Although colony size and nest number are believed to influence competitive
ability in social insects, experimental studies testing this idea are rare.
Here, we experimentally manipulated worker number and nest number in labor
atory colonies of Argentine ants, Linepithema humile, to test how these att
ributes, working alone or in combination, affected different components of
exploitative and interference ability. As expected, every measure of compet
itive performance tested increased with worker number. In contrast, the inf
luence of nest number was more complex, with colony-level performance incre
asing, decreasing, or remaining constant depending on the type of competiti
on-related test or colony-size category being considered. In the exploitati
on of randomly distributed food items, retrieval rates decreased with nest
number for 50-worker colonies yet increased with nest number for 2500-worke
r colonies. In contrast, retrieval rates decreased with nest number across
all colony sizes when resources were clumped. In two experiments on interfe
rence competition, only colonies with more than 1000 workers and that occup
ied single nests were able to maintain more than 10 workers, on average, at
baits in the presence of a competitor, Forelius mccooki. Argentine ant wor
kers initiated a majority of pairwise fights against F. mccooki workers but
often lost (i.e. either retreated or were injured), in part because their
pre-emptive attacks left them vulnerable to chemical defensive compounds us
ed against them by Forelius. These findings promise to promote a better und
erstanding of the causes of invasion success not only for the Argentine ant
but for other invasive ants as well, given that most have colony structure
s resembling that of L. humile. (C) 2001 The Association for the Study of A
nimal Behaviour.