10 
Psyche 
[Vol. 95 
mounds had a common ancestor (Mabelis, 1979b). However, 
further data are needed to confirm this for F. obscuripes. 
Discussion of several limitations of the data are in order. First, as 
noted by Scherba (1964) the census techniques used produced only 
minimum estimates of crossover of marked workers among nests, 
since only a limited number of censuses were conducted. Second, 
workers captured from a nest for marking did not necessarily 
develop within that nest. The data reported here provide evidence 
for worker movement among mounds, but not an absolute picture 
of the pattern of switching among nests. For example, if all workers 
marked on nest #6 did not originate in that nest, their recapture on 
nest #1 1 does not indicate the exact proportion of workers making 
the switch from nest #6 to nest #11. In fact, we could reverse our 
interpretation of the mark-recapture data: perhaps workers cap- 
tured on a nest for marking developed in the mound on which they 
were subsequently recaptured. This also means that the data do not 
allow us to determine whether workers originating in one nest even- 
tually acted as foragers for other nests. Finally, since the majority of 
the workers marked were probably foragers rather nest workers, the 
percentage of ants found to cross over is probably an overestimate, 
since the latter are probably more philopatric. 
In the absence of such information as the recent history of nest 
splitting (Mabelis, 1979a, b) and degrees of relatedness among nests 
and colonies (Pamilo, 1981, 1982), the pattern of worker movement 
observed remains descriptive. However, the data indicate that, in 
this population, some colonies of F. obscuripes are polydomous. 
This matches observations from an earlier study of this species in 
which Weber (1935) found “secondary nests. . .generally connected 
by a well-defined runway to the main nest”. Since the history of the 
population in the present study is unknown, the parent (“main”) 
nest within a group cannot be determined. Although the nest mounds 
varied markedly in size, it is known for F. ulkei that there is not a 
simple linear correlation between nest size and age (Dreyer, 1942). 
Colonies with multiple nest mounds connected via trail systems 
have been noted within other species of Formica (Mabelis, 1979a; 
Marikovsky, 1962; Skinner, 1980) and within species of other ant 
genera, such as Lasius and Iridomyrmex (e.g. Greenslade and Halli- 
day, 1983; Yamauchi et al., 1981). However, the existence of per- 
manent trails is not a prerequisite for exchange of workers among 
mounds. Crossover of workers among mounds occurs in the 
