The ecological and genetic basis of annual worker production in the desert seed harvesting ant, Veromessor pergandei
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Colony size is an important predictor of annual survival and reproduction in social insects. By tracking monthly forager turn over, we measured the size-specific production rates necessary to counteract forager mortality in wild Veromessor pergandei colonies. Between 31,180 and 237,980 individuals appeared as foragers annually, representing an eightfold difference in production between neighboring nests. Highly productive colonies housed 26,000 foragers at one time, turned over 34,000 foragers in a single month, and produced 470 g of worker biomass during the year. Each forager population turned over approximately 20 times during the year, cumulatively returning 3.4 kg of biomass per hectare (with 14 nests per hectare). Forager longevity was not influenced by forager number, season, or pressure from neighboring conspecifics. Genotyping at three microsatellite loci revealed that all colonies contained a single queen and one to seven patrilines (mean m observed = 3.8; m e = 2.56). The most productive colonies had significantly fewer patrilines, a larger peak forager population, and a larger annual foraging range. Colonies varied in their ability to replace lost workers, adding anywhere from 2 to 42 new foragers per death during peak forager allocation. Seasonal increases in forager population size corresponded to decreases in worker body size, suggesting a trade-off during production earlier in the year. Together, these findings demonstrate how the combination of individual colony-member characteristics influence whole colony survival across multiple years.
The survival of an animal society can depend on its size and stability. We measured the seasonal relationships between mortality and production of new colony members, with respect to foraging range size, paternity, neighbor pressure, and body size in a desert seed harvesting ant. Mortality rate was stable across seasons, but the ability of colonies to replace dead individuals corresponded to increased foraging range size, decreased patriline number, and seasonal decreases in body size. Our findings reveal the characteristics that allow colonies to respond to worker mortality on an annual scale.
KeywordsColony size Worker production Patriline number Ant biomass Mortality rate
We would like to thank Yocha DeChavez for her tireless and careful assistance in the laboratory and field and Scott Bingham for his support at the ASU DNA core facility. We are also grateful for the thoughtful discussions and advice from Robert A. Johnson, Walter Tschinkel, Ti Ericksson, Erick Lundgren, the Social Insect Research Group at ASU, and two anonymous reviewers. This study was made possible with permission from South Mountain Regional Park, Phoenix, AZ.
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