This paper deals with certain behavioral and ecological factors which may be relevant to the evolution and maintenance of social parasitism in ants. We will argue that some of the same factors which might predispose one species to evolve into a social parasite might make resistance to parasitism difficult for a closely related species.After their mating flight, the queens of most nonparasitic ant species found new colonies alone. A queen of such a species finds a suitable nesting place, excavates a small cavity, and seals herself inside. She then lays a clutch of eggs and feeds her first larvae a special "baby food" derived metabolically from the degeneration of her wing muscles and fat body. These larvae mature to become female workers which forage for food, enlarge the nest, feed the queen, and rear subsequent broods of workers and reproductives. Mature ant colonies usually occupy only one nest (monodomy). However, the number of queens in typical mature colonies varies.Colonies of some species never contain more than one functional queen (monogyny), while colonies of other species often have multiple queens (polygyny) (Buschinger 1974). However, the queens of all known obligatory slave-making, inquiline, and temporary-parasite species found colonies non-inde-1.
The ant genus Epimyrma exhibits an evolutionary transition from fully developed slave-making to a completely workerless parasitic condition. Two of the actively dulotic species, like the closely-related Myrmoxenus gordiugini, engage in mating and dispersal flights as is usual in ants, whereas in the remaining five species intranidal mating of the sexuals and thus continuous inbreeding is observed; the females shed their wings in the nest and disperse on foot. The inbreeding species are very closely interrelated, as was recently demonstrated with hybridization experiments. The reduction of slave-making and the evolution of intranidal mating in this group can hardly be explained assuming ordinary models of speciation and spreading of species. I therefore suggest the scenario of an original, widespread, polytypic species with partially isolated, host-specific races and populations in which genetic dispositions for a reduction of worker numbers and slave-raiding, and for intranidal mating, were adaptive; the latter, however, encountered problems associated with inbreeding. Such a situation then selected for a sex-determination mechanism resistant to inbreeding which spread throughout the range of the species, completing the life-pattern. Due to intranidal mating and inbreeding, however, gene flow between populations is interrupted. The inbreeding system has evidently led to the conservation of a status quo in each population it has reached. The original host-specific races are morphologically and biologically discrete entities, which should be maintained as species, as they were described, even though they consist of reproductively isolated demes which themselves are built up of isolated, clone-like lineages.
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