or triggered colony collapse when transmission rates were fast (by depleting the pool of foragers). Our findings therefore suggest a new mechanism by which colonies could defend themselves against pathogens and provide a conceptual framework for experimental investigations of the nutritional immunology of social animals. For the rest of my PhD, we investigated the regulation of nutrition in groups of specialist feeders. We developed artificial diets and experimental setups to run the first NGF study on termite macronutrient regulation. We confined termite groups to single diets with varying macronutrient compositions. Diet composition did not affect food intake, but impacted lifespan and foraging. This finding is in direct contrast observation of generalist insects studied thus far. The amount of carbohydrate eaten had a strong effect on lifespan, and foraging activity increased with global intake. We subsequently offered termites various food pairing with highly different protein:carbohydrate ratios. Foragers collected the same amount of food, regardless of protein type or group caste composition. These results validate a nutritional ecology theory predicting that animals specialised on an invariant food type would lose the ability to regulate nutrient composition and would instead only regulate the amount of food collected.