Fire Ant Colony Fitness and Worker Polymorphism

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Fire ant colony fitness and worker polymorphismWalter R. TschinkelProject Summary Darwin considered the existence of a sterile worker caste in social insects to be a challenge to his theory of evolution by natural selection. After all, how could the traits of an individual that did not reproduce be passed on to the next generation? The answer that Darwin and his followers have generally accepted is that the colony is the reproductive unit on which natural selection acts. Thus, whatever worker traits allowed the colony to place more new colonies in the next generation would be favored by natural selection. The great majority of ant species have only two physical castes: reproductive individuals (queens), and more-or-less sterile workers of a single size and shape. In about 15% of ant species, the workers are present in a great range of sizes (up to 1000-fold difference in weight) and shapes. Such worker polymorphism, as it is called, is always associated with further division of labor within the worker caste. It has long been assumed that worker polymorphism benefits the colony by increasing colony efficiency, so that the savings can be invested in additional reproductives, enhancing colony fitness (sexual production). Several tests of surrogates for fitness (foraging, brood-rearing, etc.) have yielded equivocal results, but none of these surrogates are tightly linked to sexual production. There has never been a direct test of the effects of worker polymorphism on colony fitness, that is, on the production of sexuals. The proposed experiments will be the first direct test of the value of worker polymorphism to colony fitness. In a large, 2-yr field experiment on the fire ant, Solenopsis invicta, worker pupae of selected sizes will be transferred among colonies such that one set of colonies will be enhanced in large workers, another in small workers, a third will serve as unaltered control, and a fourth as undisturbed control. The colony fitness of these experimental colonies will be measured by collecting the total annual sexual production using tent-traps. Effects (if any) of altering the worker polymorphism might be either direct through involvement of large workers in sexual production, or indirect through altered territorial defense or foraging. Therefore, the food quantity and quality, as well as territory size will also be monitored and compared to controls. The results of these field experiments will be supplemented with a series of laboratory experiments that will test each possible link (direct, food, territory) between worker polymorphism and colony sexual production. The results will address a fundamental issue of great interest to evolutionary biology and social insect biology.