The Effects of Colony Size and Social Density on Individual and Group Level Behavior and Energetics in Ants

Abstract

Social insects are used as models for understanding the evolution of sociality because they show seemingly complex behavioral and physiological traits that enforce group cohesion, collective organization, and group level reproduction. Social organization in insect societies requires workers to share information. Information sharing allows workers to efficiently perform and switch among tasks to meet colony needs. For many species that nest in preformed cavities, colony growth results in crowding inside the nest which can affect colony productivity and fitness. How does colony size and social density affect individual and collective behavior? Using a combination of laboratory and field experiments, I have begun to answer this question. In Temnothorax rugatulus ants, high social density resulted in greater colony energy use. In addition, larger colonies used proportionally less energy compared to smaller colonies, but showed reduced brood production. These results indicate that the way colonies use energy changes with social density and group size. In analyzing the effects of colony size and density on worker behavior, I found that high density increased worker connectivity and information sharing. Workers in larger colonies showed less connectivity compared to workers in smaller colonies. Interestingly, workers with more interactions spent less time in brood care. This study shows that workers' access to information and the overall pattern of information flow are affected by social density and colony size, and changes in worker connectivity can influence task behavior. The next study shows that field colonies maintained a relatively constant level of intranidal density irrespective of colony size; this suggests that Temnothorax ants actively regulate social density. When colonies were established in high density nests, they showed greater foraging and scouting activities, and this led to a higher probability for becoming polydomous, i.e., occupying multiple nests. When polydomy occurred, colonies divided evenly between two nests, but distributed fewer, heavier workers and brood to the supplemental nests. Taken together, the first four studies indicate that social density is an important colony phenotype that affects individual and collective behavior and energetics in ants, and the collective management of social density may be a group adaptation in ants and other social insects. Lastly, because crowding affects polydomy behavior, the final two experiments tested whether colony emigration and nest construction and dispersion, two strategies for reducing intranidal crowding, are influenced by food distribution. Temnothorax colonies preferred to emigrate to nests positioned closer to food, and weaver ants (Oecophylla smaragdina) positioned newly constructed nests in food-rich areas. Furthermore, weaver ants used the newly constructed nests to more rapidly retrieve and safeguard valuable food items. Thus, strategic emigrations and adaptive nest dispersion can remedy intranidal crowding and at the same time allow growing colonies to acquire adequate food to meet colony needs.