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[Vol. 93 
an important behavioral adaptation permitting range extension into 
the Sonoran Desert. Acromyrmex versicolor queens clump around 
the essential resource of favorable nest sites (tree shade with ready 
access to lower canopy leaves). The mating system of this species 
may also permit “tracking” of this resource. Unlike some desert 
species that have massive mating swarms (e.g. Pogonomyrmex spp.: 
Chapman 1957; Nagel and Rettenmeyer 1973; Holldobler 1976b, c; 
Davidson 1982), A. versicolor mates in small, localized groups at or 
near the ground in open areas between trees (Wheeler 1917; R. A. 
Johnson, pers. obs.). This behavior mimics closely the mating 
behavior of M. mimicus (M. Cazier, pers. comm.) and likely V. 
pergandei (Pollock and Rissing 1985), pleometrotic species with 
clumped natal nests. Whether such localized mating aggregations 
have led to a highly female biased sex ratio, as appears to have 
occurred in V. pergandei (Pollock and Rissing 1985), is currently 
unknown for these other pleometrotic species. 
Some other ant species with clumped, natal nests engage in inter- 
nest brood raiding in the process of establishing natal territories 
(references cited above). This may select for pleometrosis (Rissing 
and Pollock, in press) which generally results in more rapid produc- 
tion of a larger initial worker force (Waloff 1957; Stumper 1962; 
Markin et al. 1972; Taki 1976; Mintzer 1979; Bartz and Holldobler 
1982, Tschinkel and Howard 1983, Rissing and Pollock, in press). 
Colonies of these species are also territorial as adults. Brood raiding 
also seems likely in Atta texana, another pleometrotic desert leaf- 
cutter (Mintzer and Vinson 1985), which “merges” young colonies in 
the laboratory and field (Echols 1966). Adult colonies of Acromyr- 
mex versicolor are territorial (Gamboa 1974). We suggest such terri- 
toriality, when coupled with natal nest clumping through habitat 
choice, makes brood raiding and associated forms of natal nest 
competition likely for this species as well. 
A final similarity between A. versicolor and other pleometrotic 
ants with clumped natal nests discussed here is the apparent forma- 
tion of foundress associations without respect to relatedness. 
Queens collected from distant locales readily associate. Similar 
observations exist for M. mimicus (Bartz and Holldobler 1982), A. 
texana (Mintzer and Vinson 1985) and V. pergandei (Rissing and 
Pollock 1986); electrophoretic evidence indicates S. invicta queens 
also associate randomly (Ross and Fletcher 1985). This differs 
