Social organization, development, and functional neuroplasticity in the ant genus Pheidole

The degree of intra- and interspecific behavioral diversity in the hyperdiverse ant genus Pheidole is striking. Workers undergo age-related behavioral development, adding tasks to their behavioral repertoire with age; smaller minor workers attend to a wide range of tasks while larger major workers are typically behaviorally specialized; and species differ markedly in traits such as worker aggressiveness and major worker behavioral plasticity. This dissertation explores how age, experience, morphology, neuroanatomy, and neurochemistry are associated with behavioral variation among workers and provides insight into the factors underlying the remarkable sociobiological diversity of Pheidole. I first investigated age-related behavioral development in P. dentata minor workers by assaying their ability to care for developing young, a task set on which young workers are traditionally thought to specialize. Old minors were significantly more efficient brood nurses than young minors, suggesting workers actually develop increased task proficiencies with age. Pheidole dentata minors reared to equivalent age in colonies with or without brood had similar neural characteristics and did not attend to tasks differently when later exposed to brood during nursing assays, suggesting experience with brood is not required for most age-related developmental changes. I next investigated the effect of one component of neural development, increasing brain serotonin content, on an important facet of foraging behavior: pheromone trail following. Serotonin-depleted workers showed reduced trail following behavior, supporting a neuromodulatory role for serotonin in worker behavioral development and task sensitivity. Different rates of muscle development in P. dentata minors appear to partially underlie their behavioral maturation: mandible closer muscles undergo pronounced growth after adult eclosion, likely increasing the ability of older workers to manipulate, bite, and lift objects. Finally, I performed a broad-scale analysis of neuroanatomical variation among Pheidole worker groups that naturally differ in task performance (young and old major and minor workers of P. dentata, P. morrisi, and P. pilifera). Brain structure differed dramatically among age and size groups and between species. This variation was in addition to differences in brain size, and was correlated with worker behavioral specialization, suggesting intra- and interspecific variation in brain structure partially underlies adaptive behavioral differences among Pheidole workers.