Polygynous colonies of Solenopsis invicta, the red imported fire ant, evolved in the context of their native Pantanal region of South America with closely related gynes functioning as reproductives. Interestingly, polygynous queens in their invasive range are conspicuously unrelated to one another. This seems counterintuitive to theories explaining the evolution of eusociality and calls into question certain assumptions based on Hamilton’s Rule and Kin Selection Theory. If forced to rear individuals that share less than half their genes, workers would no longer benefit from their sacrifice. The lack of relatedness between workers in these colonies begs us to ask whether workers are practicing nepotism while rearing brood to adulthood.
Nepotism, or preferential care of related individuals, has been previously reported in ants. Workers can manipulate the reproductive output of queens in polygynous nests through killing offspring of unrelated queens or preferentially feeding related brood. Nepotism in S. invicta has been previously studied and found to be conspicuously absent among workers and queens; however, to our knowledge, no studies have empirically tested the interaction among workers and brood for nepotistic behavior.
Newly mated queens were collected from mating flights in Texas. The queens were allowed to found single-line colonies in the laboratory. For six colonies, workers were given sugar water dyed with food coloring, which they then transferred to brood. This marked the brood from each colony with a color.
Experimental “polygyne” colonies were created through combining brood and workers from one colony with brood from another colony so that every possible two-color colony was created. Experimental colonies were then given artificial nectar with a clear, non-toxic, fluorescent dye. After 18hrs, surviving brood from each family were counted, and the percentage of fluorescent brood among brood of each family was quantified. Another experiment was run with the same experimental design, except the fluorescent food was proteinaceous. Analyses determined if brood with related workers present survived better or were fed more than brood unrelated to the workers.
Results/Conclusions
Preferential feeding of closely related brood was found for both sugary and proteinaceous food. Survivorship of brood was also impacted by the genotype of workers and the relatedness between workers and brood. Colonies lost more adopted brood over the course of the experiment than colony brood. Nepotism, therefore, does exist in the red imported fire ant. Nepotism could potentially have implications for polygynous colonies.