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Brood adoption in the leaf-cutting ant Acromyrmex echinatior: Adaptation or recognition noise?

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Abstract

The ability to discriminate between nestmates and non-nestmates is an important prerequisite for the evolution of eusociality. Indeed, social insect workers are typically able to discriminate between nestmate and non-nestmate workers. Adult non-nestmate workers are readily detected and rejected from the colony. Whether social insects can discriminate between nestmate and non-nestmate brood, however, is less clear. Here, we show that workers of the leaf-cutting ant Acromyrmex echinatior discriminate between nestmate and non-nestmate brood, and among brood of different stages. Initially, non-nestmate brood is attacked, but it is adopted after a delay. Adoption could occur due to inefficiency of the recognition system, or it could be adaptive because it is an inexpensive way to increase the workforce. Our results suggest that brood adoption may occur accidentally. We also report how workers replace fungal hyphae on the brood’s surface before transporting the brood into their fungus garden.
Brood adoption in the leaf-cutting ant Acromyrmex
echinatior: adaptation or recognition noise?
B. Fouks1,2, P. d'Ettorre1,3 and V. Nehring1
1. Centre for Social Evolution, Department of Biology, University of
Copenhagen, Universitetsparken 15, 2100 Copenhagen E, Denmark
2. AG Molecular Ecology, Martin-Luther University of Halle-Wittenberg,
Hoher Weg 4, 06099 Halle, Germany
3. Laboratory of Experimental and Comparative Ethology (LEEC),
University of Paris 13, 99 av. J.B. Clément, 93430 Villetaneuse, France
Correspondence: Volker Nehring, Centre for Social Evolution, Department of
Biology, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen E,
Denmark. Tel. +45 35 32 12 8; Fax +45 35 32 15 50; e-mail:
vnehring@bio.ku.dk
Word count: main text 3971, total 5560, 4 figures
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Abstract
The ability to discriminate between nestmates and non-nestmates is an important
prerequisite for the evolution of eusociality. Indeed, social insect workers are
typically able to discriminate between nestmate and non-nestmate workers. Adult
non-nestmate workers are readily detected and rejected from the colony. Whether
social insects can discriminate between nestmate and non-nestmate brood,
however, is less clear. Here we show that workers of the leaf-cutting ant
Acromyrmex echinatior discriminate between nestmate and non-nestmate brood,
and among brood of different stages. Initially, non-nestmate brood is attacked, but
it is adopted after a delay. Adoption could occur due to inefficiency of the
recognition system, or it could be adaptive because it is an inexpensive way to
increase the workforce. Our results suggest that brood adoption may occur
accidentally. We also report how workers replace fungal hyphae on the brood's
surface before transporting the brood into their fungus garden.
Keywords: Acromyrmex; Brood care; Fungus-growing ants; Nestmate recognition
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Introduction
Recognition is an important factor stabilizing all kinds of social interactions, from
reciprocity in repeated encounters, to preferential treatment of kin, or inbreeding
avoidance. In social insects, an elaborate nestmate recognition system protects the
colony resources from unrelated intruders (cf. d'Ettorre and Lenoir, 2010). Typical
intruders into ant colonies are social parasite queens, slave making ant workers, or
workers from other ant colonies. It is therefore crucial that ant workers can
discriminate between nestmate and non-nestmate workers. Social insect brood,
however, is generally helpless, not mobile, and not able to survive on its own even
for short periods. Thus allocolonial brood is presumably not a common intruder
into social insect colonies and far less a danger to an insect society than
allocolonial workers are. On the contrary, it has been shown that allocolonial
brood integrates into colonies without harming them, and the newly emerging
workers will work for the adopting colony (Carlin and Hölldobler, 1986; Hare and
Alloway, 1987; Carlin and Schwartz, 1989; Fénéron and Jaisson, 1995; Isingrini
et al., 1985). Adopting pupae, for example, would promote the growth of the
adopting colony, since it saves the resources the individual would have consumed
during its larval development. This effect is exploited to reinforce Oecophylla
colonies, which are used for biological control in fruit plantations in Asia (cf.
Krag et al., 2010). One might therefore be inclined to expect that workers will
adopt allocolonial brood, particularly at a late developmental stage. This pattern
has been described for instance in Lasius ants (Lenoir, 1981). On the other hand,
ants that are threatened by social parasites may benefit from efficiently
discriminating between nestmate and non-nestmate brood to hinder the
reproduction of parasites, which would be very costly to the host colony. The
question of whether or not there is discrimination against non-nestmate brood, and
how efficient it is, is therefore very relevant to understanding how colony integrity
and efficacy are traded off.
The evidence for colony-specific brood recognition in ants is conflicting. For
many species it is known that ants react differently to allo- and concolonial brood
(Meudec, 1978; Lenoir, 1984; Bonavita-Cougourdan et al., 1987; Fénéron and
Jaisson, 1992). Leaf-cutting ants have been reported to discriminate nestmate
from non-nestmate brood as well, but also to adopt a high proportion of non-
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nestmate brood (Febvay et al., 1984; Viana, 1996; Viana et al., 2001; de Souza et
al., 2006). All studies have recorded only acceptance or rejection of brood, so that
it was not possible to examine the reasons for adoption in detail. Leafcutters grow
a fungus garden as a food resource, in which the brood lives. The ants, their
brood, and fungus have coevolved, leading to morphological and functional
adaptations by larvae to feed on gongylidia, special nutrient-rich hyphal tips. A
larva can only survive if workers place the gongylidia on the larva's mouthparts
and keep the larva clean (Wheeler, 1948; Weber, 1966). Furthermore, workers
plant fungal hyphae on the brood's cuticle; the purpose of this behaviour has not
yet been brought to light (Lopes et al., 2005). The brood is likely to communicate
its need for food or other care via semiochemicals, and these may interfere with
the chemical cues that are used for nestmate recognition (Morel and Vander Meer,
1988).
We studied the adoption of brood items by workers of the leaf-cutting ant
Acromyrmex echinatior. We hypothesized that adoptions could happen either
“intentionally”, because it is adaptive, or because of “noise”, i.e. errors in
nestmate recognition. It is possible that nestmate recognition of brood is not a
mechanism that has been selected for, since workers rarely, if ever, encounter
allocolonial brood. However, A. echinatior can be invaded by the inquiline social
parasite A. insinuator (Sumner et al., 2003) and in this case brood recognition
would be useful. In the case of same-species interactions, the adoption of later
brood stages would be more beneficial to a colony than that of early stages, since
the investment that has to be made before a worker ecloses is lower for older
brood stages. Therefore, if adoption of brood was intentional, one would expect a
higher propensity for workers to adopt pupae than larvae. We tested this by
presenting leafcutter workers in small subcolonies with brood of different stages.
We examined by detailed behavioural observations, whether the workers are able
to discriminate between nestmate and non-nestmate brood as well as different
brood stages.
Material and methods
Ten laboratory colonies of Acromyrmex echinatior were used for our experiments
(Ae168, Ae204, Ae331, Ae332, Ae334, Ae335, Ae345, Ae347, Ae356, and
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Ae372). The colonies were collected from secondary rain-forest in Gamboa,
Panama, between 2002 and 2008. After being transferred to the laboratory in
Copenhagen, the colonies were maintained at constant temperature (25°C) and
humidity (75% relative humidity) on a diet of bramble leaves, apple, and rice.
To test for colony-specific brood recognition, con- and allocolonial brood items
were introduced into small discriminator subcolonies. These were housed in Petri
dishes 8 cm, 2 cm height) and contained a small fungus garden (12 cm³), 5
large workers, 5 medium workers, and 10 small workers (cf. Bot and Boomsma,
1996). A piece of bramble leaf (16 cm²) served as substrate for the fungus, from
which we had removed all brood.
From five of the ten original colonies, we created six discriminator subcolonies
each. These were divided into three pairs, each of them receiving brood of a
different stage: one pair received pupae (3.5 - 5.5 mm long; 1 - 2.5 mm wide),
another one large larvae (3.5 - 4.5 mm long; 1.5 - 2.5 mm wide), and the third pair
received medium sized larvae (2.5 - 3.5 mm long; 1 - 1.5 mm wide). The brood
that two discriminator subcolonies of a pair received stemmed from different
sources: one received brood from the same original colony the workers and
fungus came from (concolonial), while the other one received brood from one of
the five colonies that were not used to create discriminator subcolonies
(allocolonial).
Five brood items of the same stage and from the same colony were introduced
into each discriminator subcolony sequentially, at intervals of 30 minutes, starting
one hour after setting up the subcolonies. The brood items were introduced
through a hole in the lid of the Petri dish. Following the introduction, the
behaviour of the discriminator workers towards each brood item was recorded
during 15 minutes or until the ants brought the brood into the fungus garden. The
duration of antennation, grooming (i.e. licking and other manipulation of the
brood with mouth parts, excluding carrying), and aggression (i.e. rapid movement
towards the brood with mandible opening (threat), sometimes followed by biting)
were recorded with the software EthoLog (Ottoni, 2000). The observer was
always blind to the origin (concolonial / allocolonial) of brood items. In total we
observed the behaviour of five brood items per discriminator subcolony, with six
subcolonies (30 individual brood items) per original colony. As we used five
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original colonies, we observed 150 brood items in total, namely 50 individual
brood items per each of the three brood stages.
The behavioural data were analysed with generalized linear mixed models (Bates
et al., 2008; R Development Core Team, 2010) including the colony origin as a
random factor. The stage of brood items (factor stage” with three levels pupa,
large larva, medium larva) and the combination of brood and discriminator (factor
“origin” with two levels allocolonial and concolonial) were included as fixed
factors in all models. We tested with likelihood ratio tests, which of the factors
had a significant influence on the response variable, by subsequently removing
factors and interactions between factors from a full model that included both
factors and their interaction.
The distributions of all response variables and their residuals were inspected for
symmetry, and the variables were transformed when indicated. The durations of
antennation and grooming were log-transformed and then analysed using models
with Gaussian error families and identity link functions. As aggression was rare,
we transformed the variable into a binomial distribution (presence or absence of
aggression during the observation of a single brood item) and analysed it with the
respective error family and logit link function. To analyse whether a single
antennation is enough for workers to identify the stage and origin of brood, we
compared the distribution of the first behaviour following the initial antennation
with Pearson's Chi squared test.
We also recorded when a brood item was first contacted by a worker, and when it
was brought into the fungus garden. We calculated the delay t(d) from the first
contact t(fc) with the brood item until it arrived in the fungus garden t(t), as t(d) =
t(t) t(fc). Brood items that were not brought into the fungus garden during the
first 15 minutes were assigned the value of the total observation time (t(t) = 900
sec). This variable was analysed assuming a Gaussian distribution. We also noted
whether brood was constantly contacted by workers after they discovered it and
before they brought it into the fungus garden, and analysed these observations as a
binomial variable. “Constant contact” thus means that the brood item received
continuous antennation and/or grooming from the first discovery to the transport
into the fungus garden.
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The number of brood items that were transported into the nest during the 15
minutes of observation was analysed with Poisson error families and log link
function. One hour, 3h, and 24h after this, the number of brood items inside the
fungus garden was recorded and analysed in a similar way, but adding time as a
covariate.
The amount of fungal hyphae covering the brood items was recorded before the
start of the experiments and after 24 hours. We defined four levels of fungus
coverage: (0) no fungus hyphae covering pupa or larva, (1) one to six fungus
hyphae on pupa or larva, (2) more than 1/3 and less than 2/3 of the pupal or larval
surface covered by fungus, (3) more than 2/3 of pupa or larva covered by fungus.
The fungus coverage was analysed with Poisson errors and using time (before /
after the experiment) as a third explanatory factor.
Results
Overall, allocolonial brood received more antennation than concolonial brood (χ²1
= 7.1, p < 0.01), and medium larvae were antennated less than large larva and
pupae (χ²2 = 19.3, p < 0.001, Fig. 1a). There was no interaction between the
factors (χ²2 = 3.2, p = 0.2). Workers spent more time grooming allocolonial than
concolonial larvae (Fig. 1b). Allocolonial pupae, however, where groomed less
than concolonial pupae. Comparing among concolonial brood items, pupae were
groomed more than larvae; however, this pattern is not present among allocolonial
brood items. The interaction between stage and colony origin was significant
(origin x stage χ²2 = 12.8, p < 0.01; main effects: origin χ²1 = 0.8, p = 0.37; stage
χ²2 = 19.88, p < 0.001).
The distribution of behavioural acts succeeding initial contact by workers differed
between allocolonial and concolonial brood items (χ²3 = 12.5, p < 0.01). While
concolonial brood items were mostly groomed, this behaviour was rarer for
allocolonial brood, which was ignored or even attacked. There was no difference
in behaviour towards brood of different stages (χ²3 = 2.3, p = 0.5). We also
compared allo- and concolonial brood for each stage separately (pupae χ²3 = 7.4, p
= 0.06; large larvae χ²3 = 6.4, p = 0.10; medium larvae χ²3 = 6.0, p = 0.11), and
among the three stages for con- or allocolonial brood separately (concolonial χ²6 =
14.2, p < 0.05; allocolonial χ²6 = 10.1, p = 0.12), and did not find any clear trend
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in the behaviour that followed the first contact of workers with brood items, other
that workers might be more likely to detect the difference between different brood
stages when the brood is concolonial rather then allocolonial.
During the 15 minutes of observation, concolonial brood items were brought into
the fungus garden quicker (t(d)) than allocolonial ones (Fig. 1c). Considering
allocolonial brood, medium sized larvae were brought into the fungus garden
much quicker than large larvae and pupae, while concolonial larvae of both size
classes were in the fungus garden earlier than concolonial pupae (interaction
origin x stage χ²2 = 6.2, p < 0.05; origin χ²1 = 37.8, p < 0.001; stage χ²2 = 39.4, p <
0.001).
Whether a brood item was in “constant contact” with the discriminator workers
after its first discovery depended on the brood's origin and stage and their
interaction (origin x stage χ²2 = 9.3, p < 0.01; origin χ²1 = 11.2, p < 0.001; stage χ²2
= 24.4, p < 0.001). While 60 % of the large concolonial larvae were continuously
tended, this was the case for only 8 % of the large allocolonial larvae (Fig. 2a).
Medium larvae received most attention overall and there was only a small
difference (80 % vs 60 %) between con- and allocolonial medium larvae. Only 28
% of both concolonial and allocolonial pupae were tended continuously by the
discriminator workers.
Only allocolonial brood items (12 % - 24 %) received aggressive acts by workers
(χ²1 = 19.4, p < 0.001, Fig. 2b). Neither the stage of the brood (χ²2 = 1.3, p = 0.53)
nor its interaction with the origin (χ²2 < 0.1, p > 0.99) explained any of the
variance.
After the first 15 minutes of continuous observation, the ants kept on transporting
brood into the fungus garden, so that eventually all five brood items were in the
garden in all experiments (Fig. 3). However, allocolonial brood was brought into
the garden later than concolonial brood (interaction time x origin χ²1 = 8.1, p <
0.01). The stage of a brood item did not influence when it was brought into the
fungus garden, neither on its own nor in interaction with other variables (origin x
stage x time χ²2 = 1.2, p = 0.55; stage x origin χ²2 = 2.8, p = 0.24; stage x time χ²2
= 2.15, p = 0.34; stage χ²2 = 3.6, p = 0.16; origin χ²1 = 82.9, p < 0.001; time χ²1 =
65.6, p < 0.001).
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Before the experiment, pupae were covered by more fungal hyphae than larvae. At
the end of the experiment, 24 hours after the introduction of brood items into
discriminator subcolonies, both larvae and pupae were covered by more hyphae
than before the experiment (Fig. 4). It did not matter whether the brood was allo-
or concolonial, as the factor origin and its interactions with other factors did not
contribute to the model (origin x stage x time χ²2 = 0.4, p = 0.82; origin x stage χ²2
= 2.1, p = 0.35; origin x time χ²1 = 1.2, p = 0.26; origin χ²1 = 2.5, p = 0.11). The
variation in the brood's fungal coverage was determined by an interaction of stage
and time only (stage x time χ²2 = 28.2, p < 0.001; stage χ²2 = 180.2, p < 0.001;
time χ²1 = 59.9, p < 0.001), underlining that pupae gained relatively more fungal
coverage than larvae.
Discussion
Our results demonstrate that Acromyrmex echinatior workers are able to
discriminate not only nestmate from non-nestmate brood, but also brood of
different stages. Although non-nestmate brood was rejected at first, all brood
items were transported into the fungus garden within 24 hours. We present
evidence indicating that the adoption is taking place by accident rather than
because it is adaptive.
Colony-specific brood recognition in Acromyrmex echinatior is clearly
demonstrated by the fact that only allocolonial brood received aggression. When
allocolonial brood was not attacked, it was ignored after the first antennation;
moreover, allocolonial brood was transported to the fungus garden significantly
later than concolonial brood. Although allocolonial brood was initially aggressed
or ignored, all brood items have been brought into the nest within 24 hours. This
is surprising at the first glance, but among leaf-cutting ants it is not uncommon
(Febvay et al., 1984; Viana, 1996; de Souza et al., 2006). There are two different
explanations for this phenomenon.
First, it is possible that adopting allocolonial brood is not per se harmful to a
colony. If workers that grow up in a foreign colony will integrate well and do not
jeopardize the colony's fitness, then it is adaptive to adopt allocolonial larvae and
pupae, as it for instance occurs in species that perform intra-specific brood raids
(e.g. Rissing and Pollock, 1987; Kronauer et al., 2003). A colony needs to invest
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less into these foreign workers than into their own, since they save the investment
already made by the foreign colony before adoption. Indeed, there is sufficient
evidence suggesting that non-nestmate workers integrate well into foreign
colonies and adoptions can be successful (Isingrini et al., 1985; Hare and Alloway,
1987; Carlin and Schwartz, 1989; Fénéron and Jaisson, 1995;). An alternative
hypothesis would be that workers adopt allocolonial brood by mistake, because
they cannot discriminate it from concolonial brood. As allocolonial brood does
typically not harm the colony, there may generally be only little selection pressure
on ants to detect non-nestmate brood. However, when the colony is at risk to be
infected by social parasite ants, it will be adaptive to be able to identify the social
parasite brood. Acromyrmex echinatior is host to the social parasite Acromyrmex
insinuator, and the host workers seem to be able to discriminate parasite from host
brood under laboratory conditions (Sumner et al., 2003).
Two of our observations support the “mistaken adoption” hypothesis and
contradict the “adaptive adoption” hypothesis. First, under the “adaptive
adoption” hypothesis, one would expect that workers adopted pupae and large
larvae more readily than medium larvae, because the latter require a higher
investment before they can be useful to the colony. We found the opposite:
medium size larvae are accepted more readily than larger and likely older larvae
and pupae, which is consistent with results in Acromyrmex subterraneus (Viana,
1996). Large larvae had the same low levels of fungus coverage as medium
larvae, and therefore the observed delayed adoption of large larvae cannot be due
to workers spending more time to remove fungal hyphae before transporting the
larvae into the nest.
Second, allocolonial brood was brought to the nest later than concolonial brood.
This reflects that allocolonial brood was only adopted after a delay, the reason for
this might be that the nestmate recognition label of the brood changed over time.
Social insects typically discriminate against non-nestmates using a colony-specific
label, encoded in the chemical profile located on the cuticle (Lenoir et al., 1999;
d'Ettorre and Lenoir, 2010). The nestmate recognition labels are dynamic: during
allogrooming and trophallaxis, chemicals are transferred between ants. In this way
inter-individual differences in labels are aligned, which guarantees that all
nestmates share the same nestmate recognition label (Lenoir et al., 1999; Hefetz,
2007). During the experiment, the brood was always close to the fungus garden
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and came into contact with the discriminating workers repeatedly. This could have
allowed transfer of cuticular chemical substances (Soroker et al., 1995). Thus,
even though workers correctly identified allocolonial brood as non-nestmate in the
beginning of the experiment, the chance of mistaking allocolonial brood for
nestmate may have increased with time because the label of allocolonial brood
was aligned to that of the discriminator colony. It has been shown that the
cuticular chemical profiles of brood of leaf-cutting ants, including A. echinatior,
are rather simple as compared to that of adult workers (Viana et al., 2001; Richard
et al., 2007), in that they consist of less substances and mainly of saturated
alkanes, which are generally assumed to play a minor role in nestmate recognition
(review in d’Ettorre and Lenoir 2010; van Zweden and d’Ettorre, 2010). It is
therefore possible that the brood is chemically transparent (cf. Martin et al., 2008),
i.e. it does not bear any nest-specific odour and so there is no mechanism of
colony-specific brood recognition. Interactions between brood and workers would
then be based on other semiochemicals, for example brood pheromones (Morel
and Vander Meer, 1998) and/or specific behaviours, such as begging. We can
exclude a total chemical transparency of A. echinatior brood because initially non-
nestmate brood is detected; the colony-specific profile may however be very weak
and thus fade away or be replaced fairly quickly during our experiments, which
would assist the unintended adoption process.
The alignment of cuticular profiles could also have been facilitated by workers
replacing the brood's fungal cover, as the fungus possesses a colony-specific
chemical profile that allows workers to discriminate against non-nestmate fungus
(Viana et al., 2001; Richard et al., 2007). Little is known about the exchange of
odours between workers or fungus and brood, but it seems that there is a relation
between fungus and brood chemical profiles since the two are similar to each
other (Viana et al., 2001; Richard et al., 2007). It has been noted before that
leafcutter brood is often covered by fungal hyphae, which nurse workers plant on
the surface of larvae and pupae (Lopes et al., 2005). It is not clear what the benefit
of fungal hyphae growing on brood is for the ants. In undisturbed colonies we
recorded that pupae were covered by hyphae to a greater extent than larvae, which
often do not have fungal cover. At the end of our experiments, the fungal cover
particularly of larvae, but also pupae, had increased. It might well be that brood
outside the fungus garden is likely to be contaminated by pathogens; thus workers
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clean the brood thoroughly and plant new hyphae before they transport it into
their nest, to avoid a contamination of their fungus garden.
What we recorded as “grooming” was in fact a composite behaviour. Workers did
not only lick brood, which protects larvae against diseases (Hughes et al., 2002);
we also observed that workers removed the fungus cover and planted new hyphae
before they carried a brood item into the fungus garden. The fact that more fungal
hyphae had to be removed from pupae than from larvae might explain why larvae
were generally brought into the fungus garden earlier than pupae. It is interesting
that it did not matter whether the brood was allo- or concolonial; the hyphal cover
of all brood items was replaced, irrespective of its origin.
We can only assume that replacing the hyphal cover serves to avoid contamination
that might threaten the fungus garden, which is a resource of vital importance for
the ant colony (Currie, 2001; Poulsen et al., 2005). A thorough cleaning of brood
is only possible when the brood's surface is free from hyphae; planting hyphae on
all brood items after cleaning might help to maintain the brood hygienic. It is
possible that the hyphal cover may be a permanent protection against pathogens;
the fungus may not only be a physical barrier preventing pathogens from attacking
the brood, but it may also secrete antimicrobial substances (cf. Lopes et al., 2005).
In summary, we show that Acromyrmex workers discriminate between different
brood stages as well as between allo- and concolonial brood of all stages.
Allocolonial brood is adopted after a delay, which indicates that allocolonial
brood is adopted by mistake, possibly because the profile of brood is aligned to
that of the workers. This alignment may be supported by the removal and
subsequent planting of fungal hyphae on the brood's cuticle, which is carried out
for all brood items before they are brought to the fungus garden.
Acknowledgements
This work was supported by the Marie Curie Excellence Grant CODICES (EXT-
CT-2004-014202) and partly by a ‘‘Freia grant’’ from the Faculty of Science,
University of Copenhagen, both assigned to PdE. BF was supported by the
ERASMUS programme, and VN by the German Academic Exchange Service
(DAAD). We thank the Smithsonian Tropical Research Institute (STRI) and the
Autoridad Nacional del Ambiente (ANAM) for permission to work in Panama,
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and the Danish Natural Science Research Council for financial support. The
authors also thank Hermogenes Fernandez Marin for fruitful discussions and all
the members of the Centre for Social Evolution, University of Copenhagen, for a
stimulating working environment.
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Figures
Fig. 1: The total duration of antennation (a) and grooming (b) that brood items of
different stages (ML: medium large; LL: large larvae; P: pupae) received by
concolonial (white boxes) and allocolonial (grey boxes) workers, and the delay
from first contact by the workers to the transport into the nest (c). Boxes cover the
interquartile range and whiskers all data points; the median is indicated by a
horizontal bar. Each box represents measurements from a total of 25 brood items
from 5 different colonies.
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Fig. 2: Number of brood items that received constant attention (a) and that were
attacked (b) by discriminator workers. In total 25 concolonial (white) and 25
allocolonial (grey) medium larvae (ML) and the same number of large larvae (LL)
and pupae (P) from five different colonies were observed.
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Fig. 3: Number of brood items inside the fungus garden over time. Concolonial
brood is brought into the fungus garden quicker than allocolonial brood, but after
one day all brood items are in the fungus garden. Because there were no
differences between the brood stages, items of different stages are pooled,
resulting in 15 con- and 15 allocolonial replicates per point in time. Each replicate
included five brood items. Continuous line: concolonial brood items, dashed line:
allocolonial brood items. Mean (circles) and standard errors are shown.
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Fig. 4: Level of fungus coverage of medium larvae (circles, solid line), large
larvae (squares, dashed line), and pupae (diamonds, dotted line) before and after
the experiment. A high score indicates that most of the brood's cuticle is covered
by fungal hyphae. Since there was no difference in the fungal coverage of con-
and allocolonial brood, these groups are pooled, resulting in a sample size of 50
brood items from 5 different colonies per data point. The data points indicate the
mean values and the vertical lines the standard error.
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... Les larves peuvent aussi être nourries d'oeufs trophiques pondus par les ouvrières (Le Masne 6 Par ailleurs, les ouvrières de nombreuses espèces sont capables de discriminer le couvain hétérospécifique, et parfois le couvain hétérocolonial , Fénéron & Jaisson 1992, Hare 1996, Fouks et al. 2011. Ces différents niveaux de discrimination pourraient se faire grâce aux différences morphologiques ou bien de la même manière que la reconnaissance entre ouvrières, grâce aux hydrocarbures cuticulaires D'Ettorre & Lenoir 2010), puisque les larves présentent un profil cuticulaire souvent similaire à celui des ouvrières de leur colonie, bien que la quantité de composants larvaires soit plus faible (Akino et al. 1999, Al Diri 2004). ...
... In this case, our experiment could have amplified the preference for fourth-instar larvae as ants were starved. Favouring older offspring was sometimes evidence in ants, for instance during brood transport or brood care (Lenoir 1981, Fouks et al. 2011) but also in other sub-social and social species (Glassey & Forbes 2002. ...
Thesis
L'investissement des adultes pour augmenter la survie des jeunes est souvent optimisé par une communication émise par les jeunes et informant les parents de leur niveau de besoin. Nous avons exploré dans cette thèse si les comportements des larves de fourmi pouvaient être des quémandes alimentaires telles que définies par les modèles de communication honnête. En effet, les fourmis, eusociales, possèdent un système de relations sociales et de coopérations qui diffère du modèle parental classiquement décrit. Nous avons donc testé chez la fourmi Ectatomma tuberculatum si les comportements des larves pouvaient refléter leur niveau de besoin et si les ouvrières apportaient la nourriture en fonction du signal comme prédit par les modèles. Nous montrons dans cette thèse que les mouvements émis par les larves ainsi qu'un composé chimique volatil larvaire pourraient tous deux intervenir et influencer l'apport alimentaire par les ouvrières. Les mouvements larvaires augmentent avec le stade de développement des larves et le composé chimique augmenterait avec le niveau d'affamement des larves. Les ouvrières, nourrices et fourrageuses, pourraient donc optimiser la répartition des ressources alimentaires de la colonie grâce à ces signaux ou indices des larves. Cependant, nous montrons aussi que plusieurs autres facteurs proximaux interviennent. Les mouvements larvaires font suite à des contacts fortuits avec les ouvrières, et l'organisation spatiale des ouvrières et des larves influencent les mouvements larvaires et le nourrissage. Des hypothèses alternatives ou complémentaires à la communication sont donc proposées.
... Earlier studies have shown that unrelated eggs of F. exsecta are accepted, at least in multiqueen and multinest societies (Holzer et al., 2006). Studies on other ants have shown that some species discriminate against conspecifics of earlier brood stages, eggs and larvae (Fouks, d'Ettorre, & Nehring, 2011 (Crosland, 1988;Haskins & Haskins, 1950;Souza et al., 2006). Conspecific pupae have also been shown to be either accepted (Haskins & Haskins, 1950;Maeder et al., 2005;Souza et al., 2006) or rejected (Carlin & Schwartz, 1989;Fouks et al., 2011). ...
... Studies on other ants have shown that some species discriminate against conspecifics of earlier brood stages, eggs and larvae (Fouks, d'Ettorre, & Nehring, 2011 (Crosland, 1988;Haskins & Haskins, 1950;Souza et al., 2006). Conspecific pupae have also been shown to be either accepted (Haskins & Haskins, 1950;Maeder et al., 2005;Souza et al., 2006) or rejected (Carlin & Schwartz, 1989;Fouks et al., 2011). These discriminatory rules may depend on a variety of factors and the studies cited above have different conditions and set-ups; hence it is difficult to conclude a general pattern of acceptance according to brood stage. ...
Article
The ability to distinguish friends from foe is a widespread phenomenon among social animals. In ants, recognition of intruders is important for the maintenance of colony integrity and survival. Intruders are typically adult, but the acceptance of non-nestmate brood could result in severe fitness costs, depending on the caste of the brood. Accepting non-nestmate worker brood may not carry a cost, as they should not drain resources of the adoptive colony but may instead add to the workforce. Sexual brood, however, would typically not contribute to colony performance, yet require resources, and should thus be rejected. Here, we tested whether workers of the narrow-headed ant, Formica exsecta, which strongly discriminate between adult nestmates and non-nestmates, also discriminate between nestmate and non-nestmate pupae. Furthermore, we investigated whether the caste of the brood (workers/sexuals) affects discrimination. We carried out analysis of surface chemicals to investigate whether the chemical distance between colonies was associated with the propensity to accept non-nestmate pupae. We show that worker pupae were retrieved irrespective of their origin, whereas nestmate sexual pupae were retrieved at a slightly higher rate than non-nestmates. Our chemical data, however, suggest that both the reproductive and the worker brood carry sufficient chemical information for discrimination, as they both express colony signatures. However, this information is acted upon only in the case of sexual brood. Our results thus suggest that workers selectively capitalize on the chemical information in agreement with fitness predictions, albeit to a lower extent than during discrimination between adult individuals.
... We found that worker pupae have more complex hydrocarbon profiles than adult workers. This is in apparent contrast to earlier studies on several genera of ants, which showed that brood profiles are simpler (Fouks et al. 2011;Richard et al. 2007;Viana et al. 2001), or at best match the adult profiles (Akino et al. 1999;Bagnères et al. 1991;Elmes et al. 2002;Helanterä and d'Ettorre 2014;Souza et al. 2006). The difference was due to shorter-chained compounds that were present in pupae, but not detected in adult workers. ...
Article
Full-text available
Chemical communication is common across all organisms. Insects in particular use predominantly chemical stimuli in assessing their environment and recognizing their social counterparts. One of the chemical stimuli used for recognition in social insects, such as ants, is the suite of long-chain, cuticular hydrocarbons. In addition to providing waterproofing, these surface hydrocarbons serve as a signature mixture, which ants can perceive, and use to distinguish between strangers and colony mates, and to determine caste, sex, and reproductive status of another individual. They can be both environmentally and endogenously acquired. The surface chemistry of adult workers has been studied extensively in ants, yet the pupal stage has rarely been considered. Here we characterized the surface chemistry of pupae of Formica exsecta, and examine differences among sexes, castes (reproductive vs. worker), and types of sample (developing individual vs. cocoon envelope). We found quantitative and qualitative differences among both castes and types of sample, but male and female reproductives did not differ in their surface chemistry. We also found that the pupal surface chemistry was more complex than that of adult workers in this species. These results improve our understanding of the information on which ants base recognition, and highlights the diversity of surface chemistry in social insects across developmental stages.
... The acceptance of unrelated queens was context-dependent (Sturgis and Gordon, 2012) and probably occurred when the direct benefits of acceptance, in terms of colony growth, survival and productivity, outweighed the costs of rejection. Our study hence conflicts the common view that the presence of alien, unrelated, individuals in colonies of social insects is solely due to failures in their recognition system (see Reeve, 1989;Vander Meer and Morel, 1998;Adams et al., 2007;Fouks et al., 2011). We propose that unrelated queen acceptance can be adaptive in specific contexts where peaceful cooperation is preferable to risky rejection. ...
Preprint
Full-text available
Relatedness underlies the evolution of reproductive altruism, yet eusocial insect colonies occasionally accept unrelated reproductive queens. To better understand this seemingly paradox, we investigated whether acceptance of unrelated queens by workers is an incidental phenomenon resulting from failure to recognize non-nestmate queens, or whether it is an adaptive behavior favored in specific contexts where cooperation is preferable to rejection. Our study system is the socially polymorphic Alpine silver ant, Formica selysi. Within populations some colonies have a single queen (monogynous), and others have multiple, sometimes unrelated, breeding queens .: bioRxiv preprint 2 (polygynous). Social organization is determined by a supergene with two haplotypes. In a first experiment we investigated whether workers from polygynous colonies were inherently more prone to accepting unrelated queens than workers from the alternate, monogynous social form. We found that workers rejected all alien queens, independently of their social origin and of the number of queens heading their colony. We then investigated whether queen acceptance was favored in specific conditions. We found that workers from polygynous colonies accepted alien queens when these queens were accompanied by workers. These results show that workers flexibly adjust their acceptance of alien queens according to the situation. We discuss how conditional acceptance of unrelated queens may be adaptive by providing benefits through increased colony size and/or genetic diversity, and by avoiding the rejection costs resulting from fighting.
... (E,L,P) Depends on presence of social parasite Witte et al. 2009 (H) Formica cunicularia (P) Depends on identity of intruder Pupa odor, pupa size Mori and Le Moli 1988; Mori et al. 1996 (H) Formica fusca (E,P) Depends on presence of social parasite, identity of intruder, worker post-eclosion experience Jaisson 1975;Chernenko et al. 2011Chernenko et al. , 2013 (SP) Formica rufa (P) Depends on identity of intruder, worker post-eclosion experience Pupa odor Le Passetti 1977, 1978;Rosengren and Cherix 1981;Maeder et al. 2005 (SP) Formica lugubris (P) Depends on identity of intruder, worker post-eclosion experience Rosengren and Cherix 1981;Le Moli and Mori 1982;Rosengren et al. 1994;Mori et al. 1996;Maeder et al. 2005 (H) Formica occulta (E,P) Depends on identity of intruder Zimmerli and Mori 1993;Johnson et al. 2005 (SP) Formica polyctena (P) Depends on identity of intruder, presence of social parasite, worker post-eclosion experience Jaisson 1975;Jaisson and Fresneau 1978;Rosengren and Cherix 1981 (H) Formica rufibarbis (P) Depends on identity of intruder Mori et al. 1996 (SP) Formicoxenus provancheri (L) Depends on worker post-eclosion experience Errard et al. 1992 (H) Myrmica incompleta (L) Depends on worker post-eclosion experience Errard et al. 1992 Tapinoma erraticum ( as chemically insignificant. Brood hydrocarbon profiles have indeed been shown to be less complex than adult cuticular profiles in some species (Viana et al. 2001;Richard et al. 2007;Fouks et al. 2011). However, the complexity of egg, larval and pupal profiles can also mirror that of adult profiles (Bagnères and Morgan 1991;Akino et al. 1999;Elmes et al. 2002;Souza et al. 2006;Tannure-Nascimento et al. 2009;Helanterä and d'Ettorre 2014). ...
Article
Full-text available
In social insect colonies, individuals need to communicate to coordinate cooperative tasks and protect the colony and its resources against intruders. To maintain colony integrity, it can be particularly important to recognize nestmates and discriminate against non-nestmate conspecifics and heterospecific predators and parasites. As typical intruders are either con- or heterospecific adults, the mechanisms underlying recognition and discrimination processes in interactions among adults have been well described. Ant brood (eggs, larvae, and pupae) can also play a key role in social interactions, and brood is of special importance when it comes to the priorities of worker ants. However, whether ants can, or even need to, recognize brood of different origins, is not always clear. In this review, we integrate the results of 100 years of study on brood recognition and discrimination in ants into a general framework. We begin with an overview of the proximate mechanisms involved in brood recognition and discrimination. We then discuss why brood recognition and discrimination should evolve and review the evidence for brood recognition on three organizational levels: within nests, between conspecifics and between species. We conclude by examining the constraints acting on accurate recognition and/or discrimination. With this review, we hope to inspire future research on the fascinating life of ant brood.
... This independence allows for flexibility in life trajectories and the appearance of individuals with no roles in the production, defense, or stock of agricultural products. Gowdy & Krall (G&K) compare insect colonies with human societies, but many characteristics of insect colonies could be compared with vertebrate family groups: related individuals , alloparental care (Lopes et al. 2005), or adoptions (Fouks et al. 2011). Interestingly, the social behavior of ultrasocial insects is almost entirely driven by chemical communication, which is also involved in kin recognition in vertebrates, including humans (Schaal & Porter 2003). ...
Article
Full-text available
We appreciate the depth and breadth of comments we received. They reflect the interdisciplinary challenge of our inquiry and reassured us of its broad interest. We believe that our target article and the criticisms, elaborations, and extensions of the commentators can be an important contribution to establishing human ultrasociality as a new field of social science inquiry. A few of the commentators questioned our definition of ultrasociality, and we begin our response with an elaboration of that definition and a defense of our argument that human ultrasociality began with agriculture. We then respond to the second major area of controversy, namely, our use of group selection to explain the economic drivers behind the agricultural transition. We then focus on the issue of human intentionality raised by the phenomenon of collective intelligence. The intriguing question is to what extent can an entire culture change its own destiny? We then address the issue of the division of labor raised by a number of commentators. The complex division of labor was both a driver and a defining characteristic of ultrasociality, even though it was present in simpler forms in earlier societies. The remaining issues addressed include energy and complexity, expansion and sustainability, and the accelerating evolution of human ultrasociality. These were raised by only a few commentators, but their importance warrants further elaboration.
... This independence allows for flexibility in life trajectories and the appearance of individuals with no roles in the production, defense, or stock of agricultural products. Gowdy & Krall (G&K) compare insect colonies with human societies, but many characteristics of insect colonies could be compared with vertebrate family groups: related individuals , alloparental care (Lopes et al. 2005), or adoptions (Fouks et al. 2011). Interestingly, the social behavior of ultrasocial insects is almost entirely driven by chemical communication, which is also involved in kin recognition in vertebrates, including humans (Schaal & Porter 2003). ...
Article
Given the highly social nature of the human emotion system, it is likely that it subserved the evolution of ultrasociality. We review how the experience and functions of human emotions enable social processes that promote ultrasociality (e.g., cooperation). We also point out that emotion may represent one route to redress one of the negative consequences of ultrasociality: ecosystem domination.
... This independence allows for flexibility in life trajectories and the appearance of individuals with no roles in the production, defense, or stock of agricultural products. Gowdy & Krall (G&K) compare insect colonies with human societies, but many characteristics of insect colonies could be compared with vertebrate family groups: related individuals , alloparental care (Lopes et al. 2005), or adoptions (Fouks et al. 2011). Interestingly, the social behavior of ultrasocial insects is almost entirely driven by chemical communication, which is also involved in kin recognition in vertebrates, including humans (Schaal & Porter 2003). ...
Article
The parallels between the agricultural successes of ultrasocial insects and those of humans are interesting and potentially important. There are a number of important caveats, however, including the relative complexities of insect reproduction, their more rigidly determined altricial patterns of social behaviour, the roles of post-reproductive group members, and differences in the known factors involved in ultrasocietal collapse.
... This independence allows for flexibility in life trajectories and the appearance of individuals with no roles in the production, defense, or stock of agricultural products. Gowdy & Krall (G&K) compare insect colonies with human societies, but many characteristics of insect colonies could be compared with vertebrate family groups: related individuals , alloparental care (Lopes et al. 2005), or adoptions (Fouks et al. 2011). Interestingly, the social behavior of ultrasocial insects is almost entirely driven by chemical communication, which is also involved in kin recognition in vertebrates, including humans (Schaal & Porter 2003). ...
Article
Full-text available
Experimental studies of group selection show that higher levels of selection act on indirect genetic effects, making the response to group and community selection qualitatively different from that of individual selection. This suggests that multilevel selection plays a key role in the evolution of supersocial societies. Experiments showing the effectiveness of community selection indicate that we should consider the possibility that selection among communities may be important in the evolution of supersocial species.
... This independence allows for flexibility in life trajectories and the appearance of individuals with no roles in the production, defense, or stock of agricultural products. Gowdy & Krall (G&K) compare insect colonies with human societies, but many characteristics of insect colonies could be compared with vertebrate family groups: related individuals , alloparental care (Lopes et al. 2005), or adoptions (Fouks et al. 2011). Interestingly, the social behavior of ultrasocial insects is almost entirely driven by chemical communication, which is also involved in kin recognition in vertebrates, including humans (Schaal & Porter 2003). ...
Article
Full-text available
Ultrasociality refers to the social organization of a few species, including humans and some social insects, having complex division of labor, city states and an almost exclusive dependence on agriculture for subsistence. We argue that the driving forces in the evolution of these ultrasocial societies were economic. With the agricultural transition, species could directly produce their own food and this was such a competitive advantage that those species now dominate the planet. Once underway, this transition was propelled by the selection of within-species groups that could best capture the advantages of (1) actively managing the inputs to food production, (2) a more complex division of labor, and (3) increasing returns to larger scale and larger group size. Together these factors reoriented productive life and radically altered the structure of these societies. Once agriculture began, populations expanded as these economic drivers opened up new opportunities for the exploitation of resources and the active management of inputs to food production. With intensified group-level competition, larger populations and intensive resource exploitation became competitive advantages and the "social conquest of earth" was underway. Ultrasocial species came to dominate the earth's ecosystems. Ultrasociality also brought a loss of autonomy for individuals within the group. We argue that exploring the common causes and consequences of ultrasociality in humans and the social insects that adopted agriculture can provide fruitful insights into the evolution of complex human society.
Article
Both behavioral and chemical analyses demonstrated that in the ant Camponotus vagus the colony recognition signal is strongly correlated with the composition of cuticular hydrocarbons. Variation of relative proportions of dimethylalkanes characterize the chemical signatures in different colonies.
Article
Abstract Hydrocarbons are the main lipid constituents on the insect cuticle, and generally provide the insect with a waterproof layer to prevent desiccation. In many insects this class ofchemicals,has been coopted to serve as pheromones. In so- cial insects, in particular in ants cuticular hydrocarbons (CHCs) have at least two pheromonal functions. They act as recognition cues that facilitate colony insularity, protecting it from parasites or conspecific invasions. Supporting evi- dence for this function are their extreme complexity, their colony specific composition, and in a few cases also de- monstrating elevated or reduced,aggression between,encountering ants as a function of the label (alien or nestmate) they were painted with. The second function of CHCs is in signaling fertility. In many ant species it was demonstrated that fertile individuals (queens, gamergates, or egg laying workers) have CHC profiles that are distinct from that of their sterile nestmates. This can be expressed as the augmentation of a single or a small subset of the blend components, or differences in the entire blend. The fact that these signals have an abundance of branched alkanes, which lower the break point of cuticular impermeability thus imposing cost on the individual, indicates that these may constitute honest signals. This dual function seems contradictory since nestmate recognition necessitates a uniform colony odor, i.e., uniform CHC composition, whereas fertility signal requires idiosyncrasy since the fertile individual needs to be singled out among the colony members,rather than conform to colony odor. A possible resolution is that species that use CHCs for nestmate recognition do not use them as fertility signals and vice versa. I propose an alternative solution whereby,workers have variable discrimination thresholds and response to differences in the pheromone blend, large or small, in a context- dependent manner. Key words:Cuticular hydrocarbons, nestmate recognition, fertility signals, pheromones, reproductive skew. Myrmecol. News 10: 59-68 Prof. Dr. Abraham Hefetz, Department of Zoology, George S. Wise Faculty of LifeScience, Tel Aviv University, Ramat Aviv 69978, Israel. E-mail: hefetz@post.tau.ac.il
Article
Variation in the relative size of the metapleural gland (bulla) was analyzed for workers of two sympatric but genetically differentiated species from the Acromyrmex octospinosus species complex. For the minor workers the increase of bulla width with pronotum width was about isometric, but the slope of this relationship gradually decreased through the media to have a slope of around 0.45 for the major workers of both species. Further analysis of relative bulla size in major workers showed that a significant proportion of the individual variation (35% in species 1 and 22% in species 2) was due to among colony differences. The significance of this variation is discussed, with particular reference to the antibiotic-producing function of the metapleural gland.
Chapter
The ability to recognize group members is a key characteristic of social life. Ants are typically very efficient in recognizing non-group members and they aggressively reject them in order to protect their colonies. There are a range of different recognition mechanisms including prior association, phenotype matching, and recognition alleles. The concept of kin recognition should be considered different from that of nestmate recognition. Most of the available studies address the nestmate recognition level, namely the discrimination of nestmates from non-nestmates, independently of actual relatedness. Indirect and direct evidence identify long-chain cuticular hydrocarbons as the best candidates to act as recognition cues in ants, even if other chemical substances could also play a role, at least in some ant species. The relative importance of genetic and environmental factors on the expression and variation of the cuticular hydrocarbon profile vary among species and is linked to life history strategies.
Article
The objective of the present study was to evaluate the interrelationship between the cuticular and postpharyngeal glands' hydrocarbons, both in the individual ant and during its interaction with nestmates. In vivo radiochemical assays were employed to monitor the de novo hydrocarbon biosynthesis from acetate in the ant's body. The newly synthesized hydrocarbons appeared first internally and after 24 hr they accumulated in the postpharyngeal gland and on the cuticular surface. Blocking the possibility of external transfer of hydrocarbons between cuticle and postpharyngeal gland led to a significant decrease of labeled hydrocarbons in the postpharyngeal gland. In addition, during encounters between labeled and unlabeled ants, newly synthesized hydrocarbons were transferred, mainly via trophallaxis, but also by allo-grooming and physical contact. In view of these results, we propose as a model for their dynamics that hydrocarbons are synthesized in tissues associated with the integument. Through self-grooming, there is a constant exchange of hydrocarbons between the cuticular surface and the postpharyngeal gland. Furthermore, in encounters between nestmates, hydrocarbons are exchanged among them mostly by trophallaxis, with the mediation of the postpharyngeal gland. Thus, this gland acts as a pool for mixing colonial hydrocarbons and may serve to attain a unified colony odor.