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Intraspecific Thievery in the Ant Ectatomma ruidum is Mediated by Food Availability

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Animals modify their foraging strategies in response to environmental changes that affect foraging performance. In some species, cleptobiosis represents an alternative strategy for resource access. The environmental factors that favor the incidence or prevalence of cleptobiosis, however, are poorly described. The cleptobiotic Neotropical ant Ectatomma ruidum is characterized by a high frequency of thievery behavior, a specific type of intraspecific cleptobiosis, in which specialized thief workers insinuate themselves into nests of neighboring colonies and intercept food items brought into these nests. Here, we evaluate how colonies adjust thievery behavior in response to food availability. We supplemented food availability and measured how the incidence and intensity of thievery responded to resource availability. We found that the incidence and intensity of thievery decline in response to supplemental food, suggesting that thievery behavior is a response to resource limitation at the population scale. This finding indicates that the phenomenon of intraspecific thievery, although a rare strategy in among colonies of social animals, is a viable alternative foraging tactic in the context of competition and food limitation.
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Intraspecific Thievery in the Ant Ectatomma ruidum is Mediated by Food Availability
Benoit Gu!
enard
1,2,4
,and Terrence P. McGlynn
3
1
Okinawa Institute of Science and Technology, Okinawa, Japan
2
Department of Biology and the Keck Center for Behavioral Biology, North Carolina State University, 127 David Clark Labs, Raleigh, NC,
27695, U.S.A.
3
Department of Biology, California State University, Dominguez Hills, 1000 E. Victoria Street Carson, CA, 90747, U.S.A.
ABSTRACT
Animals modify their foraging strategies in response to environmental changes that affect foraging performance. In some species,
cleptobiosis represents an alternative strategy for resource access. The environmental factors that favor the incidence or prevalence
of cleptobiosis, however, are poorly described. The cleptobiotic Neotropical ant Ectatomma ruidum is characterized by a high frequency
of thievery behavior, a specic type of intraspecic cleptobiosis, in which specialized thief workers insinuate themselves into nests of
neighboring colonies and intercept food items brought into these nests. Here, we evaluate how colonies adjust thievery behavior in
response to food availability. We supplemented food availability and measured how the incidence and intensity of thievery responded to
resource availability. We found that the incidence and intensity of thievery decline in response to supplemental food, suggesting that
thievery behavior is a response to resource limitation at the population scale. This nding indicates that the phenomenon of intraspecic
thievery, although a rare strategy in among colonies of social animals, is a viable alternative foraging tactic in the context of competition
and food limitation.
Abstract in Spanish is available in the online version of this article.
Key words: cleptobiosis; Ectatomma ruidum; food supplementation; foraging behavior; Formicidae; thievery.
FOOD RESOURCE AVAILABILITY CAN BE UNPREDICTABLE IN SPACE AND
TIME,WHETHER FOR HUNTER-GATHERER HUMANS, dung beetles, or
ants. This unpredictability has led to the evolution of strategies to
maximize the benet of foraging and to minimize the cost associ-
ated with foraging. Costs include time and energy spent during
foraging (Fewell 1988, McGlynn et al. 2003), risk of predation
(Nonacs 1990), exposure to environmental hazards (Cerd!aet al.
1998), and interference competition (Traniello 1989). Because of
these costs, natural selection acts heavily on strategies to maxi-
mize food discovery and retrieval, and in some cases, the evolu-
tion of alternative foraging tactics involving less foraging effort
and risk may be favored (e.g., Mathot & Giraldeau 2008).
In social insects, foragers collect food items within a home
range and bring them to the nest. This leads to central-place for-
aging, where the central placeis the nest where successful for-
agers converge with food resources. While central place foraging
may ameliorate the unpredictability of food resources (McGlynn
et al. 2003), it also may incur costs. Specically, sedentary nests
are potentially susceptible to parasitic species that, rather than
forage for food, exploit the aggregation of resources within nest
sites (Buschinger 1986). Cleptobiosis is a particular form of social
parasitism, in which thieves intercept the food collected by
workers at areas around the nest or on trails used by foragers
(Holldobler 1986, Richard et al. 2004, LaPierre et al. 2007, Breed
et al. 2012) and, in doing so, reduce their own foraging costs
(Curio 1976, Passera & Aron 2005). In ants, this behavior has
been described several times at an inter-specic level (Holldobler
1986, Perfecto & Vandermeer 1993, Espadaler et al. 1995, Gras-
so et al. 2004, Richard et al. 2004). In at least two cases, however,
intraspecic cleptobiosis has been observed, in Messor aciculatus
(Fr. Smith) in Japan (Yamaguchi 1995) and in the Neotropical
Ectatomma ruidum (Roger) (Breed et al. 1990, 1992, 1999). A key
difference between cleptobiosis between species and that within
species, is that intraspecic cleptobiosis can be symmetrical,
which is to say that any given colony can be the villain and the
victim.
Ectatomma ruidum (Formicidae, Ectatomminae) is widespread
throughout the Neotropics, from Mexico to Brazil (Fern!andez &
Sendoya 2004). Colonies of this species are monodomous (single-
nested) and monogynous, containing 50300 workers (Lachaud
et al. 1984, Breed et al. 1990, 1992, 1999, Schatz & Lachaud
2008, Lenoir et al. 2011). Nests have a single entrance, about
34 mm wide dug in the ground (Lachaud et al. 1984, Breed
et al. 1990). This species exhibits a unique form of intraspecic
cleptobiosis, which has been called thievery behavior, in which a
worker of one nest will enter a neighboring nest of the same spe-
cies, wait within the foreign nest, intercept food brought by a for-
ager, and then leave the foreign nest with the food and bring it
to its own nest (Breed et al. 1990, 1992, 1999). While resident
E. ruidum workers tend to repel foreign workers outside of their
nests and territories without injuries (Breed et al. 1990, 1992),
thieves are able to inltrate colonies through chemical camouage
and no specic aggressiveness is directed toward them (Breed
Received 22 August 2012; revision accepted 18 November 2012.
4
Corresponding author; e-mail: zeroben@gmail.com
ª2013 The Author(s) 497
Journal compilation ª2013 by The Association for Tropical Biology and Conservation
BIOTROPICA 45(4): 497–502 2013 10.1111/btp.12031
et al. 1992, Jeral et al. 1997). Thievery behavior is common within
populations of E. ruidum and has been noted at several locations
within the speciesrange (De Carli et al. 1998, Breed et al. 1999).
To date, studies of thievery have focused on a precise
description of the behavior (Breed et al. 1990, 1992, 1999, Jeral
et al. 1997) and the ecological consequences on foraging and nest
distribution for the different protagonists (Yamaguchi 1995). Lit-
tle is known about the factors that inuence the frequency or
even simply occurrence of intraspecic cleptobiosis in ants. In
1990, Breed and collaborators hypothesized that thievery within
populations of E. ruidum might be a facultative response to high
colony densities or to food competition. Later on, it was shown
that the rate of thievery was independent of nest density (Breed
et al. 1999), but the possibility that thievery rates are related to
food supplies remains untested. Here, we hypothesize that food
supplementation will modify the intensity of thievery observed in
a population of E. ruidum. In this study, we modied food avail-
ability with daily additions of food and measured the foraging
responses of E. ruidum. We expect that, as a result of this manip-
ulation, colonies receiving supplemental food will experience a
reduction in the rate of thievery, as well as lower rates of thievery
relative to surrounding nests that do not receive supplemental
food.
METHODS
SITE LOCATION AND DESIGN.Work was conducted in the arbore-
tum of La Selva Biological Station, Heredia Province, Costa
Rica in June 2008. Six sites were selected. Each site had an area
of 452 m
2
, comprised of a circle with a 12 m radius. Each plot
was separated from one another by at least 50 m, from edge to
edge, far greater than the maximal foraging distance of any E.
ruidum worker. For each site, a focal nest of E. ruidum was
located at the center of a 5 m radius circle in the middle of
the site and corresponded to the treated area receiving food
supplementation. Each inner circle was delimited with agging
and each plot was exhaustively searched (for a minimum of
twenty person-hours) until all colonies of E. ruidum were found,
following a protocol involving direct searching for nests, feeding
foragers, and baiting, as conducted in previous work (McGlynn
et al. 2010). Nests outside the inner circle, when discovered,
were also marked (up to 12 m from the center) (Fig. 1). During
the food supplementation, any additional nests that were
detected were marked.
PRE-TREATMENT ASSAY.Forty-eight hours after we completed the
establishment of sites with marked nests, six to seven study nests
per site were randomly selected to measure the rate of thievery.
Three to four nests were located within the inner circle areas,
and two to three were located outside the inner circle. In total,
twenty nests within treatment areas were measured for thievery
(including the focal nests), and nineteen were measured outside
treatment areas.
For each nest, ten food items (4-mm long cylinders of con-
densed white bread) were presented at the nest entrance. Over a
10-min duration, the number of items collected outside of the
focal nest was recorded. Three different events were observed
and recorded: (1) Collecting eventthe food item was collected
by a resident worker and brought within the focal nest; (2) Col-
lecting and thievery eventsthe food item was collected and
brought by a resident worker within the focal nest, but then inter-
cepted within the focal nest by a thief which left the focal nest
and transport the food item into a different nest; and (3) Exploit-
ing eventthe food item was collected by a worker from a for-
eign nest (later qualied as exploiters) which transport it to its
nest. The number of thieves departing the nest with the supple-
mental food was recorded and followed (by a second observer)
to the destination nest. For each thievery event, distances
between origin nests and destination nests were measured. For
each study nest, we calculated the proportion of food collected
(number of food items collected/number of food items offered)
and the proportion of thievery observed (number of food items
stolen within the nest by a thief/number of food items collected).
FOOD SUPPLEMENTATION PROCEDURE.Over seven consecutive
days, about 3 mg of food was deposited daily adjacent to the
entrance of each nest present within the treated area (within the
5 m radius circle). The supplemental food was alternated, either a
mix of tuna and honey, or peanut butter, both of which are
known to be rich in proteins, lipids, and carbohydrates. Further-
more, to adjust for the possibility that we might have not
detected some nests within our treatment area, we added extra
food that could be exploited by all the nests present in the area.
Within the treated area, we added nine stations where food was
deposited daily. The stations were separated by 1.5 m from each
other within a square grid conguration (see Fig. 1). After a week
of food provisioning, 24 hr elapsed without any food provision-
ing before the commencement of the post-treatment assay.
POST-TREATMENT ASSAY.Using the same methods as in the pre-
treatment assay, we measured thievery rate for areas inside and
outside our treatment area on the same study nests. Four study
FIGURE 1. Design of the treated area for one selected site.
498 Gu!
enard and McGlynn
nests (three inside and one outside the supplementation area) that
were used for pre-treatment measurements were not active or
had relocated to new positions, so these nests were excluded; to
accommodate this reduction in sample size, eight arbitrarily
selected nests that had not been measured during the pretreat-
ment assay were incorporated in the post-treatment measurement
(three inside and ve outside).
THIEVERY OCCURRENCE.Thievery was considered to occur if at
least one event of thievery was observed for the observed nest
during the 10 min period of our observation. We used a
chi-square test to compare the occurrence of thievery inside and
outside of the treated areas before and after the application of
treatments.
FOOD ITEMS COLLECTED BY FOREIGN WORKERS (EXPLOITERS).We
monitored the number of food items that were collected by
exploiters while the food was standing outside of the focal nest
entrance. Exploiters are individuals foraging on the territory of
others, in this case at the vicinity of other nest entrances.
STATISTICAL ANALYSIS.We evaluated the effect of food addition
on the different foraging behaviors observed in E. ruidum. All
our analyses used non-parametric Wilcoxon tests to compare the
effects of the treatments. Non-parametric tests were used on
account of ordinal and non-normally distributed data. We com-
pared the proportion of food collected before and after the food
addition for the areas located inside and outside the treated areas.
We then compared the rate of thievery observed before the treat-
ment between the treated areas and the non-treated areas. Simi-
larly, we compared the effect of food addition on thievery rate
after the food addition. Then we compared the rate of thievery
observed before and after the food addition within the two dif-
ferent treatments with matched-pairs Wilcoxon signed-rank tests.
Similar analyses were conducted on the proportion of total food
and the proportion of food collected by exploiters. For these
analyses, we used a subset of seventeen nests within the treated
areas and of 18 nests within the non-treated areas for which we
had data prior and after food supplementation. The larger dataset
(20 nests inside the treated areas and 23 nests within the non-
treated areas) included supplemental nests that did not have data
for both periods.
RESULTS
NEST DENSITIES.Within our six treatment areas, the mean nest
density was 0.22 nest/m
2
(range: 0.100.37 nest/m
2
), which cor-
responds to a mean 17.3 nests per treated area (range: 829 nests
per 5 m radius area).
MEAN DISTANCE TRAVELED BY THIEF ANTS.Forty-three thievery
events were observed in their entirety, to measure the distance
traveled by thieves. The distance traveled by thieves between the
origin and destination nests ranged from 40 to 480 cm
(mean !SD =148 !111 cm).
PERCENTAGE OF FOOD ITEMS COLLECTED.Food supplementation
did not modify the proportion of food items collected before
and after within (mean !sd: Interior zone: X
before
=82.9 !
2.6; X
after
=71.3 !5.4; N=17; Z ="18.5; P=0.40, Wilco-
xon matched-pairs signed-ranks test) or outside (mean !SD:
Exterior zone: X
before
=89.2 !4.2; X
after
=82.3 !4.8; N=
18; Z ="9.5; P=0.54, Wilcoxon matched-pairs signed-ranks
test) the treated areas.
OCCURRENCE OF THIEVERY.Thievery was observed from 67 per-
cent of the 39 nests surveyed prior to the application of food
supplementation. No difference between the nests inside and out-
side the treated areas was observed (N=39, v
2
=0.22,
P=0.63). After food supplementation, the overall occurrence of
thievery observed was lower, with only 42 percent of occurrence
observed. The twenty nests within the treated areas had a lower
thievery frequency (25%) than the 23 nests outside the treated
areas (57%) (N=43, v
2
=4.37, P=0.037).
INTENSITY OF THIEVERY.Prior to food supplementation, no differ-
ence in the percentage of food robbed was observed (N=39;
Z="0.07; P=0.94) between the nests located inside or outside
the treated areas (mean !SD inside =19.4 !4.5;
outside =15.3 !4.9). After food supplementation, the percent-
age of food robbed was signicantly lower (N=43; Z ="1.95;
P=0.05) for the nests located inside than outside the treated areas
(mean !SD inside =7.2 !4.1; outside =17.8 !5.2) (Fig. 2).
When comparing the proportion of food robbed before and after
food supplementation, we observed a signicant decrease for the
nests located within the treated areas (N=17; Z ="27;
P=0.01, Wilcoxon matched-pairs signed-ranks test), but no
change in the nests located outside of the treated areas (N=18;
Z="4.5; P=0.36, Wilcoxon matched-pairs signed-ranks test).
FIGURE 2. Percentage of food items stolen per nest before and after the
treatment. In black are presented the nests located within the food-supple-
mented area and in light gray, the nests located outside the treated area. Sig-
nicant differences between pairs are represented by a star.
Food Limitation Enhances Thievery in Ants 499
FOOD ITEMS COLLECTED BY FOREIGN WORKERS (EXPLOITERS).Prior
to food supplementation, the fraction of food items collected by
exploiters was greater inside the treated areas (N=39, Z =2.10,
P=0.04); as these areas were assigned independent of treatment,
this P-value below alpha presumably is an instance of Type I
error. After food supplementation, this difference was not
observed, with a similar effect size, and the fraction of stolen
items was not signicantly different (N=43, Z =1.65;
P=0.10; Fig. 3). When comparing the percentage of exploiters
for a similar area before and after food supplementation, we
observed no difference between inside (N=17, Z =18.5;
P=0.40, Wilcoxon matched-pairs signed-ranks test) and outside
the treated areas (N=18, Z =9.5; P=0.54, Wilcoxon matched-
pairs signed-ranks test).
DISCUSSION
MANY ANT SPECIES HAVE THE CAPACITY TO EXPRESS FLEXIBILITY IN
THEIR FORAGING STRATEGIES,ACROSS SPACE AND TIME, as a function
of local conditions. Foraging decisions are context-dependent in
relation to the properties of food and environmental conditions
outside of the nest (Carroll & Janzen 1973, Traniello 1989). For
instance, ant colonies can adjust their foraging strategies to
optimize food intake on the basis of food quality or quantity
(Bernstein 1975, Breed et al. 1987), competition (Vepsäläinen &
Savolainen 1990, Andersen & Patel 1994, Cerd!aet al. 1998) or
predation pressure (Feener 1988, LeBrun 2005). The importance
of those factors has been highlighted over the past decades, but
their importance on the expression of social parasitismsuch as
cleptobiosisis still uncertain. In this study, we experimentally
demonstrated that the rate of cleptobiosis observed in popula-
tions of E. ruidum is directly related to food limitation.
Thievery was frequent among the nests of E. ruidum that we
surveyed. Prior to our treatment, under natural conditions, at least
two-thirds of the nests were experiencing some impact of thievery
from workers of surrounding nests. These results are similar to
those of Breed and collaborators who found that 7 of 10 nests
suffered from thievery (1990). After only a week of food addition,
and while no difference in foraging activity (number of food items
collected per nest) was observed, the frequency of thievery
decreased to be observed in only a quarter of the nests surveyed.
Non food-supplemented nests maintained a similar rate of thiev-
ery to that observed prior to the experiment. Similar to frequency,
thievery intensity responded negatively to food addition (Fig. 2).
Fifteen to twenty percent of the food items collected by foragers
were stolen in areas without food supplementation, but thievery
rate was reduced by half in areas with food supplementation.
The nest density of E. ruidum found in our study of 0.22
nest/m
2
is similar, although in the lower tail of other studies
reporting nest density for E. ruidum (Table 1). Despite this rela-
tively low density of nests, thievery behavior was common in the
population surveyed, further indicating that, as suggested by
Breed et al. (1999), thievery is not limited to populations with
high nest density.
In Mexico, E. ruidum had a strong predatory effect on the
insect community in a coffee plantation, with a daily estimate per
hectare of 150,000 insects collected (Lachaud et al. 1990). If rate
of thievery is constant and estimated to be 15 to 20 percent of
the prey brought to the nest as our results indicate, this could rep-
resent 22,000 to 30,000 prey items that could be stolen each day
in similar conditions. As indicated by previous studies and our
results, however, thievery rate is not uniform among colonies of
E. ruidum (Breed et al. 1990) or as observed for the other known
intraspecic cleptobiotic species M. aciculatus (Yamaguchi 1995),
and this asymmetry could lead to strong difference in colony
growth performance between colonies within a population. Two
different evolutionary strategies could emerge within the popula-
tion. First, colonies may invest in foraging and defensive strategies
to protect the food collected from thieves; and second, colonies
may invest more in thievery and less in foraging. In this context,
thievery and foraging behaviors in E. ruidum relate to the pro-
ducer-scrounger game developed for other organisms such as
birds. In bird population, some individuals nd food resources
(producer tactic), while other individuals exploit their discoveries
(scrounger tactic) (Barnard & Sibly 1981, Giraldeau & Beauchamp
1999). The expression of one of these strategies can be individ-
ual-dependent (Morand-Ferron et al. 2011, David & Giraldeau
2012) or context-dependant (Coolen 2002, Barta et al. 2004). As a
counterpoint to work on birds, thievery in E. ruidum represents a
great social insect model to study evolutionary aspects of foraging
and of alternative strategies, and the factors that determine the
use of thievery by specic colonies. In this regard, our results
tend to show that the use of thievery as an alternative strategy is
reduced by an increase of food within the population.
The mean distance covered by a thief to its destination nest
was about 1.5 m with a maximum distance observed of 4.8 m.
This corresponds to the foraging distance observed in other
FIGURE 3. Mean number per nest of items stolen by exploiters outside the
nest entrance before and after the treatment. In black are presented the nests
located within the food-supplemented area and in light gray, the nests located
outside the treated area. Signicant differences between pairs are represented
by a star.
500 Gu!
enard and McGlynn
study of about 3 m although occasional distance up to 30 m has
been noted (Lachaud et al. 1984). Several authors have noticed
that the high nests density should favor the overlapping of
foraging territory (Lachaud et al. 1984, Breed et al. 1999, Schatz
& Lachaud 2008), with the exception of the results of Breed and
collaborators (1990) who found that only few workers foraged
outside of their territories. Here, we conrm those observations
with the demonstration of a consequent percentage of food items
(1030%) collected by exploiters directly at the vicinity of other
nest entrances. Our results indicate that the addition of food did
not modify the intensity of exploiters and so we did not see a
switch from thievery (collection of food inside the nest) to
exploiting (collection at the vicinity of other nest entrance).
Future studies focusing on worker polyethism, perhaps through
the use of marking techniques and in laboratory colonies, should
emphasize the exibility of workers to exploit food within their
environment either as thieves or exploiters.
In conclusion, colonies of E. ruidum use a complex foraging
strategy mixing social parasitism and exploitative competition,
which is responsive to the availability of food in the environment.
The rarity of intraspecic thievery, compared with heterospecic
food robbing, remains a conundrum. We propose that the evolu-
tion of intraspecic thievery may not be favored when encounters
between thief and victim result in injury or death. In most social
insects, interactions between workers from separate colonies at
the nest site typically result in major aggression, although this is
not the case in most Ectatomma species (Breed et al. 1990, 1992,
Zinck et al. 2008), nor in M. aciculatus where intraspecic cleptobi-
osis is common (Yamaguchi 1995). The new question posed by
our ndings is to understand how thievery represents an evolu-
tionarily stable strategy, and whether this phenomenon functions
as a form of parasitism or a form of mutualism.
ACKNOWLEDGMENTS
We thank Stephanie Mattingly, Lauren Valbracht and Gilbert Lam
for assistance in eldwork. We also thank Rob Dunn for useful
comments on the manuscript, and Melody Castellon and Roder-
ick Castro for their assistance in translating the abstract. Finally,
we thank three anonymous reviewers for their insightful com-
ments on previous version of the manuscript. Work was con-
ducted under support of the National Science Foundation
(OISE-0854259 and OISE-1130156), a CSUDH Sally Casanova
Memorial RSCAAP Fellowship to TPM, and a Harkema Award
offered through the NCSU Department of Biology to BG.
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502 Gu!
enard and McGlynn
... Our focal species was the thieving ant, Ectatomma ruidum . This is one of the most common species in the Neotropics, ranging from northern Mesoamerica down to western Amazonia, often occurring extraordinarily high densities (Guénard & McGlynn, 2013;Santamaría, Armbrecht, & Lachaud, 2009). This species has been characterized as thermophilic, as it is successful in areas with ample sunlight and foraging occurs even during the hottest times of the day (McGlynn et al., 2010). ...
... Workers in E. ruidum forage outside their nests throughout a home range that typically covers several square meters, and foragers are typically generalists that mostly hunt for arthropods, but also collect seeds and nectar (Jandt et al., 2015). From earlier experiments, it is known that foragers represent a only a small fraction of the total number of ants in the colony, and based on marking of individuals, foragers will remain in this role for at least several weeks (Guénard & McGlynn, 2013). While workers are observed foraging at all times of day, it is not known if there is a division of labor between foragers that forage diurnally and those that forage nocturnally. ...
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1. Insects spend energy to function in high temperature environments, and because social insects employ a division of labor, it is likely that thermal tolerance varies among individuals in the colony, based on the tasks that they perform. 2. Foraging workers of the ant Neotropical ant Ectatomma ruidum are known to show temporal differences in thermal tolerance, with greater tolerance in hot afternoons, relative to cool mornings. 3. We developed three hypotheses that can account for temporal differences in thermal tolerance among workers: Thermal Acclimation, Division of Labor, and Circadian Rhythm. 4. We tested these hypotheses with a pair of experiments that involved the measurement of thermal persistence of ants at a constant temperature in time-to-failure assays. The first experiment compared ants with different behavioral roles in colonies, and the second compared colonies subjected to thermal manipulations, then iteratively sampled at daily thermal maxima and minima. 5. We found robust support for the Circadian Rhythm and Thermal Acclimation Hypotheses, and little support for the Division of Labor Hypothesis. Colonies of this species integrate multiple mechanisms of adapting to thermal challenges including time of day, ambient temperature, and the behavioral context of individual workers.
... Although in some populations these ants have a relatively permissive non-nestmate acceptance threshold (Breed et al. 1990;De Carli et al. 1998;Jandt et al. 2015), high nest density and thievery might have promoted stricter acceptance thresholds in other populations. Intruder acceptance thresholds are usually plastic and are adjusted to local food availability (d'Ettorre et al. 2004;Jandt et al. 2015), as well as thievery frequency in E. ruidum ants (Guénard and McGlynn 2013). ...
... Savolainen and Vepsäläinen 2003) and the maintenance of variation in recognition cues (Crozier 1986). E. ruidum ants exhibit cleptobiosis (Breed et al. 1990;De Carli et al. 1998), which is a form of parasitic interactions (Breed et al. 2012;Guénard and McGlynn 2013). For the foregoing reasons, it is suggestive to draw an evolutionary scenario where high colony density and high risk of colony intrusion by intraspecific thief ants (cleptobiosis) may have promoted polymorphism in recognition cues (via disruptive selection) and this, in turn, may have favored genetic divergence within the species complex. ...
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In social insects, chemical communication is the main communication mode among colony members, which use the blends of cuticular hydrocarbons as recognition cues to discriminate between nestmates and non-nestmates and to prevent the exploitation of their nest resources by aliens. The aim of this study was to assess the variation of nestmate recognition cues in the ant Ectatomma ruidum, a species complex with a considerably conserved morphology and one of the few ant species where intraspecific thievery, a form of cleptoparasitism, has been reported. We analyzed the cuticular hydrocarbon profiles of ants collected from a number of geographically separated populations and examined DNA sequence data to assess their species identity. We focused on one species of the complex, E. ruidum sp. 3–4, whose species delineation remains controversial. We documented that several quantitative and qualitative traits of the cuticular hydrocarbon profiles varied significantly between populations, indicating that this species harbors more cuticular chemical phenotypic diversity than expected within a single species. In particular, there was a striking divergence among populations in the proportion of methylalkanes, alkenes, alkadienes and odd-chain components, which likely play a major role in nestmate/non-nestmate discrimination, a process which might have been crucial in these cleptobiotic ants. Further investigations are needed to test the hypothesis that biotic pressures, such as the need to discriminate conspecific intruders and limit thievery, could have played an important role in promoting the evolutionary divergence between populations in this ant species complex.
... A) Bahia (Brasil). B) Tapachula (México (BREED et al., 1990;BREED et al., 1992BREED et al., , 1999GUÉNARD;McGLYNN, 2013) e são apenas encontradas em populações do México (LACHAUD et al., 1999a, b Emery (1901). Esta espécie foi revisada por Kugler;Brown (1982) e atualmente é considerada um sinônimo júnior de E. ruidum. ...
... A) Bahia (Brasil). B) Tapachula (México (BREED et al., 1990;BREED et al., 1992BREED et al., , 1999GUÉNARD;McGLYNN, 2013) e são apenas encontradas em populações do México (LACHAUD et al., 1999a, b Emery (1901). Esta espécie foi revisada por Kugler;Brown (1982) e atualmente é considerada um sinônimo júnior de E. ruidum. ...
... ruidum can move seeds that were initially moved by other ants and move seeds from one E. ruidum colony in another direction (4 observations) including moving seeds from one colony into another (1 observation). Ectatomma ruidum workers engage in thievery, where foragers from one colony are able to infiltrate another colony and intercept food items brought into the nest (Guénard & McGlynn, 2013), potentially explaining our observations that seeds removed into one colony can move to another. If seeds remain attractive over long time scales, these secondary dispersal events can shape dispersal kernels as shown by Jansen et al. (2012) with rodents stealing cached seeds from other rodents. ...
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Plants are often dispersal limited relying on passive or active agents to find suitable microhabitats for germination. Seeds of pioneer tree species, for example, require light gaps for growth but have short median dispersal distances and often do not provide a food reward to encourage animal dispersal. Zanthoxylum ekmanii seeds are frequently moved by ants but evaluating the effectiveness of ant-mediated seed removal requires knowledge of the species moving the seeds, how far they are moved, and the deposition site. To assess the effectiveness of ants as seed dispersers of Z. ekmanii, we utilized the seed dispersal effectiveness framework. We tracked the movement of seeds from caches on the forest floor, revealing that foragers of Ectatomma ruidum moved 32.8% of seeds an average first distance of 99.8 cm with 68.3% of those seeds taken into a colony. The quality of deposition location was assessed using a seedling emergence study where freshly germinated seeds were buried at different depths. Seedlings were primarily able to emerge from the shallowest depths. Wax castings of E. ruidum colonies demonstrated that seeds brought into the colony were deposited in chambers that had larvae present and experienced more damage than seeds unhan-dled by ants. Foragers, however, did not have a strong enough bite force to rupture Z. ekmanii seeds likely because their muscle morphology is not structured to maximize force generation. Overall, E. ruidum may help fine tune deposition location, incorporating seeds into the topsoil, though few seeds will likely emerge if soil bioturbation is low.
... In Mexico, Costa Rica, and Panama, this species has been considered monodomous (each colony occupies a single nest structure) due to the architecture of its nests being characterized by only one nest per colony and each nest being an individual entity independent from other, neighboring nests (each nest contains at least one queen and shows territoriality against neighboring homospecific nests) [30,42,43]. However, the common occurrence of intraspecific cleptobiosis, that is, the theft of food resources between neighboring nests [42,44,45], and, on some occasions, the transport of workers and even brood between them [46], suggest the possible existence of a more complex nesting structure. More specifically, in southwestern Colombia, this assertion is not at all clear given that various fragmentary observations [47][48][49] have reported quite frequent surface relationships between neighboring nests and the absence of queens in most of them, suggesting a polydomic nesting strategy (a colony occupies two or more spatially separated nests) [6,28]. ...
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Nest architecture plays a fundamental role in the adaptation of ants to their habitat, favoring the action of economically important species. Ectatomma ruidum sp. 2 (ruidum species complex) is a biological control agent in Neotropical agroecosystems, exhibiting high bioturbation impact due to high nest densities. The architecture and composition of 152 nests were studied in two Andean populations of southwestern Colombia, 24 of them being cast using the paraffin wax technique. Nest entrance was a single, circular, 4 mm hole at ground level, without any special external structure, connected to a single vertical tunnel communicating with successive half ellipsoidal chambers. Nests were extremely shallow (depth range: 28.7-35.4 cm), with an average of six chambers and an overall volume of 92.2 cm3 per nest. The deeper the chamber, the smaller its volume. Nest building was independent of plants or roots, and no surface or underground physical connections were found between neighboring nests. Few nests possessed a queen, and neither ergatoids nor microgynes were recorded. Despite significant interactions between localities and the number of both males and workers, queen presence had an overall highly positive effect on the number of workers and larvae and a negative one on the number of gynes. Overall, the studied Colombian populations of E. ruidum sp. 2 retained the simple nest structure described for other species of this species complex and for colonies of the same species from other geographical areas, though they constrasted in their extreme shallowness. Our data suggest that E. ruidum sp. 2, at the local level, does not follow the usual monodomic pattern of this species with facultative polygyny but, rather, has a polydomic pattern with monogyny, perhaps related to the extreme shallowness of the nests due to soil structure, which could significantly enhance the queen's reproductive inhibition previously reported for this species.
... The first clade included samples from a Mexican population of E. ruidum without microgynes together with samples from Venezuela, while samples from a Mexican population with microgynes grouped with samples from Ecuador and Panama. However, to date, no microgynes have been reported in E. ruidum from Ecuador or Panama (Breed et al. 1990(Breed et al. , 1999Guénard and McGlynn 2013). E. ruidum microgynes have only been found in samples from Mexico (Lachaud et al. 1999a, b). ...
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Due to its high biodiversity and its complex climatic and geological history, the Neotropical region has caught the attention of evolutionary and conservation biologists. The Neotropics have an understudied and probably extensive cryptic diversity, stemming from old lineages that have persisted through time with highly similar morphology or from new morphologically undifferentiated sibling species. The wide-ranging Neotropical ant genus Ectatomma currently has only 15 described species, some of which present limited distribution. These ants provide an excellent system for the study of diversification and cryptic diversity in the Neotropics. Ectatomma also displays queen-size dimorphism in some northern populations of its two most common species: a case of true microgyny and a recently described parasitic species. We performed a phylogenetic and biogeographic analysis of Ectatomma species using two mitochondrial genes and one nuclear gene. We also explored the relationship between the history of the genus and the appearance of minia-turized queens. Our analysis recovered a monophyletic Ectatomma that originated in the Parana region of South America. We recorded three likely events of colonization of the Caribbean-Mesoamerican region. We also detected ample evidence of cryptic divergence that deserves a full taxonomic revision of the genus. Miniature queens-microgynes and parasites-represent two independent evolutionary events that appeared in the recent history of the genus.
... Ectatomma ruidum is a common Neotropical ant that nests in secondary forests, attaining densities up to 1 colony m −2 at La Selva (McGlynn et al. 2010). Colonies consist of a single queen and occupy a single nest (i.e., are monogynous and monodomous), with one 3-4 mm wide entrance dug into the ground (Guénard and McGlynn 2013). ...
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Article
In the tropics, daily temperature fluctuations can pose physiological challenges for ectothermic organisms, and upper thermal limits may affect foraging activity over the course of the day. Variation in upper thermal limits can occur among and within species, and for social insects such as ants, within colonies. Within colonies, upper thermal limits may differ among individuals or change for an individual throughout the day. Daytime foragers of the Neotropical ant Ectatomma ruidum have higher critical thermal maxima (CTmax) than nocturnal foragers, but whether these differences occur among or within colonies was not previously known. We investigated the potential mechanisms accounting for day/night variation in CTmax of E. ruidum foragers by testing whether CTmax varied among or within colonies or due to individuals within colonies acclimating to changes in temperature over a short time scale (3 h). We found within- but not among-colony differences in CTmax on a diel cycle, and we found no evidence for among- or within-colony partitioning of foraging times by individual workers. Individuals did not acclimate to experimental manipulations of temperature, although additional experiments with more ecologically relevant temperature manipulations are needed to rule out this mechanism. In summary, we have shown that day/night differences in upper thermal limits can occur within ant colonies, but further investigation is needed to elucidate the mechanisms driving this variation.
... The first clade included samples from a Mexican population of E. ruidum without microgynes together with samples from Venezuela, while samples from a Mexican population with microgynes grouped with samples from Ecuador and Panama. However, to date, no microgynes have been reported in E. ruidum from Ecuador or Panama (Breed et al. 1990(Breed et al. , 1999Guénard and McGlynn 2013). E. ruidum microgynes have only been found in samples from Mexico (Lachaud et al. 1999a, b). ...
... 'ruidum 1' inspected for the presence/absence of microgyny lack it. These include populations situated in south-eastern Mexico, Costa Rica, Venezuela, French Guiana and eastern Colombia (Breed et al., 1990(Breed et al., , 1992(Breed et al., , 1999Gu enard & McGlynn, 2013;J.-P. Lachaud, unpubl. ...
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Nuclear mitochondrial (mt) paralogues (numts) are non-functional fragments of mtDNA integrated into the nuclear genome that can overestimate the number of species in analyses based on mtDNA sequences. As numts have relatively slow mutation rates, they can pass undetected by conventional procedures such as inspecting for internal stop codons, indels or apparent polymorphism in chromatograms. Species boundaries based on mtDNA markers therefore require a thorough assessment of numts, especially in insects, where this phenomenon appears to be relatively frequent. Ectatomma ruidum is a widely distributed Neotropical ant species that is distributed from northern Mexico to northern Brazil. Previous behavioural and molecular evidence suggests that this species actually represents a composite taxon. Here we assessed the species boundaries in E. ruidum based on two mt (COI, cyt b) and one nuclear (H3) marker, as well as on external morphology. Ancient and recent mt paralogues were detected in several specimens, although pre-PCR dilution of DNA template helped to recover most of the mt orthologues. Based on the congruence found between our species delineation obtained from the mt genealogies and the discriminated morphospecies, we propose that E. ruidum is actually composed of at least three species. Two of these species have a wide geographical distribution in the Neotropics, whereas the remaining one was restricted to localities situated near the Pacific coast in south-east Mexico. We also found extensive intra- and interspecific variation in the barcoding locus. Moreover, the nuclear evidence suggests the existence of hybrids between two of these species in Oaxaca, south-east Mexico. This study agrees with previous studies of other closely related animal taxa, which have revealed a complex evolutionary history and overlooked species diversity in the latter region.
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Animal behaviour can lead to varying levels of risk, and an individual’s physical condition can alter the potential costs and benefits of undertaking risky behaviours. How risk-taking behaviour depends on condition is subject to contrasting hypotheses. The asset protection principle proposes that individuals in better condition should be more risk averse, as they have higher future reproductive potential (i.e. more to lose). The state-dependent safety hypothesis proposes that high-condition individuals that are more likely to survive and maximise the benefits of risky situations may make apparently riskier choices, as their individual risk is in fact lower. We systematically searched for studies that experimentally manipulated animals’ nutritional or energetic condition through diet treatments, and subsequently measured risk-taking behaviour in contexts relating to predation, novelty and exploration. Our meta-analysis quantified condition effects on risk-taking behaviour at both the mean and variance level. We preregistered our methods and hypotheses prior to conducting the study. Phylogenetic multilevel meta-analysis revealed that the lower-nutritional-condition individuals showed on average ca. 26% greater tendency towards risk than high-condition individuals (95% confidence interval: 15% – 38%; N = 126 studies, 1297 effect sizes). Meta-regressions revealed several factors influencing the overall effect, such as the experimental context used to measure risk-taking behaviour, and the life stage when condition was manipulated. Meta-analysis of variance revealed no clear overall effect of condition on behavioural variance (on average ca. 3% decrease in variance in low- versus high-condition groups; 95% confidence interval: –8% – 3%; N = 119 studies, 1235 effect sizes), however, the experimental context was an important factor influencing the strength and direction of the variance effect. Our comprehensive systematic review and meta-analysis provide insights into the roles of state dependency and plasticity in intraspecific behavioural variation. While heterogeneity among effect sizes was high, our results show that poor nutritional state on average increases risk taking in ecological contexts involving predation, novelty and exploration.
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When they forage in groups, animals can search for their own food (producer tactic) or exploit the discoveries of others (scrounger tactic). Previous experimental inquiries have demonstrated that individuals vary in their tendency to play either tactic but the extent to which individual factors influence variation in foraging behavior are little studied. In the present study, we have assessed the influence of natural variation in body condition on the differential use of social foraging tactics and their resulting payoffs in zebra finches (Taeniopygia guttata). The producer tactic was found to yield more consistent and predictable rewards across trials than the scrounger tactic. The use of producer was related to reduced variation in food intake and an increased amount of food consumed. We found that poor-condition birds were more likely to produce and so consumed more seeds than good-condition birds. The results are consistent with theoretical models of variance-sensitive social foraging but suggest that scrounging may not represent a variance-averse option in all situations. We propose that in a producer-scrounger context, the variance-averse option may depend on group size and food clumping. Finally, we discuss our results in relation to interindividual differences in metabolism and behavior.
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The effects of predation on the use of social foraging tactics, such as producing and scrounging, are poorly known in animals. On the one hand, recent theoretical models predict increased use of scrounging with increasing predation risk, when scroungers seeking feeding opportunities also have a higher chance of detecting predators. On the other hand, there may be no relation between tactic use and predation when antipredator vigilance is not compatible with scanning flockmates. We investigated experimentally the effects of predation risk on social foraging tactic use in tree sparrows, Passer montanus. We manipulated predation risk in the field by changing the distance between shelter and a feeder. Birds visited the feeder in smaller flocks, spent less time on it and were somewhat more vigilant far from shelter than close to it. Increased predation risk strongly affected the social foraging tactic used: birds used the scrounger tactic 30% more often far from cover than close to it. Between-flock variability in scrounging frequency was not related to the average vigilance level of the flock members, and within-flock variability in the use of scrounging was negatively related to the vigilance of birds. Our results suggest that in tree sparrows, the increased frequency of scrounging during high predation risk cannot simply be explained by an additional advantage of increasing antipredator vigilance. We propose alternative mechanisms (e.g. increased stochasticity in food supply, and that riskier places are used by individuals with lower reserves) that may explain increased scrounging when animals forage under high predation risk.
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A case of interference competition in which some ant species (Messor barbarus and Aphaenogaster senilis) rob seeds of Euphorbia characias from other ants (especially Tapinoma nigerrimum) is reported. Seed robbing is here regarded as a possible natural impediment to myrmecochory as it may interfere with the seedling recruitment process of the myrmecochorous plant.
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The foraging of the submissive Myrmica, which rank lowest in a 3-level competition hierarchy, was investigated with bait experiments. The activity (forager traffic) of M. scabrinodis and M. ruginodis was related to temperature, time of day, and the activity and presence of other ant species. In the absence of competitors, Myrmica tended to be active independently of the time of day, but preferred slightly lower temperatures. On baits, Myrmica were more often present at night, especially at high temperatures. Most potential competitors were day-active and preferred higher temperatures. The territorial Formica truncorum and F. exsecta reduced the presence of Myrmica on the baits, but not their traffic to the square. Myrmica shifted their foraging at the baits to lower temperatures and used the baits mainly at night. Of the 2 aggressive but non-territorial species, the large Camponotus ligniperda did not affect the foraging of Myrmica, but circumstantial evidence indicated exclusion by the similarly sized Lasius niger. The submissive F. fusca had no effect on Myrmica. Earlier studies implied that, in spite of interference and reduced foraging success on the baits, Myrmica are able to coexist with most of the aggressive ant species. Coexistence may be facilitated by: phylogenetically based differences in morphology and size; small colonies and foraging areas of Myrmicinae in contrast to most of the aggressive Formicinae; inclination to forage at low temperatures at night; and a tendency to forage in the litter and nest underground. -from Authors
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Foraging strategies of three species of desert ants were studied along gradients of altitude and season, both of which affect food density. When food density was low, colonies foraged in long, narrow columns extending linearly outward from the nest entrance. The columns rotated around the nest, and, in time, a circular area was covered. Both the length of the column and the rate of rotation increased with decreasing food density. Under conditions of relatively high food density, the foragers searched for seeds individually, without apparent behavioral cohesion. Each of the three species decreased intraspecific competition by regular spacing of colonies. Interspecific competition was reduced by partially distinct population distributions along the altitudinal gradient. For two species, colonies had a constant foraging area size regardless of altitude or season. For these colonies, time spent searching for food is a good indication of available food. As food density increases, colonies of species with smaller foraging areas become more abundant.
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Describes the pattern of dispersion of 15 ground ant species on Barro Colorado Island, Panama. All 4 plots showed significant evidence of both intra- and interspecific overdispersion. The observed pattern of dispersion may be the result of competition among all members of this set of species with similar requirements. Mechanisms which could produce this pattern are nest movement and the preferential predation on conspecific founding queens and incipient colonies by established colonies.-from Authors