Do polygyne ants cooperate? : Colony boundaries and larval
discrimination in multiple-queen colonies of the red imported fire ant
(Solenopsis invicta)
Abstract
Unicoloniality, or the absence of behavioral boundaries between nests,
is thought to promote ant abundance due to reduced intraspecific
competition. Workers within unicolonial populations may increase their
own inclusive fitness by preferentially caring for more related
individuals (nepotism), but nepotism has only rarely been documented in
ants. We tested for unicoloniality and nepotism in polygyne red imported
fire ants (Solenopsis invicta; hereafter fire ants). Fire ants
occur in two social forms: monogyne (i.e., colonies with a single
egg-laying queen) and polygyne (i.e., colonies with multiple egg-laying
queens). Introduced populations of polygyne fire ants are commonly
referred to as unicolonial, but cooperation between and within colonies
is poorly documented. To delimit boundaries between colonies in the
field, we quantified the exchange of a 15N-glycine
tracer dissolved in a sucrose solution and correlated this exchange with
colony genetic structure. We also quantified within-colony conflict
between workers and larvae using close siblings (i.e., from the same
mother) and non-siblings (i.e., from a different mother). Counter to our
expectations, polygyne colonies did not exchange resources or workers,
indicating distinct colony boundaries. Polygyne workers also
preferentially fed larval sibling and may have preferentially
cannibalized non-siblings. Polygyne colony behavior was correlated with
higher levels of within-mound relatedness between workers in the field
than those previously reported in North America (mean ± SE: 0.269 ±
0.037). Our study challenges fundamental assumptions about introduced
populations of polygyne fire ants and suggests that polygyne colonies
are multicolonial and likely engage in high levels of intraspecific
competition.