LEPTOTHORAX DULOTICUS AND THE BEGINNINGS OF SLAVERY IN ANTS

Slave-making in ants, sometimes referred to as dulosis, is a form of social parasitism. The slave-makers conduct raids against neighboring colonies of other, related species, overwhelm the defending workers, and carry off their pupae and larger larvae. When the young captives eclose as adult workers, they accept the slave-makers as nestmates and join in the labor of the parasite colony. The workers of species in early stages of dulotic evolution are still relatively self-reliant. They assist their slaves in ordinary tasks and in some cases are able to survive without them. But those of the morphologically most advanced forms are completely helpless, having been transformed into specialized fighting machines capable only of conducting efficient slave raids. Beyond its considerable intrinsic interest slavery in ants holds general significance for the study of behavioral evolution. Detailed comparison of slave-making species with closely related, free-living forms will identify the behavioral elements modified or lost during the early evolution of social parasitism, while the observation of the slave-makers with and without their slaves is a convenient means for revealing the amount of flexibility in the modified traits. For example, some behaviors are greatly attenuated or absent in slave-makers when the functions are filled by slaves. We may ask whether they will be evoked when the slaves are removed and survival demands a full, "normal" repertory. Slavery has been discovered in six genera of ants: Lepiothorax, Harpagoxenus, and Strongylognathus in the Myrmicinae; and Formica, Polyergus, and Rossomyrmex in the Formicinae (Huber, 1810; Kutter, 1969; Buschinger, 1970; Wilson, 1971a). Each of these taxa may contain at least one phyletic line that has evolved slavery independently of the others; it is at least safe to conclude that the habit originated three times, in Leptothorax-Harpagoxenus, Srongylognathus, and the Formicinae. Leptothorax duloticus represents a case of special interest. Its original discoverer, L. G. Wesson (1937, 1940), recognized that it represents an early stage in the evolution of dulosis. His observations on captive colonies revealed that although the species is an obligatory parasite on other Leptathorax (L. curoispinosus, L. longispinosusv, its external morphology is only slightly modified and the organization of its slave raids are relatively crude. The behavior of duloticus has not been studied since Wesson's original work, partly because the species is rare and local. Wesson's type population appeared to be limited to an area of only about 100 square meters in an oak forest in Ohio. Subsequently Talbot (1957) discovered a more extensive population in the E. S. George Reserve of Michigan and subjected it to a demographic study, in the course of which she confirmed the obligatory nature of the dulosis. In 1973, Dr. Talbot re-examined her study site and kindly supplied me with two queenright and four queenless mixed colonies of L. duloticus and its slave L. curuispinosus (see Fig. 1), together with unenslaved colonies of L. curuispinosus and the related species L. ambiguus. I took this unusual opportunity to make an intensive study of a rare and scientifically valuable species.