Statary Behavior in Nomadic Colonies of Army Ants: The Effect of Overfeeding

Nearctic colonies of the army ant Neivamyrmex nigrescens Cresson (subfamily Ecitoninae) exhibit behavioral cycles consisting of alternating nomadic and statary phases. During the statary phase, a colony remains at the same nesting site and forages irregularly for food. The nomadic phase, by contrast, is characterized by night-long raids and frequent emigrations to new bivouacs. According to Schneirla (1957, 1958), the nomadic phase is triggered by stimulation arising from newly-eclosed callows, and is subsequently maintained by comparable excitation from the developing larvae. Experimental support for brood-stimulation theory stems from studies showing: (1) an abrupt reduction in nomadism after removing a portion of a larval brood (Schneirla and Brown, 1950); and (2) the eclosion of a pupal brood (in the absence of newlyhatched larvae) is indeed sufficient to initiate a nomadic phase (Topoff et al., 1980a). Recent studies have suggested, however, that brood stimulation may in turn depend on the degree of brood satiation. Thus, in a preliminary field study involving food augmentation, Mirenda et al. (in press) was able to halt the occurrence of emigrations during a portion of the nomadic phase in colonies of N. nigrescens. This was followed by more prolonged laboratory studies (Topoff and Mirenda, 1980 a,b) showing that the frequency and direction of nomadic emigrations are indeed influenced by the amount and location of food. This paper reports findings from our continued studies of food augmentation for colonies of N. nigrescens. In previous studies, larval stimulation was reduced by artificially feeding colonies early in the nomadic phase, after callow eclosion. Because an additional