Behavioral Ecology Vol. 9 No. S: 301-308
Group defense by colony-founding queens in
the fire ant Solenopsis invicta
Christopher A- Jerome, Donald A- Mclnnes, and Eldridge S. Adams
Department of Biology, University of Rochester, Rochester, NY 14627, USA
Mutualistic associations among nonkin can form when animals in groups have a greater chance of overcoming challenges
presented by the environment than do solitary animals. Colony founding by small groups of unrelated queens, a habit
documented in several species of ants, is often interpreted as a mutualistic interaction selected by intense competition among
incipient colonies. However, many new colonies in these species are founded in areas where their chief enemies are mature
ant colonies, rather than other newly founded colonies. In this study, we tested whether group nest-founding in the fire ant
Solenopsis invicta improved the ability of queens to survive attacks by mature colonies. In the laboratory, queens in groups
of three were more likely than solitary queens to survive attacks by workers of the native fire ant Solenopsis grminata. When
newly mated queens were established experimentally in the field, workers from mature S. invicta colonies caused the majority
of queen deaths. Queens in groups of two, but not in groups of four, had higher survival rates than did solitary queens
during the period between colony establishment and the appearance of the first workers. The advantage of cooperative
defense approximately counterbalanced the disadvantages caused by competition within foundress associations of two to three
queens. Previous studies have shown that colonies founded by multiple queens produce larger worker populations than
colonies founded by solitary queens; however, experimentally increasing worker number in incipient colonies had no effect
on colony survival in the field. Key words: cooperation, group defense, pleometrosis, queen number, Solenopsis invicta.
[Behav Ecol 9:301-308
(1998)]
T
he structure of animal societies is shaped in part by two
conflicting forces. On the one hand, group formation is
favored when collective action improves the animal.*' abilities
to meet challenges presented by the environment. This can
lead to cooperation via "by-product mutualism," in which
benefits to other individuals arise as side effects of individual
selfishness (Dugatkin, 1997; Mesterton-Gibbons and Dugatkin, 1992; West-Eberhard, 1975). By-product mutualism requires neither genetic relatedness nor reciprocity among
group members. On the other hand, the proximity of animals
within groups fosters competition for limited resources or opportunities to mate, reducing the incentive for cooperation
or group membership. A major goal of studies on animal social behavior is to understand the conditions under which the
advantages of group living are sufficient to counterbalance
the disadvantages caused by selfish behavior among interacting animals.
Although social insects are well known as subjects for studies on kin selection, they also furnish examples of cooperation
among unrelated individuals (Bourke and Franks, 1995; Crozier and Pamilo, 1996; Holldobler and Wilson, 1990). In several species of ants, colonies are often founded by multiple
queens, a process known as pleometrosis (reviewed by Choe
and Perlman, 1997; Heinze, 1993; Herbers, 1993). Each
queen contributes to brood rearing and other tasks; however,
unlike the social wasps, ant co-foundresses are usually unrelated (Hagen et aL, 1988; Sasaki et al., 1996). Furthermore,
foundress associations in ants are usually temporary, lasting
only as long as it takes to rear the first few workers (e.g., Barn
and Holldobler, 1982; Rissing and Pollock, 1986; Tschinkel
and Howard, 1983). Cooperation among queens then breaks
down, and supernumerary queens are eliminated either by
Address correspondence to E. S. Adams, who U now at the Department of Ecology and Evolutionary Biology, University of Connecticut, 75 North EaglevUle Road, U-43. Stom, CT 06269-3043, USA.
Received 22 July 1997; accepted 10 December 1997.
O 1998 International Society for Behavioral Ecology
direct fighting or by attacks by the workers (reviewed by Heinze, 1993). Thus, any advantages due to pleometrosis must
stem from processes that occur early in the life of the colony,
although if the presence of multiple queens increases worker
production in young colonies, this may continue to produce
advantages much later in life.
Studies on cooperation have shown that the effects of group
behavior on individual fitness vary with ecological conditions
(e.g., Caraco, 1979; Packer et aL, 1990). Most previous studies
on the advantages of pleometrosis in ants have focused on
competition among young colonies, which often takes the
form of "brood raiding," or reciprocal stealing of brood (e.g.,
Adams and Tschinkel, 1995b; Bartz and H6Udobler, 1982; Rissing and Pollock, 1987). However, many colonies—perhaps
most colonies—are founded within or near the territories of
mature colonies, which may contain hundreds of thousands
of workers (HoUdobler and Wilson, 1990; Markin et aL, 1972;
Tschinkel, 1993b). Because mature colony workers attack newly mated queens and incipient colonies (Nichols and Sites,
1991), it is important to know how queen number affects colony survival in this context
We tested whether small groups of queens of the fire ant
Solenopsis invicta are better able than solitary queens to defend themselves from attacks by workers from mature colonies. There are two possible sources of defensive advantages
for queen groups. First, the queens themselves defend the
colony before the development of the first workers and
queens in groups may fight more effectively than solitary
queens. In laboratory and field experiments, we compared
the survival rates of solitary queens and of queens in foundress associations exposed to attacks by mature-colony workers. Second, the larger worker force produced by groups of
queens may improve colony defense, even after queen number is reduced to one. In a field experiment, we tested whether queen survival is enhanced by such an increase in worker
number. Finally, we comment on whether these defensive advantages are sufficient to favor the formation of foundress associations.
Behavioral Ecology VoL 9 No. 3
302
MATERIALS AND METHODS
Study apecies
After its introduction earlier this century from South America,
the fire ant SoUnopsis invitta Buren spread throughout the
southern United States and is currently abundant from Florida to Texas (Lofgren, 1986). Mature colonies of S. invicta
have two distinct social forms: monogyne, with a single queen,
and polygyne, with multiple queens (Ross and Fletcher,
1985a). The population of 5. invicta near Tallahassee, Florida,
where we conducted experiments, appears to be exclusively
monogyne. Haplometrosis, or colony founding by a solitary
queen, and pleometrosis occur in the same populations of S.
invicta (Tschinkel and Howard, 1983). Queen number in
newly founded colonies varies with the density of newly mated
queens and other ecological factors; however, newly founded
colonies average approximately 2.5 queens and commonly
contain 2-4 queens (Tschinkel and Howard, 1983). In the
monogyne social form, newly mated queens usually found colonies claustralry; that is, queens dig a chamber and seal themselves within the chamber for 2-7 weeks during the development of the first brood (Markin et aL, 1972; Tschinkel, 1992a,
1996). Workers edosing in the first brood are called minims,
due to their small size, and are behaviorally distinct from the
workers of mature colonies (Balas and Adams, 1996b; H61ldobler and Wilson, 1990). Colonies often engage in brood
raiding during the first 10 weeks of postdaustral development,
particularly when colonies are close together (Adams and
Tschinkel, 1995a,b; Tschinkel, 1992a,b). Development of permanent foraging territories follows the period of brood raiding (Tschinkel, 1992a; Wilson et aL, 1971).
Experiment 1: predation by SoUnopsit geminata in the
laboratory
In die first experiment, we exposed 5. invicta queens, singly
and in groups of three, to predation by laboratory colonies of
the native fire ant SoUnopsis geminata. S. gettdnata is a common ant in northern Florida that competes with S. invicta
(Mclnnes, 1994). Newly mated & invicta queens, collected
from Tallahassee, Florida, parking lots, were placed into polystyrene cluster culture trays (Bioquip; Gardena, California).
Each tray contained 24 compartments in 4 rows of 6. Alternate
compartments received an inaer sloeve of plastic tubing (9.5
mm inner diam) that reduced the cell volume by two-diirds.
To hold moisture in die compartments, dental plaster was
added to each compartment to a depth of approximately 1.5
mm. The final compartment volumes were 3.2 ml and 1.1 ml.
We placed three queens into each large compartment and
one queen into each small compartment. Rubber bands secured the lids of the culture trays. To allow the passage of 5.
geminata workers, we drilled a 0.8-mm hole in the lids at the
center of each compartment.
Each culture tray, containing 12 groups of 3 queens and 12
solitary queens, was set on a ring stand of either 20 or 30 cm
and placed into a photo tray containing a S. geminata colony.
We placed culture trays one or two at a time into the 5. geminata colonies. The & geminata colonies ranged in size from
5000 to 20,000 workers and had been maintained in die laboratory for at least 9 mondu. Before die experiment, colony
diets consisted of sugar, crickets, beede larvae, and a crushed
bird sead mixture. We presented 26 culture trays (1244
queens) to 22 & geminata colonies. As controls, we reared S
trays of queens (144 queens) without exposing them to predadon.
All stages of die experiment were conducted at 29°C We
censused die trays and gave die queens water dairy. The plaster is each compartment was moistened by injecting.water
through the lid openings. We recorded die number of queens
alive in each compartment at each census, until all of die
queens were dead.
Field experiment
Three experiments examined die effect of predation by mature colonies on the survival of incipient S. invicta colonies
in die field. We conducted our experiments on cattle pasture
at Southwood Farms near Tallahassee, Florida. Territories of
mature S. invicta colonies covered die study sites, except for
narrow gaps between adjacent boundaries. Newly mated
queens were collected several kilometers from die study site.
In all experiments, queens were placed in artificial nest chambers made of transparent 15-ml centrifuge tubes, which were
13 cm long when capped (Lab Products; Rochester, New
York). The walls of each tube were pierced by 50-75 holes
large enough to permit passage by workers but too small to
allow queens to escape. Hole diameter was 1.1 mm in experiment 3 and 0.8 mm in experiments 2 and 4. We filled each
tube with 7-10 ml of locally collected, gray soil in which
queens constructed brood chambers. Each tube was capped.
In die field, nest chambers were buried in holes so that the
top of die cap was 1-3 cm below die surface of die soiL Fire
ant workers forage below ground (MacKay et aL, 1991) and
could enter these tubes through die numerous holes in die
side walls, similar to die manner in which they normally prey
on founding queens. We marked each colony location with a
wooden stake. Nest tubes were planted in pairs, as described
below. We planted experimental colonies within 4 m of mature colonies to ensure that experimental colonies were in
areas foraged by S. invicta workers. The volume of a fire ant
colony correlates closely with die biomass of workers in die
colony (Tschinkel et aL, 1995). We selected mature colonies
that were of moderate size (mound volume ranged from 6 3
1 to 52 1; mean 22 1) and that lacked neighbors within 4-6 m.
We collected data on queen or colony status by periodically
removing the artificial nests from die ground and visually inspecting the contents. When queens were not visible, the tube
was gently shaken to displace soil until at least one live queen
was sighted. In cases where one colony was greatly disturbed,
we disturbed die paired colony in a similar manner. Colony
death was scored when no queens remained alive. We censused colonies in experiments 2 and 3 until 19-21 days after
planting, which was when die first workers completed development. The last experiment, which had workers present
from die start, was followed for die same length of time.
Experiment 2: two-queen groups venus solitary queens in
the field
Experiment 2 compared die survival of solitary queens and
two-queen groups in the vicinity of mature fire ant colonies.
Newly mated queens were collected within hours of a mating
Sight on 5 June 1996 and were planted die next day. Queens
were randomly assigned to treatment groups. We planted 71
replicate pairs around 6 mature colonies. Each replicate pair
was planted at a randomly determined angle and distance between 0 3 and 4 m from die nest of a mature colony and
consisted of a two-queen colony and a one-queen colony separated by 20 to SO cm. Replicates were separated by at least
20 cm. We examined colonies at 3-day intervals between days
3 and 9 and thereafter at 2-day intervals until day 19. Colonies
were left undisturbed for an additional 24 days, then removed
and censused. During die experiment, we noted when dead
queens were spotted within each chamber.
Jerome et aL • Group defense by fire ant queens
Experiment 3: four-queen groups versos solitary queens in
the field
Experiment 3 compared the survival of four-queen groups
with the survival of solitary queens. Queens were collected
after a mating flight on 21 May 1996. Due to unusually dry
conditions at the field site, planting was delayed 9 days until
just after a heavy rain to prevent desiccation of the queens.
During this 9-day period, queens were refrigerated (at approximately 9°C) to delay egg laying and physiological development We randomly assigned queens to nests singly or in
groups of four. Replicate pairs, consisting of one four-queen
and one single-queen colony, were planted at a random distance (ranging from 0.5 to 4 m) and angle from the center
of a selected mature nest. The two colonies in each replicate
pair were planted 20-30 cm apart, and pairs were separated
by at least 20 cm. We planted 119 replicate pairs (238 colonies) around 12 mature colonies. We censused experimental
colonies on alternate days, starting on the first day after planting and ending on day 21. At the end of the experiment, we
emptied the chambers and counted the remaining live
queens.
Experiment 4: worker number and predation
Experiment 4 tested the effect of worker number on colony
survival within the foraging territories of mature S. invicta
colonies during die period immediately after supernumerary
queens were eliminated. Newly mated queens were collected
on 21 May 1996 then sealed individually in test tubes with a
source of water. To stagger colony development, half the
queens were kept in a heated room (29 ± 1°C) for the whole
period before planting and half were kept at a cooler temperature (approximately 22°C) for 12 days and then placed
in the heated room. About 20% of die matings in the North
American population of 5. invicta produce some diploid male
progeny in the first brood (Ross and Fletcher, 1985b). We
discarded queens producing diploid males and unhealthy or
unfertilized queens. After 30 days, many of the first brood
were about to edose from the pupal stage.
In most laboratory colonies with multiple queens, queen
number is reduced to one within 3 weeks after edosion of die
first workers (Balas and Adams, 1996a), and the surviving
queen inherits the workers and brood produced by the group.
To create colonies with increased worker number, the brood
produced by two queens, selected randomly 1 day before
workers edosed, was combined into one queen's nest tube
(worker-supplemented queen). The remaining queen was discarded. Simultaneously, we randomly selected control queens,
whose worker number was not altered. All three queens had
been maintained at the same temperature. After workers began edosing, colonies were transferred to die perforated nest
tubes and planted in replicate pairs. Each replicate pair consisted of one control colony and one worker-supplemented
colony. The numbers of workers and pupae were counted in
30 colonies from the experimental group and 30 control colonies 2 days before planting. Because four-queen groups of S.
invicta queens produce approximately as many workers as two
solitary queens (Tschinkel and Howard, 1983), the number
of workers in a worker-supplemented colony was approximately equal to die number of workers reared by a pleometrotic group of four queens.
We planted 119 replicate pairs around 12 mature colonies
in die field. Paired colonies were planted at least 105 cm apart
to inhibit brood raiding between die incipient colonies (Adams and Tschinkel, 1995a). Two replicates were planted at a
distance of 1 m from each mature colony. Four pairs were
planted 2.5 m from the mound, and four pairs were planted
303
4 m from die mound. One of die 2.5-m pairs was lost during
planting. At each distance, we placed the first pair at a randomly determined angle and placed diree other pairs at 90°
arcs around die mound at the same distance from the center
of die mound. Only two pairs were placed at a distance of 1
m to allow at least 105 cm separation between planted colonies. We censused colonies every third day starting on day 3
and ending on day 21.
Reactions of mature-colony workers to incipient colonies
To observe die interactions of workers from mature colonies
and workers and queens in incipient colonies, 10 test tubes
containing incipient colonies were opened and placed individually in die nest tray of 1 of 7 mature laboratory colonies
of S. invicta. We observed interactions for evidence of fighting
or brood raiding after 5 min, after 1 h, and after 16 h.
Statistical analysis
Because of die brief and variable duration of die laboratory
trials, we compared die survival rates of solitary queens and
three-queen groups at 24 h from die start of each experiment
and at die point when 50% of queens had died. For die laboratory study, die proportions of haplometrotic and pleometrotic queen survival for treatment queens were compared using one-tailed Wilcoxon signed-ranks tests. Because of die
small sample size, control queen survival rates were compared
using Z tests of die difference between proportions. We used
a chu-square test to compare die observed distribution of surviving queen number at die 50% mortality point with die distribution of surviving queen number expected if queen deaths
are independent events. The expected distribution was generated from die binomial probability distribution using die
total proportion of queens surviving in die pleometrotic
groups.
We used methods described by Dawson-Saunders and Trapp
(1994) to summarize and compare colony survival rates. Survival curves and confidence intervals were calculated using
actuarial methods, which account for periodic sampling. Linear regressions on log-transformed data were used to test die
hypothesis that exponential deadi rates described die colony
survivorship data. To be consistent with die paired-sample design, when die fate of one colony in a replicate pair was unknown, information on die fate of die other colony was not
used to calculate survivorship and regression curves. Actuarial
methods account for lost samples by using information up to
die day when die sample was lost. When exponential curves
closely fit survivorship data, die mortality schedules can be
summarized by die rate of death per colony per day. The number of colony deadis observed divided by die number of days
colonies were observed alive gives die rate of death per colony
per day. The reciprocal of this number is die mean survival
time. Proportions of queens alive in treatment groups at die
end of each field experiment were compared using Z tests of
die difference between proportions. To test for independence
of queen mortality, die proportion of foundresses alive in multiple-queen chambers at die end of die claustral period in
experiments 2 and 3 was used to calculate die expected distributions of surviving queen number (see above).
In experiments 2 and 3, we planted colonies at randomly
chosen distances from selected mature colonies (see above).
Logistic regression analysis was used to test for an effect of
this distance on die probability of colony survival to the day
when 50% of colonies had died, hi experiment 4, we planted
colonies at one of diree distances from mature colonies. The
chi-square test was used to test for an association between distance and probability of survival to die 50% mortality point
Behavioral Ecology Vol. 9 No. 3
304
150
• Expected
D Observed
w
a. 120
3
2
"o
CD
.Q
90
O)
c
CO
c
o
'•c
o
60
E
3
• O 1 Queen
OL
o
30
• 2 Queen
0.1
0
1
Surviving queen number
Figure 1
Frequency distributions of the final number of queen* alive in
groups founded by three queens. The significant difference
between the observed and expected values indicates that queen
deaths were not independent events.
RESULTS
Experiment 1: predation by Solenopsis gemmata in the
laboratory
The holes in the chamber lids allowed entry of all but the
largest S. geminata workers and prevented escape of the 5.
invicta queens. After 24 h, 66.6%±6.1% (mean ± SE) of the
queens in pleometrotic groups were alive. A significantly lower
proportion (43.3% ±7.3%) of the solitary queens survived the
first 24 h (Z = -3.54; p = .0004; Zcorrected for ties). More
of the pleometrotic queens also survived until the census
nearest to 50% mortality. At these censuses, 45.3% ±4.7% of
the pleometrotic queens were alive, while only 16.5%±3.2%
of the solitary queens survived (Z = -3.99; p - .0001). At
the 50% mortality point, the surviving queens were significantly clumped in groups compared with the expected distribution of surviving queen number determined by the binomial probability distribution (Figure 1; x* = 1^2; 2 dt, p <
.001).
In most of the predation trials, S. gtminata workers killed
half of the queens within the first 2 days. On average, half of
the queens were dead by 42 h (±8.2 h) after a trial was initiated. In the control trials, in which the queens were not
exposed to predation, the 50% mortality point occurred 334
h (±140 h) after trials began. Pleometrosis had no effect on
the survival rates of control queens. In the absence of predation, 46.3%±3.3% of the pleometrotic queens and
55.6%±11.1% of the solitary queens were still alive at the 50%
mortality point (two-tailed Z test of the difference between
proportions; Z - 0.97; p - .34). After 24 h, 94.4% ±2.8% and
88.9% ±11.1% of the pleometrotic and solitary queens, respectively, reaaaiaod alive in the controls (Z = 1.12; p m .26).
Experiment 2: two-queen groups versus solitary queen* in
die field
Exponential curves closely fit colony survivorship data for
both one- and two-queen groups (Figure 2; one-queen colo-
3
i
\
6
9
i
i
r
12 15 18
Day
Figure 2
Survivorship curves for experiment 2. At the end of the
experiment, survival rates for two-queen colonies were significantly
lower than survival rates of one-queen colonies (n ~ 71 replicate
pairs). Dashed lines are 95% confidence intervals.
nies: i? •= .98; 8 df; p < .001; two-queen colonies: Ff = .91;
8 df, p < .001). Since there was no significant effect of distance to the mature colonies on the probability of survival for
either two-queen colonies (logistic regression; G =» 0.78; 1 df,
p = .38) or for one-queen colonies (G = 1.80; 1 df, p = .18),
further analysis ignored distance. Survivorship curves in all
three field experiments could be approximated by the following exponential model:
log. (proportion surviving) = rD,
(1)
where r is a constant and D is time measured in days. The
value of r was estimated by the slope of the regression of logtransformed survivorship against D (Table 1). Regression
slopes for two-queen colonies were significantly lower than
slopes for one-queen colonies (one-tailed t test; t = 5.85; 18
df; p < .001) indicating a higher rate of survival for two-queen
groups. At the end of the experiment, the proportion of twoqueen colonies surviving was significantly greater than the
proportion of one-queen colonies surviving (one-tailed Z test
of the difference between proportions; Z ™ 2.73; p = .003).
The ratio of two-queen group to one-queen group survival at
the end of the experiment was 1.67 (95% (3 = 1.13-2.47).
Individual queen mortality was lower in two-queen colonies
compared with solitary foundresses (one-tailed; Z " 3.02; p •=
.001). One nest chamber was lost during the study due to
damage by cattle. For queens with known fates, 79 of 140
(56%) queens in two-queen colonies survived, while 24 of 71
(34%) solitary queens survived to day 19. The ratio of the
proportion of pleometrotic queens that survived to the proportion of solitary foundress that survived was 1.63 (95% (3
™ 1.14-2.33). The distribution of the number of pleometrotic
queens alive per colony at the end of expenaoat 2 differed
significantly from that expected under the hypothesis of independent queen H^»tti« with an excess of colonies with both
queens alive and colonies with both queens dead (Figure 3;
X* •» 513; 1 df; p < .001. Table 1 mimmariTM survivorship
statistics for all three field experimenu.
Jerome et al. • Group defense by fire ant queens
305
Table 1
Survival statistics for field experiment* on group defense in incipient colonies
Workers
and
pupae
n
Proportion
surviving*
1
2
0
0
71
71
0.34
0.57
1
4
0
0
119
119
0.06
0.12
119
119
0.16
0.18
Queen
number
Experiment 2
Haplometrouc
Pleometrotic
Experiment 3
Haplometrotic
Pleometrotic
Experiment 4
Control
Worker-supplemented
1
1
21 (5.3)
38 (8J)
Regression
slope"
P
00H
0
lfi
nifl
U.12
Mean survival
time' (days)
P
-0.054 (0.002)'
-0.035 (0.003)
1
no
.20
(per day)
„.
16.02 (0.58)
34.80 (1.08)
-0.134 (0.005)
-0.102 (0.007)
-0.081 (0.004)
-0.077 (0.003)
Death rate"
p
6.20 (0.28)
11.47 (0.34)
W1
9.43 (0.31)
10.03 (0.32)
31
,.
0.062
0.029
0.16
0.087
0.11
0.10
* Proportion of colonies surviving; to 19 days for experiment 2; 21 days for experiments 3 and 4.
Slope from linear regressions on log-transformed survivorship data.
c
Assumes exponential death rate. Calculated from the death rate; see Dawson-Saunders and Trapp (1994).
d
Assumes exponential death rate. Probability of death per colony per day, see text and Dawson-Saunders and Trapp (1994).
• SEs in parentheses.
b
Experiment 3: four-queen groups versus solitary queens in
the field
There was no significant effect of distance to mature colonies
on the probability of survival either of four-queen groups (logistic regression; G = 1.00; 1 df;; p = .32) or of solitary queens
(G = 0.18; 1 df; p = .67). At the end of experiment 3, fourqueen group survival did not differ significantly from solitary
foundress survival (two-tailed; Z = 1.39; p = .16). However,
over the whole experiment, regressions on the colony survivorship data indicated that the death rate for four-queen
groups was lower than for lone foundresses (Figure 4). The
survivorship curves of one-queen and four-queen colonies fit
exponential curves (one-queen: / ? =• .99; 11 df; p < 0.001;
four-queen: Ff = .95; 11 df; p < .001). The slope for fourqueen colony survivorship was not as steep as the slope for
one-queen colonies (Table 1; one-tailed t test; t = 5.07; 22 df;
p< .001).
Due to the size of the holes in the centrifuge tubes, 37
colonies were lost when queens escaped. The analysis of survivorship curves assumed paired samples, but allowed information on a pair to be used up to the date when either colony
was lost (see Materials and Methods). Because the comparison
of the percentage of queens surviving to the end of the experiment did not assume paired samples, we used all colonies
for which we were confident that no queens escaped in order
to maintain the largest appropriate sample size (112 fourqueen colonies and 87 single-queen colonies). For these replicates, we found no difference between per-queen survival in
groups compared with solitary queen survival at the end of
1.00
50
Expected D Observed
£40
30
20
O 1 Queen
10
4 Queen
E
i
0
0
i
i
12 15 18 21
1
Surviving queen number
Figure 3
Frequency distributions of the final number of queens alive in
groups founded by two queem. The observed distribution of
surviving queen number differed significantly from the random
expectation.
Day
Figure 4
Survivorship curves for experiment 3 (n => 119 replicate pairs).
Dashed lines are 95% confidence intervals. Survivorship rates
differed significantly during the experiment; however, the
proportion of colonies surviving at the end of the experiment in
the two groups did not differ significantly.
Behavioral Ecology Vol. 9 No. 3
306
100
• Expected
• Observed
a. 80
5 60
40
20
Worker-supplemented
0.1
0
1
Surviving queen number
Figure 5
Frequency distributions of the number of queens surviving in
originally fdur-queen groups at the end of experiment 3. The
significant difference between the observed and expected values
indicates that deaths were not independent events.
0
i
i
3
6
i
\
i
i
9 12 15 18 21
Day
Figure 6
Doubling the number of workers in small colonies had no effect on
the survival rate of solitary queens (n ~ 119 replicate pairs).
Dashed lines represent 95% confidence intervals.
Penetration of nest chambers by mature-colony workers)
the experiment (two-tailed; Z = 0.102; p = .92). Only 29 of
448 (63%) queens in four-queen colonies survived, and only
5 of 87 (5.7%) single foundresses survived to day 21. However,
queen deaths in four-queen colonies were not independent
occurrences. Figure 5 shows the distribution of surviving
queen number per colony at the end of the experiment (day
21). This distribution differed significantly from the random
expectation (x* = 59.0; 1 df; p < .001), indicating that when
one or more queens died within a colony, there was an increased chance that the remaining queens died.
Experiment 4: worker number and predation.
Manipulating woker number did not affect queen survival
rates (Figure 6). Two days before die colonies were planted
in the field, control queens averaged 21 ±5.3 minims and pupae. Worker-supplemented queens averaged 38±8.S minims
and pupae ( n = 3 0 for both control and worker-supplemented groups). There was no significant association between distance to the closest mature colony and probability of survival
of experimental colonies (x* •• 1.40; 2 df, p = .50). Survivorship of control colonies and of worker-supplemented colonies
fit exponential curves (control colonies: B? = .99; 7 df; p <Z
.001; worker-supplemented colonies: (F? «- .99; 7 df; p < .001)
Regression slopes for the two groups did not differ significantly (one-tailed t test; * = 1.11; 14 df; p » .29). Five nest
chambers were lost during the experiment due to damage by
cows.
Following most queen deaths (n ™ 193), the dead quoon
was found without live brood or minims. In 37 cases, a live
queen was found without minima or brood. Thirty-three of
these queens died during the following 6-day period. This sequence of worker and brood loss followed by death of the
queen is consistent with brood raiding and attack by maturecolony workers (see below).
Queens placed in the artificial nests dug chambers and laid
their brood in a common pile, as seen in naturally founded
colonies (Tschinkel and Howard, 1983; Tschinkel 1993a).
Minims, distinguishable from mature-colony workers by their
smaller size, were seen burrowing in and foraging outside of
dieir pest chambers. Workers from mature colonies investigated the sites of some experimental colonies within 1 min of
planting.
Three lines of evidence showed that mature-colony workers
attacked queens in the field experiments. First, die nest chambers were filled with gray sand, and die soil in die study area
was red. When workers burrowed into die nest chambers from
die surrounding soil, visible trails of red soil indicated their
activity. Second, we observed four instances of workers biting
live or dead queens during colony censuses. Third, fighting
between experimental groups and mature-colony workers often left fragments or bodies of workers in die nest tubes. Of
die 268 colony deaths observed in experiments 2 and 3, 71%
of die nest chambers contained fragments of mature-colony
workers and burrows into the sand; another 17% showed evidence of predation such as dismembered queens. In 13 of 83
(16%) cases when at least one queen survived to die end of
die experiments, chambers contained evidence of predation,
indicating that some queens survived attack. Of the 193 colony deaths in experiment 4, 65% of die nest chambers contained worker fragments or dismembered queens, though
counts were made conservatively to avoid counting fragments
of minims as fragments of mature-colony workers.
Reaction of mature-colour workers to
colonies
Workers from mature colonies raided die brood from die incipient colonies in all 10 laboratory trials. Workers attacked
die minim* and queen in die incipient colonies widiin 5 min
of discovery. Minims always attacked die invaders and, in eight
cases, die queen abandoned die nest tube. Workers removed
S07
Jerome et al. • Group defense by fire ant queens
the brood from the incipient colony within 1 h in seven of
the replicates and within 16 h in the remaining three cases.
DISCUSSION
Newly founded colonies suffered high rates of mortality due
to attacks by mature colonies. Before the first workers completed development, survival rates of colonies initiated by two
to four queens exceeded survival rates of solitary queens. This
is expected even if queen mortality is unaffected by group size
because all the queens in the group must die for the p l e o
metrotic colony to be killed (Queller, 1996). However, the perqueen survival rate was higher for pairs of queens in the field
and three-queen groups in the laboratory than for solitary
queens. In contrast, when they are not exposed to predation,
solitary 5. invicta queens and queens in groups are equally
likely to survive the claustral period (experiment 1 of this
study; Tschinkel and Howard, 1983; Tschinkel, 1993a). Because queens fought with invaders and some queens survived
attacks, we attribute the increase in survival rates for pleometrotic queens to group defense.
If die probability of death per day is equivalent for all
queens in groups of a particular size, then the distribution of
the number of queens per group surviving to the end of that
period will fit the binomial distribution. The observed distributions of the number of surviving queens had higher variances than expected for binomial distributions in experiments
1, 2, and 3 (Figures 1, 3, and 5), indicating that queen mortality rates were greater for some groups than for others.
There are at least two possible causes of this deviation. First,
if the defensive ability of a group increases with the size of
the group, then when some queens die, the remaining queens
in the group are more likely to die. Second, heterogeneity in
the rate of attacks, or other sources of mortality, between nest
sites can cause clumping of queen mortality. We suspect that
both mechanisms contributed to the patterns of mortality observed in the laboratory and field experiments.
In experiment 3, the survival rates for queens in groups of
four did not differ significantly from survival rates of solitary
queens. The large difference between solitary-queen survival
rates in experiments 2 and 3 suggests that environmental conditions varied considerably between experiments (see Figures
2 and 4). Unfortunately, die magnitude of this variation confounds direct comparison of the survival rates of queens in
two- and four-queen groups. In a similar study in which workers from S. invicta colonies preyed upon newly mated queens,
MacKay and co-workers (1991) also found high rates of predation and variation in rates of attack due to environmental
conditions. Workers from mature monogyne and polygyne 5.
invicta colonies found up to 100% of individual queens
placed in 2-ml cryogenic tubes that were punctured nine
times and planted directly in and around die mounds of mature colonies; however, die frequency of attack varied, probably due to changing soil moisture conditions. Experiments 2
and 3 were conducted far enough apart (approximately 200
m) that local changes in soil moisture, topography, or vegetation could account for die difference between solitary
queen survival rates in die two experiments.
The increased survival rate of queens initiating colonies in
small groups does not necessarily favor pleometrosis because
only one queen per group survives to reproduce. In die case
of pain of queens, for example, a queen is favored to join
another queen during colony founding if die survival rate of
queens in pairs is more dian twice die survival rate of solitary
queens (Bartz and H6Udobler, 1982). Our estimate of die ratio of survival of queens in pairs to survival of solitary queens
was 1.67, and die confidence interval extended above two. In
die laboratory, groups of diree queens had survival rates diat
were diree times higher dian diose of solitary queens. Thus,
die advantages of group defense against workers from mature
colonies approximately counterbalance die costs of competition among co-foundresses. However, die magnitude of die
payoff is variable and may often fall short of complete compensation.
In general, animals are expected to cooperate when die
combined benefits of group activity outweigh die costs of widiin-group competition (e.g., Packer et aL, 1990; Wade, 1977).
Solitary S. invicta queens often establish new colonies successfully. Nonedieless, die benefits of joining a group may favor pleometrotic behavior in some ecological circumstances.
If queens join odler queens diat have begun to excavate burrows after die mating flight, diey may get below ground faster,
avoiding predation and water stress (Pfennig, 1995). During
brood raiding, colonies widl high worker number usually have
an advantage over smaller colonies (e.g., Adams and Tschinkel, 1995b; Bartz and Holldobler, 1982; Rissing and Pollock,
1991; Tschinkel, 1992b). Pleometrotic groups are more successful dian solitary foundresses at resisting usurpation by
queens attempting to relocate during brood raids (Balas and
Adams, 1997). Finally, under conditions odler dian brood
raiding, or later in development, colonies may benefit from
having a large number of workers early in development
(Tschinkel and Howard, 1983). One possible benefit of having
a large worker force early in development is diat die colony's
ability to resist predation is increased. We tested diis in our
last experiment.
Doubling die number of workers in small colonies did not
affect queen survival. Although workers from incipient and
mature colonies fought, increased worker number may have
had little effect because all die experimental colonies were
small compared to mature colonies, which contain many
diousands of workers (Marian et al., 1972; Tschinkel, 1993b).
Colonies diat are initially larger probably outgrow smaller colonies (Tschinkel and Howard, 1983) and gain odler demographic advantages, such as earlier reproduction (Vargo,
1988); however, dlis is difficult to test in die field due to heavy
mortality in young colonies (Tschinkel, 1992a) and because
colonies occasionally relocate (Hays et al., 1982).
We gratefully acknowledge Walter Tschinkel for the invaluable use of
his laboratory and the assistance of his graduate students. We thank
Florida State University for the use of facilities and Southwood Farms
for the ute of their land as a field site. Deby Cassill gallantly collected
queens, and Tobin Foster and Keith Mason provided excellent help
in the field. This research was funded by a Fellowship in Science and
Engineering to E.SA. from the David and Lucile Packard Foundation.
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