Introduction
Invasive ants often reach extremely high densities, outcompete and prey
upon native species, disrupt mutualisms, and lower ecosystem
biodiversity (Berman, Andersen, Hély, & Gaucherel, 2013; Holway, 1998;
Holway, Lach, Suarez, Tsutsui, & Case, 2002; LeBrun, Abbott, &
Gilbert, 2013; McGlynn, 1999; Porter & Savignano, 1990). Understanding
the factors that promote the success of invasive ants is critical to
defining and managing their ecological impacts. One factor thought to
play a role in invasive ant success is their proclivity for
intraspecific cooperation (Holway et al., 2002; Passera, 1994). Many
invasive ant species are unicolonial in their introduced range, meaning
that populations are characterized by an absence of behavioral
boundaries among nests (Helanterä, Strassmann, Carrillo, & Queller,
2009; Holway et al., 2002; Krushelnycky, Holway, & LeBrun, 2010). By
cooperating instead of competing with conspecifics, unicolonial ants
avoid the costs of intraspecific competition, which is thought to allow
them to reach higher densities (Giraud, Pedersen, & Keller, 2002;
Porter, Fowler, & Mackay, 1992) and achieve greater ecological
dominance by more effectively outcompeting other species (Holway et al.,
2002; Holway & Suarez, 2004; LeBrun et al., 2013). For example, the
number of Argentine ant workers (Linepithema humile ) was
approximately 50-fold higher in sites where nests cooperated compared
with sites where nests competed (Holway & Suarez, 2004). Consequently,
cooperative Argentine ants more effectively outcompeted native ants, as
native ant species richness was reduced by over 50% when Argentine ant
nests cooperated compared with when they competed with each other
(Holway & Suarez, 2004). The positive correlation between
instraspecific cooperation and ecological impact has also been
implicated in other social insects (Hanna et al., 2014; Korb & Foster,
2010; Perdereau et al., 2015, Wilson, Mullen, & Holway, 2009).
However, unicoloniality does not explain the success of all invasive ant
species (Holway et al., 2002; Passera, 1994). The dark rover ant
(Brachymyrmex patagonicus ), for example, has invaded large
portions of the southern USA (MacGown, Hill, & Deyrup, 2007), but
introduced populations of this species are multicolonial, with distinct
nests usually headed by a single egg-laying queen (monogyne; Eyer,
Espinoza, Blumenfeld, & Vargo, 2020). Moreover, despite the ecological
benefits, when many unrelated queens cooperate and produce workers
within a single colony, relatedness between nestmates can decrease to
effectively zero (Crozier & Pamilo, 1996; Keller, 1995; Queller &
Strassmann, 1998). As a consequence, polygyny and especially
unicoloniality increase the likelihood of workers interacting with and
caring for less related individuals, which in turn reduces their
inclusive fitness (Helanterä et al., 2009; Schmid-Hempel, 1997;
Schmid-Hempel & Crozier, 1999). Inclusive fitness may be maintained,
however, if workers discern and preferentially direct care towards kin
over non-kin (Hamilton, 1964; Keller, 1995). For example, although
Argentine ant supercolonies are characterized by cooperative,
interconnected nests (Giraud et al., 2002; Tsutsui, Suarez, Holway, &
Case, 2000), workers spend more time investigating (i.e., antennating)
non-nestmates than nestmates from the same supercolony
(Björkman-Chiswell, Van Wilgenburg, Thomas, Swearer, & Elgar, 2008),
indicating nestmate recognition despite a lack of aggression within the
same supercolony in this species. Nestmate recognition may reduce or
eliminate indiscriminate sharing between Argentine ant nests, as sharing
was consistently limited to distinct clusters of nests within a single
supercolony over a three-year-period (Heller, Ingram, & Gordon, 2008).
Kin recognition can also impact sharing between individuals within the
nest in what is known as nepotism. For example, in polygyne ant colonies
of Formica argentea , workers preferentially care for brood that
are more closely related to them (Snyder, 1993). By discriminately
sharing food resources with more related individuals from neighboring
mounds or from within the same mound, workers may increase their
inclusive fitness in polygyne colonies (Hamilton, 1964; Helanterä et
al., 2009). Despite the evolutionary benefits of preferential care
towards kin over non-kin, it has only rarely been documented in polygyne
or unicolonial ant species and may be less common than previously
thought (Boomsma & d’Ettorre, 2013; Keller, 1997).
We tested for unicoloniality and within-colony nepotism in polygyne red
imported fire ants (Solenopsis invicta ; hereafter fire ants).
Fire ants occur in two social forms: the polygyne form (i.e., colonies
with multiple egg-laying queens) and the monogyne form (i.e., colonies
with only a single egg-laying queen; Gotzek, Shoemaker, & Ross, 2007;
Ross, 1993; Ross, Vargo, & Keller, 1996; Tschinkel, 2006). Polygyne
fire ant colonies are considered highly cooperative, and they are often
referred to as unicolonial throughout their invaded range (e.g.,
Greenberg, Vinson, & Ellison, 1992; Holway et al., 2002; Morel, Vander
Meer, & Lofgren, 1990; Plowes, Dunn, & Gilbert, 2007; Porter et al.,
1992; Vander Meer, Obin, & Morel, 1990). Cooperative behavior in
polygyne fire ants is thought to correspond with a greater abundance
compared with the monogyne form due to reduced intraspecific competition
(Porter, Bhatkar, Mulder, Vinson, & Clair, 1991). For example, polygyne
mounds were over twice as abundant on average compared with monogyne
mounds in Texas (mean ± SE: 680 ± 475 polygyne mounds/ha vs. 295 ± 240
monogyne mounds/ha; Porter et al., 1991). The greater abundance of the
polygyne form may increase the likelihood of ants interacting with and
preying upon native species, thereby increasing their ecological impact
(Allen, Epperson, & Garmestani, 2004; Porter & Savignano, 1990).
Despite the assumption that polygyne mounds cooperate and are highly
interconnected (see Bhatkar & Vinson, 1987), the physical exchange of
workers and resources between polygyne mounds in the field is actually
poorly documented. Moreover, although polygyne workers from different
nests do not aggressively attack each other outside of the nest (Vander
Meer et al., 1990), their interactions within the nest are relatively
unknown (but see DeHeer & Ross, 1997), particularly in the case of
worker-brood interactions. Within-colony relatedness between polygyne
workers is often near zero throughout their invaded range (DeHeer &
Ross, 1997; Goodisman, Sankovich, & Kovacs, 2007; Ross, 1993; Ross &
Fletcher, 1985; Ross et al., 1996), but workers may increase their
inclusive fitness by preferentially caring for more related brood.
Our study tests fundamental assumptions about inter- and intracolonial
cooperation in introduced populations of polygyne fire ants. To delimit
boundaries between colonies, we quantified the exchange of a15N-glycine tracer dissolved in a sucrose solution and
correlated this exchange with colony genetic structure. Using a labeled
resource in combination with genetics data allows for two different ways
to define colony boundaries (Ellis, Procter, Buckham-Bonnett, &
Robinson, 2017). We also quantified within-colony conflict between
workers and larvae by comparing brood-tending behaviors towards close
siblings (i.e., from the same mother) and non-siblings (i.e., from a
different mother). Studies that actually document cooperation between
and within colonies are critical to understanding the ecology and
success of invasive species.