Social complexity, diet, and brain evolution: modeling the effects of colony size, worker size, brain size, and foraging behavior on colony fitness in ants

Animals may face a tradeoff between enhanced cognitive ability and the cost of maintaining larger and more energetically expensive brains. In social species, this tradeoff could be influenced by energy-reducing benefits of cooperation and collective cognition. Greater social complexity in ants is hypothesized to be associated with smaller brain size, a pattern opposite that found in some social vertebrates. Ants vary in worker and colony size, and worker size-frequency distributions; larger workers have larger, more costly, brains. Colony-level foraging performance and its energetic consequence to fitness depend on individual cognitive capabilities as well as collective behaviors. To explore the impact of diet, behavioral ability, and social organization on brain evolution, we developed a model incorporating food availability, foraging behavior and related energetic gain, colony size, worker size and polymorphism, and brain size. Colonies could increase energy intake through foraging performance by increasing worker size, and by correlation, brain size and expanded task capability, or by increasing colony size and collective foraging. Results show that resource-poor environments favored small colonies of relatively large-bodied and thus large-brained pluripotent workers that had higher energetic costs. In contrast, large colonies of relatively small-bodied and small-brained workers in resource-rich environments harvested food with lower brain investment and hence decreased metabolic costs. Worker size-related behavioral specialization and covarying brain investment also influenced fitness. The energetic advantage of polymorphism depended on the degree of association of the navigational abilities employed during foraging and worker brain size. Our study suggests that diet, colony size, and worker size may influence tradeoffs between individual and collective cognition and the evolution of brain size and division of labor in eusocial insects such as ants.