Behavioral Ecology Vol. 8 No. 1: 99-103
Intraspecific usurpation of incipient fire ant
colonies
Michael T. Balas and Eldridge S. Adams
Department of Biology, University of Rochester, Rochester, NY 14627, USA
Brood raiding, or reciprocal stealing of brood, is common among incipient colonies of the Ere ant SoUnopsis invicta. Paradoxically, workers from a colony that loses its brood during a raid often abandon their nest and join the winning colony. Queens
abandoned by their workers may then migrate from their original nest site and attempt to forcefully usurp another incipient
colony by displacing that colony's queen or queens. This study examined factors that influence the success of usurpation
attempts. Queens attempting to usurp a nearby colony after laboratory brood raids were successful in less than 30% of trials.
Usurpation attempts were more successful if workers familiar to the migrating queen were present in the nest, as would happen
if a queen were to find the colony to which her workers had moved. Cross-fostering experiments showed that this effect was
due to familiarity rather than relatedness. Usurpation attempts were less successful if they were delayed by 16 h. The probability
of usurpation success was not reduced by doubling the number of defending workers in the invaded colony. However, colonies
founded by three queens were almost always able to resist usurpation attempts. These results support the hypothesis that workers
abandon their natal colony after losing a brood raid to increase the likelihood that their queen can usurp the colony to which
they migrate. These results also provide the first evidence that colonies of ants founded by several queens are better able to
resist usurpation attempts than colonies founded by a single queen. Key words brood raids, fire ants, kin recognition, SoUnopsis
invicta, usurpation. [Bthav Ecol 8:99-103 (1997)1
I
ntraspecific competition among incipient colonies of social
insects is often intense, resulting in high mortality of
queens before the colonies reach reproductive maturity (H61ldobler and Wilson, 1990). In some ant species, much of this
mortality is associated with brood raiding. During brood raids,
incipient colony workers remove eggs, larvae, and pupae from
neighboring colonies and take them to their home colony,
thereby supplementing the worker force (Bartz and Holldobler, 1982; Rissing and Pollock, 1987; Stamps and Vinson, 1991;
Tschinkel, 1992a,b). Brood raids by the fire ant SoUnopsis invicta (Buren) cause a substantial reduction in the density of
incipient colonies and queens (Adams and Tschinkel, 1995a).
Queens whose colonies have been depleted of brood through
these raids often attempt to usurp nearby colonies whose
brood population has been increased through raiding (Adams
and Tschinkel 1995a,b; Tschinkel, 1992b).
Success in brood raids rises with the size of the worker population (Rissing and Pollock, 1991; Stamps and Vinson, 1991;
Tschinkel, 1992b). One way a colony can produce more workers in its first brood is through pleometrosis, or colony foundation by several queens (reviewed in Rissing and Pollock,
1988). In three species for which raiding has been described,
Mymucocystus mimicus (Bartz and H&lldobler, 1982), Mtssor
prrganda (Rissing and Pollock, 1987, 1991), and £ invicta
(Adams and Tschinkel, 1995b; Tschinkel, 1992b), pleometrotic colonies are more likely to win brood raids. Tschinkel
(1992b) demonstrated that this effect was due to increased
worker number and not increased queen number.
A peculiar behavior associated with brood raiding is the
abandonment of an incipient colony by its workers after its
brood has been removed. When abandonment occurs, workers leave their colony's queen or queens at the original nest
Address correspondence to E. S. Adams.
M. T. -Balai u now at St. John'i University, 300 Howard Ave., Staten
Island, NY 10301, USA.
Received 31 August 1995; revised 27 March 1996; accepted 29
March 1996.
1045-2249/97/15.00 O 1997 International Society for Behavioral Ecology
and peacefully join the colony that successfully raided it (Bartz
and HoUdobler, 1982; Rissing and Pollock. 1987; Tschinkel,
1992b). However, a queen from a losing colony will usually
attempt usurpation shortly after her workers abandon her
(Adams and Tschinkel, 1995a,b; Bartz and HoUdobler, 1982;
Rissing and Pollock, 1987; Tschinkel, 1992b). It has been proposed that workers' relocation is adaptive if the workers'
mother queen has a high probability of joining them in the
new colony, a process termed "hopeful co-emigration" (Pollock and Rissing, 1989).
Queens attempting to enter other incipient colonies may
encounter resistance. Workers of fire ant incipient colonies
recognize familiar queens and often expel or kill unfamiliar
queens (Balas and Adams, 1996b). However, migrating queens
occasionally succeed in displacing the resident queen or
queens, which may be killed and discarded (Adams and
Tschinkel, 1995a,b; Rissing and Pollock, 1987; Tschinkel,
1992a). In addition, Adams and Tschinkel (1995b) showed
that if a queen attempts to usurp a colony that has won a raid,
her probability of success is greater if the workers that joined
the colony are her daughters. However, it is not known whether this effect is due to the presence of familiar workers or
simply to the smaller number of defending workers in die
invaded colony.
This study examines whether workers of S. invicta are able
to assist their mothers in colony usurpations. First, we tested
whether workers of S. invicta are better able to usurp colonies
to which their daughters have moved. Second, we tested
whether the effect of daughters on usurpation success is due
to familiarity with the queen or to genetic relatedness. Aggressive behavior during these usurpation attempts was observed to determine if the presence of familiar or related
workers influenced the aggressive response toward either the
resident queen or the queen attempting usurpation.
In addition, preliminary experiments suggested that a
queen's probability of usurping the colony to which her
daughters have moved may decline if she does not migrate
immediately after a brood raid. This may be because queens
rapidly lose familiarity to their former nestmates. We tested
Behavioral Ecology Vol. 8 No. 1
100
this possibility by delaying usurpation attempts for 16 h after
the conclusion of brood raids.
Finally, since colonies with more workers have an advantage
in brood raids (Tschinkel, 1992b), we tested the possibility
that increasing worker number in a colony increases that colony's ability to resist usurpation. Since worker number covaries with queen number, we also tested whether incipient
colonies with several queens resist usurpation attempts more
successfully.
METHODS
from the queens' paint marks. Hereafter, any queens that
moved to or were placed at the entrance of other colonies in
order to simulate a usurpation attempt (see below) are referTed to as "migrating queens." We observed the colony 1 h
after queen migration for aggressive behavior (e.g., biting or
stinging) by the workers against either queen. Each colony
was scanned once a day for 3 days after the raid, and any
evidence of queen injury, such as missing appendages or lethargic movements, was recorded. We noted dead or expelled
queens as well. The experiment was terminated after 3 days,
and the fates of both queens were determined.
Effects of *"""*"• or related workers on usurpation
We collected mated queens after nuptial flights in Tallahassee,
Florida, USA, during April and May 1994. Queens were placed
in glass nest-tubes 8-10 mm in diam and 75 mm long that
were half-filled with water retained by a cotton plug and also
plugged with cotton at the open end. These colonies were
maintained at 29°C during the experiment. As toon as three
workers had edosed in a colony, the nest tube was opened
and placed in a plastic box 10.5 X 10.5 X 3.4 cm, the walls
of which were coated with Fluon (Northern Products Inc.,
Woonsocket, Rhode Island) to prevent escapes. Colonies were
fed crickets and diluted honey immediately after the first
worker edosed and 1 day before the experimental trial. To
assure that the colonies were healthy, we used only colonies
that had at least eight workers 5 days after the first worker
edosed. This is approximately the average number of workers
present per single-queen nest at the time brood raids begin
in the field (Adams and Tschinkel, 1995a). We distinguished
queens by unique paint marks on the queens' alitrunks. These
marks have no effect on aggression against familiar or unfamiliar queens (Balas and Adams, 1996a).
Tests of usurpation «"«y»««
Brood raids were staged in the laboratory in two different
containers: the colonies' nest-boxes, in which the interior of
each nest was visible for behavioral observations, and cups
filled with sand, which more closely mimic natural conditions.
In the nest-boxes, raids were initiated by removing a colony's
nest tube from its box and placing it beside a second colony's
nest tube. To prepare raids in cups of sand, 1 frounce cups
were filled with damp sand with a small hole about 7 cm deep
poked into the center to serve as an artificial nest site. We
then gently tapped a colony's queen, workers, and brood out
of the nest tube and onto the sand's surface. The workers and
queen quickly found the hole and moved their brood into it
We allowed the colony to settle into the hole overnight. The
next morning, raids were initiated by placing a second colony's nest tube on the sand's surface. With either nest design,
raids were usually completed between 2 and 4 h after the
colonies were placed together. In a minority of trials (26 of
478; 5.4%), raids had not begun after 4 h. We discarded these
trials to avoid isolating the queen for longer than the experimental treatment indicated.
Umxunipulated raids
The unmanipulated raids experiment measured the success
rate of queen migration and usurpation after raids between
two one-queen incipient colonies. We placed colonies in pairs
in plastic nest-boxes and observed them at 20-min intervals.
The colony that contained all the brood at the raid's conclusion was recorded, as well as any migrations by the losing colony's queen to the winning colony's tube. In some cases,
queen migration was not observed directly but was inferred
m w
We tested whether queens are better able to usurp a colony
when workers familiar with her are present in that colony and
whether this effect depends on genetic relatedness between
the queens and workers. This was achieved using a two-factor
design that varied both workers' previousfamiliaritywith and
genetic relatedness to a queen. Before raids were initiated in
this experiment, one queen from each pair of colonies was
chosen at random and held aside in a dean test tube during
the raid. Raids took place between die queenless colony and
an unmanipulated one-queen colony. After the raid's conclusion, usurpation attempts were initiated by placing die isolated queen at the opening of the consolidated colony.
We introduced queens either to colonies containing their
former workers or to colonies in which all workers were unfamiliar with her. Also, by cross-fostering queens between colonies shortly before their workers edosed, replicates were created in which workers were not familiar with dieir biological
mother as adults. The five experimental treatments were (1)
the migrating queen was both related to and familiar to the
raided workers (familiar kin treatment); (2) the queen was
neither related to nor familiar to the raided workers (unfamiliar nonkin treatment); (3) the queen was familiar to the
raided workers but was not related to them (familiar nonkin
treatment). These were queens that were cross-fostered with
unrelated pupae just before the workers edosed; (4) the
queen was related to die raided workers but was unfamiliar
to diem (unfamiliar kin treatment). In this treatment, a queen
tried to usurp a colony that contained her daughters, but her
daughters had been cross-fostered with an unrelated queen;
(5) the migrating queen was introduced mto a colony that
had not raided (no raid treatment).
One hour after the queens' introductions, half of the colonies were observed for any incidents of aggression. The other raids took place in sand as stated previously; therefore, the
queens were usually not visible. We determined die fates of
queens after 3 days by observing dead or expelled queens or
by excavating colonies in sand.
Influence of
of queen migration on usurpation
We tested whether delaying a usurpation attempt by 16 h reduced usurpation success. Raid trials were conducted as in the
previous experiment, except diat in this experiment die isolated queens were released into die enlarged colony about 16
h after die raid's condusion. Only die familiar kin and unfamiliar nonkin treatments were induded in diis experiment
Influences of resident queen and worker number
Finally, we tested die effects of increasing queen or worker
number in a resident colony on die success rate of usurpation
attempts after raids. Raids were induced under diree experimental conditions: (1) die resident colony's worker number
101
Balas and Adams • Usurpation in fire ants
Familiar Kin
Relationship
ofjoined
workers to
usurping
queen:
r
(44)
• (43)
Unfamiliar Kin
Unfamiliar Non-kin
No raid prior to attempt
Attempt
delayed
by 16
hrs:
Increased
worker
number
Familiar Kin
Unfamiliar Non-kin |laoncseoo
ao
(41)
Enlarged through raiding
Enlarged with pupae
Three queens
• B (43)
10
IS
20
figure 1
Percentages of fire ant queens
that successfully usurped the
queen or queens of alien incipient colonies. Only those trials
in which there was one dear
winner are included. The categories on the left describe the
conditions under which the
queen attempting usurpation
was introduced into the alien
colony and are described in
Methods. Sample sizes for each
category are given in parenthe-
Percentage of successful usurpations
was approximately doubled by adding pupae from another
incipient colony just before worker eclosion (enlarged with
pupae treatment); (2) the resident colony's worker number
was enlarged after work eclosion by allowing it to raid a
queenless incipient colony of the same age (enlarged through
raiding treatment). This colony then underwent a second raid
that served as the experimental trial; (3) the resident colony
was founded by three queens and thus had both an increased
number of workers and queens (three-queen treatment).
Rough counts of worker number showed that the above procedures created colonies with approximately 15-25 workers in
all three treatments, which was roughly twice the number of
workers in the colonies they raided.
RESULTS
Unmsoiipulftted raids
Most attempted usurpations were unsuccessful. Only 10 out
of 52 (19.2%) migrating queens successfully displaced queens
in other nests. Thirty out of 52 (57.7%) migrating queens
were either expelled or killed within 3 days. In 11 (21.2%)
trials, both queens were dead or badly injured after 3 days.
Both queens were alive and uninjured after 3 days in the final
case. Thus, in the 40 trials with a clear winner after 3 days
(Figure 1), the probability of usurpation by the migrating
queen was less than 50% (binomial test, p =• .001).
The winning queen was injured in 8 out of 40 (20%) trials
with a single surviving queen. Successfully usurping queens
were as likely to be injured (2 of 10 trials) as resident queens
that successfully resisted usurpation (6 of 30 trials; G test, G
= 0).
We observed the migration of the queen in 34 of these 40
colonies. One hour after the queen migrated, the number of
workers biting each queen was counted. Results were divided
into two categories: low aggression, in which the introduced
queen was bitten by at most one resident ant, and high aggression, in which the introduced queen was bitten by two or
more resident ants. Queens eliciting low aggression were
more likely to usurp (7 of 21 trials) than queens eliciting high
aggression (1 of 13 trials), but the difference was not significant (two-tailed Fisher's Exact Test, p = .116).
Effects of familiar or related workers on usurpation success
In 215 (86%) of 250 trials, only 1 queen was left in the colony
after 3 days. In 17 trials (6.8%), both queens were alive and
uninjured after 3 days. Both queens were injured or dead in
the other 18 (7.2%) cases. Remaining analyses consider only
the 215 cases with a single surviving queen.
Usurpation success was greater if workers familiar with the
migrating queen were present in the colony she entered, but
did not depend on relatedness (Figure 1). The log-linear
model (Agresti, 1984) that provides the best fit to the data
shows conditional dependence between the percentage of
usurpation successes and familiarity of the workers to the
queen, given kinship, but no relationship between usurpation
success and kinship, given familiarity (G* • 0.29, p =» .963).
This model is a slight improvement over a simpler model that
assumes that familiarity, kinship, and percentage of usurpation successes are independent of each other (C 1 = 3.21, p
= 323; difference between the two models, C - 2.92, .1 >
p > .05). Therefore, usurpation success is improved slightly
by the presence of workers familiar with the migrating queen,
but not by the presence of unfamiliar yet related workers.
The percentage of successful usurpations by migrating
queens entering colonies that had not raided was significantly
lower than the percentage of successful usurpations by migrating queens entering colonies with familiar workers (G^
m
5.02, p < .05). However, the percentage of successful usurpations by migrating queens entering colonies that had not
raided was not significantly different from that of migrating
queens entering colonies that had raided and acquired workers unfamiliar with her (G^ = 1.02, p > .1). Therefore, increased usurpation success is due to the presence of workers
familiar with the migrating queen rather than to variation in
the number of workers unfamiliar with the migrating queen.
Migrating queens confronted with high aggression by the
residents 1 h after introduction were less likely (3 successes
out of 49 observations) to usurp the colony than migrating
queens treated with low aggression (16 of 61; G^n m 8.24, p< .005). In addition, migrating queens were less ukety to be
confronted with high aggression when familiar workers were
present (IS of 47) than when no familiar workers were present in the colony (33 of 63; Gy, =• 4.58, p < .05).
102
Resident queens elicited high aggression in 7 out of 91
(7.7%) trials when workers unfamiliar with the resident queen
were present in the colony before the migrating queen was
introduced. High aggression against resident queens was not
associated with an increased likelihood that the resident
queen would be usurped (2 usurpations out of 7 cases of high
aggression against resident queens versus 14 usurpations out
of 84 cases of low aggression; G^ = 0.49, p > .1).
Influence of rimfatg of Queen migration on usurpation
When introductions of migrating queens were delayed by 16
h after conclusions of brood raids, the percentage of successful usurpations declined, whether or not workers familiar with
the migrating queen were present (Figure 1). These percentages were significantly lower for both the familiar kin and
unfamiliar nonkin treatments, compared with the similar
treatments when migrating queens were introduced immediately after a raid's conclusion (familiar kin, Gu = 4.92, p <
.05; unfamiliar nonkin, G^ = 7.41, p < .01). As in the previous experiment, migrating queens usurped colonies more
often if workers familiar with the migrating queen were present than if no workers familiar with the migrating queen were
present (Figure 1), but this difference was only weakly significant (G^ = 3.60, .1 > p > .05).
Influences of resident queen and worker number
Migrating queens were just as likely to usurp colonies with
artificially supplemented worker populations as they were to
usurp a colony with an equal number of workers (Figure 1;
unmanipulated raids versus enlarged through raiding, G^ =
0.22, p > .30; unmanipulated raids versus enlarged with pupae, G^g = 0.004, p > .95). However, a migrating queen was
significantly less likely to usurp a colony that contained three
cofounding queens than colonies that contained one queen
10.38, p < .005).
DISCUSSION
Usurpation attempts by queens whose colonies have lost
brood raids are usually unsuccessful. However, evidence from
this study and elsewhere (Adams and Tschinkel, 1995b) indicates that usurpation attempts are more successful if the
migrating queen enters a colony containing familiar workers.
The percentages of successful usurpations reported in the two
studies do not differ significantly (familiar kin: G^ = 0.96, p
> .1; unfamiliar nonkin: G^ = 1.44, p> .1), although a weaker effect was found in this study than by Adams and Tschinkel
(1995b). When the data from these two studies are pooled,
migrating queens usurped invaded colonies in 35 out of 124
attempts (28.2%) when workers familiar with her were present, but in only 15 out of 124 attempts (12.1%) if no workers
familiar with her were present. This difference is significant
(Gkg ** 10.1, p < .001), indicating that a migrating queen is
2.3 times as likely to usurp a colony if it contains familiar
workers.
The increase in usurpation success associated with the presence of familiar workers supports the "hopeful co-emigration" hypothesis proposed by Pollock and Rissing (1989). This
hypothesis suggests that the workers and queen of a colony
that has lest all of its brood attempt to make the best of a bad
situation by usurping a surviving colony. In general, workers
will usually attempt to win a raid first by stealing another colony's brood. If a colony loses a raid, the workers defect to
the winning colony (Bartz and HoUdobler, 1982; -Kissing and
Pollock, 1987; Stamps and Vmson, 1991; Tschinkel, 1992b).
Behavioral Ecology Vol. 8 No. 1
It has recently been suggested that queens move freely
among incipient nests in S. mvicta (Pfennig, 1995) and that
brood raiding may actually be a form of cooperative nest consolidation (Nonacs, 1993). Yet this study and others (Adams
and Tschinkel, 1995a,b; Balas and Adams, 1996b) show that
workers actively resist entry by unfamiliar queens, which they
often kill. The phenomenon of "traitor" raiders that move
brood away from their home nest (Tschinkel, 1992b) may suggest that cooperative nest consolidation (Nonacs, 1993) rather
than raiding is taking place. However, traitors have only been
reported for & mvicta and are less common than workers that
maintain fidelity to their nest site (Tschinkel, 1992b). Furthermore, most attempted usurpations are unsuccessful (Figure 1). Thus, although queens can occasionally usurp other
nests after raids, usurpation appears to be an attempt to recover from what would otherwise be a competitive loss.
The presence of familiar workers improves migrating
queens' usurpation chances. Figure 1 shows that migrating
queens are better able to usurp colonies containing workers
familiar with her than either colonies that have not raided or
colonies that acquired workers unfamiliar with the migrating
queen. The mechanism by which familiar workers confer this
advantage is not fully understood. They may attack the resident queen when their former nest mate queen arrives. However, aggression against resident queens was uncommon and
was not associated, with their subsequent death or expulsion.
Alternatively, by joining surviving colonies, the joining
workers may lower the aggressive response by the resident
workers toward their own invading queen. In support of this
hypothesis, migrating queens elicited lower aggression in colonies containing workers familiar with her, and low aggression
was associated with greater usurpation success. One possible
mechanism for this effect is that the original residents of the
colony adjust their recognition template to include the colony
odor carried by the new workers. When the new workers'
mother queen arrives shortly thereafter, they may be slightly
less likely to judge her as unfamiliar. However, the usurpation
success was lower if the attempt was delayed, and the window
of opportunity for this advantage is less than 16 h. This may
be because workers quickly acquire die odor of the new colony after their migration.
Doubling the worker number in a single-queen resident colony did not increase the chance that the colony would successfully resist a usurpation attempt, even though increasing
worker number increases the likelihood that a colony may win
a raid (Adams and Tschinkel, 1995b; Rissing and Pollock,
1987; Stamps and Vinson, 1991; Tschinkel, 1992b). However,
colonies founded by three queens successfully resisted usurpation in all but one trial. This shows that one advantage to
cooperative colony foundation in S. mvicta is the increased
ability of multiple-queen colonies to resist usurpation attempts. This is die first report of this phenomenon in ants,
although two cases of such an advantage gained by pleometrotic associations are known within the wasp genus PoHstts
(Gamboa, 1978; Klahn, 1988).
Usurpation success was not improved by the presence of a
migrating queen's biological daughters if the daughters were
unfamiliar with her. Although heritable cues have been implicated in recognition by workers of mature & mvicta colonies, they are neither necessary nor sufficient for acceptance
of queens in incipient colonies (Balas and Adams, 1996b). hi
addition, environmentally correlated cues play a much larger
role than heritable cues in recognition in mature fire ant colonies, and these cues are learned by the workers (Obin and
Vander Meer, 1988, 1989a,b). Nest mate recognition cues appear to be learned by incipient colony workers as well, because migrating queens had a usurpation advantage even if
their former nest mate workers were unrelated to them. This
Balas and Adams • Umrpadon in lire ants
adds to the large body of evidence that nest mate recognition
is based on cues learned after worker edosion (reviewed in
Breed and Bennett, 1987; see also Alloway and Ryckman,
1991; Breed and Julian, 1992; Errard, 1994; OWn and Vinder
Meer, 1988, 1989a,b; Stuart, 1988). Heritable cues may be involved in nest mate recognition (Carlin, 1989), but there is
little evidence that ants have the ability to recognize unfamiliar kin (but see Provost, 1991).
We are grateful to Walter Twhinkcl and the Department of Biological
Sdence at Florida State University for their assistance during the
course of this project Steven Kissing and Larry Wolf provided helpful
comments on the manuscript. This work was supported by a Fellowship in Science and Engineering from the David and LucUe Packard
Foundation to ZSJ
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