Influence of Demography, Development and Death on Seasonal Labor Allocation in the Florida Harvester Ant (Pogonomyrmex Badius)

Eusocial insect societies are analogous to organisms in that the demography, development and regulation of workers within are shaped by selection acting on whole colony characteristics. Just as relative investment varies across the lifetime and reproductive cycle of a traditional organism, adaptive patterns of worker allocation are expected to vary with colony development and need across each annual cycle. Despite these predictions, adaptive patterns of labor allocation remain un-described for most social insect societies. This dissertation identifies a seasonal pattern of forager allocation in colonies of the Florida harvester ant (Pogonomyrmex badius) and describes its relationship to colony demography, size, reproduction, worker development rate, death rate, longevity, and neighborhood dynamics. Aging P. badius workers progress through a sequence of interior labor roles before leaving the nest to forage. By marking and recapturing foragers, forager population size was estimated and foragers were identified as a discrete, age-correlated labor group that resides only in the top 12cm of nests that may be more than 200cm deep. Excavation and census of whole colonies revealed that foragers were present in a consistent ratio to the colony's larval population from May through August, but that forager allocation was not a response to larval presence. Proportional allocation to foraging followed an annual pattern, shaped by the interaction of seasonal phases of colony growth and worker development rate. Forager allocation began in March or April and increased to a peak of approximately 40% of the colony in June, as colonies provisioned alates for mating flights in the days surrounding the summer solstice. In spring, proportion foraging increased due to an increase in forager number combined with a reduction in colony size. Beginning in late summer, proportional allocation to foraging decreased, as colonies grew through new worker birth and forager replacement declined. This annual pattern was shaped by a five-fold difference in the age of summer and autumn-born workers when they entered the forager population (43 vs. 200+ days). The chronological age of foragers was revealed by collecting whole colonies across two annual cycles, marking age cohorts with colored wire-belts, releasing each colony into a field nest created from melting buried ice chambers, then monitoring the forager population for the appearance of each marked cohort. Slow-developing workers, produced from late August until mid-October each year, dominated the forager population the following March through mid-July; while fast developing workers appeared in early June and developed rapidly to become foragers the following month, overlapping with their older sisters. While wild foragers of both types lived an average maximum of 27 days after entering the forager population, these same foragers were capable of surviving for hundreds of days in the laboratory. Likewise, restricting the foraging range of wild foragers increased forager longevity by 57%, demonstrating that foraging carries mortality risks and the observed age at death was not part of a developmental program involving senescence in P. badius. By removing neighboring colonies, this study also showed that interactions with conspecific neighbors can influence the labor thresholds of individual workers, and the demographic structure of whole colonies, as neighbors account for 30% of forager mortality in the spring. At the colony level, increased forager longevity suppressed the movement of new workers into the forager population, increasing their time in earlier labor roles and promoting colony growth. In contrast, both removing 50% of the forager population and doubling the larval population did not induce forager replacement or increase the daily rate of new foragers added within seven days. Together, these results suggest a unidirectional control of labor allocation in P. badius, where the forager population size is not maintained by workers detecting colony need and filling vacancies, but by workers developing at a rate selected to allow forager replacement. In essence, the annual cycle of forager allocation emerges as P. badius workers 'age' into behavioral roles at environmentally appropriate times, in the same proportions, on nearly the same dates each year and experience a predictable death rate. This process allows colonies to divide a limited number of workers between competing functions without a leader.The findings of this study reinforce our understanding of the organism-like nature of social insect colonies. Like cells in a body, the thousands of individual insects in a P. badius colony are organized into functional labor groups, which are responsive to cycles of growth, reproduction and dormancy through self-regulating processes. The emergence of measurable, colony-level traits from the accumulation of thousands of transient individuals, from multiple generations is one of the most striking feats of social organization across taxa.