Causes and Consequences of Intraspecific Variation in Behavior of the Red Imported Fire Ant

Abstract

Organisms vary at the individual and population level in many ecologically relevant traits. This study documents and quantifies colony-level variation in ecologically important behaviors of a widespread invasive social insect, demonstrates multitrophic ecological effects of this colony-level variation, and explores genetic factors that may affect and predict behavior at the colony-level.

I quantified significant, persistent regional and colony-level variation in the red imported fire ant (Solenopsis invicta) in behaviors such as extra-nest activity, exploration, and resource discovery speed and recruitment effort. Colony behavior correlated with both colony productivity and colony growth. Using single-lineage colonies, I estimated broad-sense heritability of between 0.45 and 0.5 for the observed colony behaviors.

I created experimental microcosms comprised of fire ant colonies, plants, and insect herbivores. Differences in fire ant colony behavior linked to carbohydrate attraction directly impacted herbivore mortality and indirectly impacted plant damage.

I quantified colony differences colony differences in the expression of the fire ant foraging gene (sifor) as well as colony-level differences in behavior for fire ant colonies collected from across a large area of Texas. Expression of sifor was more than three-fold higher in fire ant foragers than in fire ant workers in the interior of the nest, and colony-level differences in sifor expression of foragers and interior workers correlated with colony behavior. Higher sifor expression in foragers correlated with higher foraging activity, exploratory activity, and recruitment to nectar in fire ant colonies.

Finally, I explored the hypothesis that fire ant foundress groups could maximize inclusive fitness benefits and alter cooperative and competitive behaviors in response to cues indicating higher relatedness of foundresses. I found that group and queen performance was significantly affected by group composition. Groups composed of foundresses that were less likely to be related produced no more workers than queens founding alone, while groups composed of foundresses from the same site produced the most workers of all group types.

The conclusions of this study have widespread implications for many social insects and their ecological interactions. By further exploring these effects at the mechanistic, organismal, and ecological level we will improve our understanding of collective behavior, social evolution, and intraspecific variation.