2 
Psyche 
[Vol. 95 
nest splitting in a population which defines the spatial correlates of 
relatedness among nests and colonies (Mabelis, 1979a, b; Pamilo, 
1981) and may be regulated by the patterns of foraging trails 
(Holldobler, 1974, 1976; Holldobler and Lumsden, 1980; Skinner, 
1980; Brian, 1983). Competition among ant colonies for limited 
resources may also play a role in defining the nest distribution (Lev- 
ings and Traniello, 1981; Ryti and Case, 1986) and nature of interac- 
tions (Mabelis, 1979b; Skinner, 1980). 
Formica obscuripes Forel is a common ant of the northcentral 
and western United States, where it constructs mound nests covered 
with twigs, grass stems, and other plant material. Nests are con- 
nected to foraging areas via a system of permanent trails over which 
foragers return to nests with honeydew, prey, and carrion (Weber, 
1935). This paper reports the results of a study of the pattern of 
trails among a group of 45 nest mounds and the movement of 
workers among nests in relation to trail location in F. obscuripes. 
Here, the term “nest” refers simply to a distinct above ground 
mound, without implying that each mound represents a discrete 
colony. 
Materials and Methods 
Formica obscuripes was studied at a site along an unused railroad 
track near the outskirts of Bozeman, MT from 16 June through 11 
September, 1986. There were 45 nests active during the study 
period. Twenty-nine were chosen for more careful study. A map of 
the study site was constructed to include the location of all of the 
nests, trails used by the worker ants, and the major patches of plants 
visited by the ants. The site was surveyed at least once per week for 
the presence of new trails and nests. The mounds of plant material 
constituting the above ground portion of nests of F. obscuripes at 
the research site ranged in height from four to 33 cm (mean = 9.9; 
SD = 7.8; N = 29) with the maximum width of mounds ranging 
from 15 to 130 cm (mean = 54.9; SD = 28.8; N = 29). Since there 
were few plants growing on nests, the mounds were conspicuous. 
Thus, it is likely that all of the mounds within the research site were 
located. 
To determine the movement patterns of workers, a large number 
were marked using two different techniques: 1) between 17 July an^ 
18 August, 1300 workers from five nests were marked on the dor- 
