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Sick ants become unsociable

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Sick ants become unsociable

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Parasites represent a severe threat to social insects, which form high-density colonies of related individuals, and selection should favour host traits that reduce infection risk. Here, using a carpenter ant (Camponotus aethiops) and a generalist insect pathogenic fungus (Metarhizium brunneum), we show that infected ants radically change their behaviour over time to reduce the risk of colony infection. Infected individuals (i) performed less social interactions than their uninfected counterparts, (ii) did not interact with brood anymore and (iii) spent most of their time outside the nest from day 3 post-infection until death. Furthermore, infected ants displayed an increased aggressiveness towards non-nestmates. Finally, infected ants did not alter their cuticular chemical profile, suggesting that infected individuals do not signal their physiological status to nestmates. Our results provide evidence for the evolution of unsociability following pathogen infection in a social animal and suggest an important role of inclusive fitness in driving such evolution.
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Sick ants become unsociable
N. BOS*, T. LEFE
`VRE,A.B.JENSEN*,à&P.D’ETTORRE*,§
*Centre for Social Evolution, Department of Biology, University of Copenhagen, Copenhagen, Denmark
Biology Department, Emory University, Atlanta, GA, USA
àCentre for Social Evolution, Department of Agriculture and Ecology, University of Copenhagen, Copenhagen, Denmark
§Laboratoire d’Ethologie Expe
´rimentale et Compare
´e (LEEC), Universite
´Paris 13, Villetaneuse, France
Introduction
Living in groups confers several advantages to the
individuals involved, including increased foraging effi-
ciency, cooperative brood care, enhanced reproduction
and predator defence (Maynard Smith & Szathmary,
1995; Bourke, 2011). Group living, however, is also
subject to significant costs mostly in the form of increased
disease risk because of higher frequencies of social
interactions and typically a high level of relatedness
within the group (Bull et al., 1991; Myers & Rothman,
1995; Schmid-Hempel, 1998; McCallum et al., 2001;
Boomsma et al., 2005). In response to parasite (broadly
defined here to include both macroparasites and micro-
parasites such as fungi) pressure, group living organisms
have evolved several lines of defence. In addition to their
individual physiological immune mechanisms, they are
able to engage in a group-level immunity consisting of a
series of behaviours that reduce the risk of infection or
minimize the parasite-induced fitness losses (Cremer
et al., 2007; Cremer & Sixt, 2009; Wilson-Rich et al.,
2009). In social insects, group-level defences include
avoidance behaviours (Oi & Pereira, 1993; Cremer et al.,
2007; Cotter & Kilner, 2010), hygienic behaviours
(reviewed in Wilson-Rich et al., 2009), collection of
antimicrobial resin (Christe et al., 2003; Castella et al.,
2008; Simone et al., 2009), spread of metapleural gland
secretions (Yek & Mueller, 2010), behavioural fever
(Starks et al., 2000) and allo-grooming (Yanagawa &
Shimizu, 2007; Walker & Hughes, 2009).
Allo-grooming is a behaviour found in many social
animals, from primates to social insects, and is a mean of
removing parasites from another individual. This social
interaction is involved in disease resistance in at least two
ways. First, allo-grooming seems to be a very efficient
way of physically removing infective fungal conidia from
nestmates (Rosengaus et al., 1998; Hughes et al., 2002;
Yanagawa et al., 2008; Walker & Hughes, 2009). Second,
allo-grooming and other social contacts have been shown
to boost host resistance to parasites by ‘social transfer’ of
disease resistance (Traniello et al., 2002; Ugelvig &
Cremer, 2007).
Recently, it has been shown that moribund ants
(Heinze & Walter, 2010) and honeybees (Rueppell et al.,
2010) leave their nest and die in isolation. Previously, it
has been found that infected ants spend less time with
the brood (Ugelvig & Cremer, 2007). These results could
Correspondence: Nick Bos, Centre for Social Evolution, Department of
Biology, University of Copenhagen, Copenhagen, Denmark.
Tel.: +45 35 32 12 38; fax: +45 35 32 12 50;
e-mail: nbos@bio.ku.dk
ª2011 THE AUTHORS. J. EVOL. BIOL. 25 (2012) 342–351
342 JOURNAL OF EVOLUTIONARY BIOLOGY ª2011 EUROPEAN SOCIETY FOR EVOLUTIONARY BIOLOGY
Keywords:
ants;
chemical communication;
host-parasite interaction;
infection;
insects;
life history evolution.
Abstract
Parasites represent a severe threat to social insects, which form high-density
colonies of related individuals, and selection should favour host traits that
reduce infection risk. Here, using a carpenter ant (Camponotus aethiops) and a
generalist insect pathogenic fungus (Metarhizium brunneum), we show that
infected ants radically change their behaviour over time to reduce the risk of
colony infection. Infected individuals (i) performed less social interactions
than their uninfected counterparts, (ii) did not interact with brood anymore
and (iii) spent most of their time outside the nest from day 3 post-infection
until death. Furthermore, infected ants displayed an increased aggressiveness
towards non-nestmates. Finally, infected ants did not alter their cuticular
chemical profile, suggesting that infected individuals do not signal their
physiological status to nestmates. Our results provide evidence for the
evolution of unsociability following pathogen infection in a social animal
and suggest an important role of inclusive fitness in driving such evolution.
doi: 10.1111/j.1420-9101.2011.02425.x
be predicted by inclusive fitness theory, where a mori-
bund ant increases its inclusive fitness by leaving the
nest, to prevent nestmates and brood from getting
infected. Although the behavioural changes observed in
social insects following parasite exposure and infection
are likely to provide fitness benefits to the colony, and
hence represents host adaptations, they could also be the
consequence of parasitic manipulation whereby parasites
modify the host phenotype (e.g. behaviour) in a way that
increase their transmission (Schmid-Hempel, 1998;
Moore, 2002; Andersen et al., 2009; Lefevre et al.,
2009). For example, the fungal parasite Ophiocordyceps
manipulates the behaviour of its ant host causing it to
abandon its colony, and bite and cling onto the underside
of leaves where temperature and humidity conditions are
optimal for fungal growth (Andersen et al., 2009). An
increasing number of studies on generalist pathogens
provide evidence in favour of behavioural modifications
as host adaptations (Rosengaus et al., 1998; Hughes et al.,
2002; Traniello et al., 2002; Ugelvig & Cremer, 2007). For
instance, a recent study by Heinze & Walter (2010)
elegantly ruled out the alternative hypothesis of parasitic
manipulation by exposing healthy ants workers to CO
2
,
hence reducing their prospective lifespan and observing
that they desert their nest and die in seclusion exactly
like their nestmates infected by a fungus.
The behavioural changes performed by uninfected
colony members towards their infected conspecifics (e.g.
exclusion, intensive care, allo-grooming) could be based
on possible changes in the recognition cues of infected
individuals, although this has not yet been tested.
Communication in social insects occurs predominantly
through the use of chemicals. In ants, cuticular hydro-
carbons play a major role in a wide range of communi-
cation and interaction processes including nestmate and
caste recognition (reviewed in van Zweden & d’Ettorre,
2010) and home range markings (Lenoir et al., 2009).
Thus, we might expect that the cuticular hydrocarbon
profile of infected individuals would undergo a modifi-
cation triggering a change in the behaviour of their
nestmates, although other chemicals might be involved.
However, behavioural changes observed in infected
individuals, such as self-exclusion from the colony, do
not necessarily have to involve chemical communication.
To date, the different behavioural mechanisms used by
social insects to fight their parasites have mostly been
studied in isolation (but see Wilson-Rich et al., 2007). For
instance, Walker & Hughes (2009) focused on allo-
grooming, whereas Heinze & Walter (2010) focused on
ant location. Here, we present an integrated study,
investigating social interactions and chemical recognition
cues in the ant Camponotus aethiops following infection.
We exposed experimental colonies of C. aethiops to
Metarhizium brunneum, a generalist entomopathogenic
fungus commonly used as an experimental parasite of
ants and termites (Hughes & Boomsma, 2004; Calleri
et al., 2006; Chapuisat et al., 2007). First, we recorded the
location and behaviours exhibited by infected and unin-
fected ants until the death of infected individuals.
Second, we investigated whether the cuticular hydrocar-
bon profile of infected individuals were different from
that of uninfected nestmates. Finally, we tested whether
infection changes the behaviour of ants with regard to
nestmate recognition. We hypothesized that terminally
sick individuals might become more aggressive against
non-nestmates, as they have ‘nothing to lose’, and by
repelling potential colony invaders inclusive fitness
benefits could still be gained.
Methods
Study organisms
Five queenright colonies of C. aethiops (Latr.) were
collected in April 2009 in the Italian Apennines. Each
colony was housed in a Fluonª(De Monchy, the
Netherlands)-coated plastic box (27 ·17 ·9.5 cm) with
a plaster floor serving as the nest area. This was
connected with a short plastic tube to another plastic
box (27 ·17 ·9.5 cm) serving as a foraging arena. The
ants were fed twice a week with diluted honey and
mealworms (Tenebrio molitor); water was provided ad
libitum. The nests were kept in a climate room, at
25 ± 2 C, D:L 12:12 h. Subcolonies were created
2 weeks before the start of the experiment, by housing
50 ants (40 minor and 10 major workers, taken all from
the foraging arena and thus presumably of similar age)
and 10 larvae in a plastic box (17 ·11 ·6 cm) with a
plaster floor, serving as a nest. The lid of the nest box was
covered with opaque tape to darken the nest area. The
nest box was connected to another plastic box of the
same size, serving as a foraging arena, also closed with a
lid perforated with 10 holes of 2 mm diameter each.
Relative humidity inside the boxes was ± 52%. For
experiment 1, two subcolonies (test and control) were
created from each of the five colonies, for a total of 10
subcolonies. For experiments 2 and 3, four subcolonies
(two tests and two controls) were created from each of
three different colonies, for a total of 12 subcolonies.
Treatment
Conidia suspensions of M. brunneum (strain KVL 04-57,
formerly named Metarhizium anisopliae, Bischoff et al.,
2009) were created according to Hughes et al. (2002).
Conidia were harvested from SDA agar plates and diluted
in 0.05% Triton-X. Their viability was checked by plating
100 lLof1·10
5
conidia per millilitre solution on SDA
agar plates, incubating for 24 h at 23 C and recording
germination at a·400 magnification (the per cent of
conidia germination was 98%). The conidia solution was
diluted to a final concentration of 1 ·10
8
viable conidia
per millilitre, assuring effective infection (c.f. Rosengaus
et al., 1998). Ants to be treated (minor workers) were
Sick ants become unsociable 343
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marked individually with dots of enamel paint. The day
after, individual marked ants were exposed to M. brun-
neum (hereafter ‘treated ants’), by submerging them for
5 s in the conidia suspension (previously vortexed). After
this, each ant was allowed to walk on a clean filter paper
to remove excess liquid. The ant was then put back in the
foraging arena of its subcolony. For the control, individ-
ual marked ants were submerged into 0.05% Triton-X
solution (hereafter ‘control ants’).
In summary, for experiment 1, five ants per subcolony
were marked and treated. Half of the subcolonies (n=5)
were treated with conidia (test) and the other half
(n= 5) with Triton-X (control), for a total of 25 treated
and 25 control ants. For experiments 2 and 3, eight ants
were marked and treated per subcolony. Six subcolonies
were treated with conidia (tests) and the other six with
Triton-X (control). Three of these ants per subcolony
were used for experiment 2, whereas the other five were
used for experiment 3, for a total of 48 treated and 48
control ants.
Experiment 1: Behavioural study
During 6 days, starting on the day after exposure to
M. brunneum, the behaviour of each marked ant (5 ants for
each of the 10 subcolonies) was recorded for 4 min twice a
day, once in the morning (between 9 and 12
AMAM
) and once
in the afternoon (between 2 and 5
PMPM
), and the order in
which the different subcolonies were observed was ran-
domized. The following behaviours performed by other
workers towards the marked ants (either treated or
control) were recorded using the software E
THOTHO
L
OGOG
(Ottoni, 2000): (i) No contact, (ii) Antennation, (iii) Allo-
grooming, (iv) Stomodeal trophallaxis (exchange of liquid
food). No aggressive acts (mandible opening, biting and
gaster flexing) were observed. Additionally, the following
behaviours performed by the marked ant itself were
recorded: (i) self-grooming, (ii) allo-grooming, and (iii)
stomodeal trophallaxis. As it was not always clear in which
direction trophallaxis took place, both ‘donating’ and
‘receiving’ trophallaxis were pooled together for analysis.
The location of the ants was recorded, divided into
three categories: (i) nest (excluding brood pile), (ii)
foraging arena, and (iii) brood pile. All observations were
done blind with respect to treatment. On the sixth day of
observation, only two of the infected ants were still alive,
and thus only the results from the first 5 days of
observation are reported.
When marked cadavers of both treated and control ants
were found, they were surface sterilized for 5 s in 10%
sodium hypochlorite (Lacey & Brooks, 1997) and placed in
individual medicine cups containing a ball of moist cotton
wool. These cadavers were checked for the appearance of
M. brunneum conidia to make sure the cause of death was
indeed infection. Six of 25 treated ants escaped from the
subcolonies or had their colour mark rubbed off. Of the 19
remaining focal ants that had been treated, a total of 15
(79%) became infected and none of the control ants were
found infected. All subsequent analyses are restricted to
these infected (n= 15) and control (n= 25) ants. Overall,
analyses of ant behaviours and locations were based on
248 four-minute observations of control individuals and
127 four-minute observations of treated individuals
(because infected individuals died over the course of the
experiment).
Experiment 2: Chemical analysis
It takes 2–4 days for the fungus to establish an infection
and involves attachment of the conidia to the cuticle,
conidia germination, formation of appressoria and pene-
tration through the insect cuticle by enzymatic degrada-
tion and mechanical pressure (Gillespie et al., 2000). Thus,
on day three after inoculation, three marked ants per
subcolony (n= 6 test and 6 control subcolonies) were put
in individual vials and frozen. Chemical extracts were
made by submerging each ant in 150 lL of solvent
(pentane; Sigma-Aldrich, Copenhagen, Denmark) for
10 min, after having removed the gaster, which contained
the marking. The extract was transferred to a 200-lL glass
insert, the solvent was allowed to evaporate and the
extract was re-dissolved in 50 lL of solvent. Gas chroma-
tography–mass spectrometry (GC-MS) was performed as
described in Van Zweden et al. (2009), using a Agilent
Technologies (Horsholm, Denmark) 6890N gas chromato-
graph (capillary column: Agilent HP-5MS, 30 m ·
25 lm·0.25 lm; split–splitless injector; carrying gas:
helium at 1 mL min
)1
) coupled with an Agilent 5975
mass selective detector and 70eV electron impact ioniza-
tion. We obtained chemical profiles very similar to those
found in the study of Van Zweden et al. (2009), and
identified and integrated 36 GC peaks and used them for
statistical analysis. Integration of chemical data was
performed blind with respect to treatment. Before day
three, 2 of 36 treated ants died, making a total of 34 ants
analysed.
Before the start of the experiments, a pilot study was
conducted to establish whether treatment itself (sub-
merging the ant in test and control solution) affects the
chemical profile of ants. Five ants were treated with the
conidia solution, five with Triton-X, and five were not
treated at all. After 1 hour, chemical extracts were
prepared and injected in a GC-MS (same method as
above) for chemical analysis. Treatment itself (conidia
solution or Triton-X) did not affect the chemical profile
compared with nontreated ants (
MANOVAMANOVA
, Pillai V = 1.22,
F
14,12
= 1.36, P= 0.30).
Experiment 3: Aggression tests
On day three after treatment, aggression tests were
conducted by placing a treated or control ant (focal
individual) and a non-nestmate (target individual) into a
circular neutral arena (50 ·60 mm), where they were
344 N. BOS ET AL.
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allowed to habituate for 1 min. The target ant was
separated from the focal ant by a plastic cylinder. After
1 min, the cylinder was removed and the observations
started. The antennae of the target ant were abscised
prior to the test, so that the target ant was alive but
passive. This allowed us to record the behaviour of the
focal ant against a live ant, without the risk of the target
ant interfering. The following items were recorded during
3 min: (i) no contact, (ii) antennation, (iii) mandible
opening, (iv) biting and (v) gaster flexing. For each
aggression test, an aggression index was calculated
according to Errard & Hefetz (1997). In addition, avoid-
ance behaviour was calculated, as defined by the dura-
tion of ‘no contact’ after the first contact, divided by the
total time after the first contact. Before day 3, five of
the treated ants died, making a total of 55 aggression
tests conducted.
Statistical analyses
Analyses of all data were carried out in R version 2.8.1(R
Development Core Team, 2008). Generalized Linear
Mixed Models (GLMM) were used to investigate the
effect of treatment, colony, and time on the duration spent
by focal ants (treated and controls) in a given location and
on the duration spent performing a given behaviour
(trophallaxis, antennation, self-grooming, being
groomed, grooming other). A GLMM with a binomial
error structure and logit link function was used to
examine the effect of treatment, time and colony on the
likelihood that focal ants were observed in a given location
and engaged in a given behaviour. Finally, locations and
behaviours frequencies were analysed using GLMM with
Poisson errors. For all GLMM analyses, we used the lmer
function of the lme4 library. Time was treated as a
continuous variable, and colony and treatment were
treated as categorical variables. Full models included
treatment, colony and time as explanatory variables,
and all relevant second-order interactions. Time and
treatment were specified as fixed effects and colony as a
random effect. We also included ant identity as a nested
random factor within colony to account for the repeated
behavioural measurements performed on individual ants
(i.e. pseudoreplications) (Pinheiro & Bates, 2000).
From the full models, we removed nonsignificant
(P> 0.05) terms, starting with the least significant
second-order interaction. Only terms for which removal
significantly reduced the explanatory power of the model
were retained in the minimal model (Crawley, 2007).
Model selection was based upon the likelihood ratio test
(LRT) statistics. Terms significance was also estimated
with an alternative method by Markov chain Monte
Carlo sampling (using the function mcmcsamp from
lme4 (Baayen et al., 2008)). The two approaches yielded
the same results, and only the LRT tests are reported on.
Peak areas of the integrated cuticular hydrocarbon
profiles were normalized according to Aitchison (1986)
and used as variables in a principal component analysis,
followed by
MANOVAMANOVA
, with both treatment and colony as
categorical predictors. Effect size was calculated using the
software G
**
P
OWEROWER
3.1.3 (Erdfelder et al., 1996).
Results from the aggression tests were square root
transformed, and afterwards analysed using a general
linear model, with both treatment and colony as cate-
gorical predictors.
Results
Experiment 1: Behavioural study
All data presented are based on a total of 127 observation
periods of 240 s for treated ants and 248 observation
periods of 240 s for the control ants.
The survival of infected ants was much lower than
control, uninfected ants (Fig. 1). Although all infected ants
died by day 7 post-exposure, none of the control ants died
over the course of the period of survival observation
(7 days). We found no effect of treatment on antennation
time (LRT v
2
= 0.35, P= 0.55, Fig. 2a, see Table S1 for full
model details). However, antennation significantly varied
among ant colonies (LRT v
2
= 12.77, P= 0.01) and
decreased over the course of the experiment (LRT
v
2
= 19.77, P< 0.001, Fig. 2a). Treated ants spent signif-
icantly less time performing trophallaxis than their unin-
fected counterparts (mean ± SE; 8 ± 0.7 s and 16.7 ± 1 s
respectively; LRT v
2
= 4.9, P= 0.02, Fig. 2b). We also
found that trophallaxis decreased over time in both treated
and control workers (LRT v
2
= 4.7, P= 0.03, Fig. 2b).
Treated ants were groomed significantly more often by
nestmates than control ants (10.4 ± 2.90 s and
01234567
0.0
0.2
0.4
0.6
0.8
1.0
Time (days)
Survival
Infected ants
Control ants
Fig. 1 Survivorship of infected and uninfected individuals. Propor-
tion of infected and control ants that survived during the first 7 days
of observation.
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ª2011 THE AUTHORS. J. EVOL. BIOL. 25 (2012) 342–351
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3.98 ± 0.99 s for treated and control individuals respec-
tively, LRT v
2
=8,P= 0.005, Fig. 2c), although this only
occurred on days 1 and 2 post-treatment (time by treat-
ment interaction: LRT v
2
= 10.1, P= 0.02, Fig. 2c). We
also found a significant colony main effect with individuals
from colonies denoted 1 and 3 spending more time being
groomed than individuals from other colonies (LRT
v
2
= 9.7, P= 0.04). There was a significant colony-by-
treatment interaction indicating that the intensity of care
directed towards treated individuals varied among colonies
(LRT v
2
= 10.3, P= 0.04). Treated individuals almost
never performed allo-grooming towards nestmates from
day 1 post-treatment until death, although the difference
with control workers was not statistically significant
(1 ± 0.09 s and 3.2 ± 0.2 s for treated and control indi-
viduals respectively, LRT v
2
= 2.2, P= 0.13, Fig. 2d).
There was a strong effect of infection on self-grooming,
with treated workers spending significantly more time
performing this activity than control workers (26 ± 2.3 s
and 15 ± 1 s respectively, LRT v
2
= 11.8, P< 0.001,
Fig. 2e). We also found a significant effect of time on self-
grooming with both treated and control ants self-grooming
less and less over the course of the experiment (LRT v
2
=4,
P= 0.04, Fig. 2e). The observed decrease in self-grooming
over time was much steeper in treated individuals than in
uninfected counterparts (time by treatment interaction:
LRT v
2
=7, P= 0.008, Fig. 2e). Finally, self-grooming
significantly varied among colonies with individuals from
colony 4 spending less time self-grooming than individuals
from other colonies (LRT v
2
= 11, P= 0.03).
The probability of becoming infected upon pathogen
inoculation was not influenced by self-grooming and or
(a) (b)
(d)
(c)
(e)
Fig. 2 Behaviours of infected and uninfected ants overtime. Infected ants did not differ from control ants in antennation time. They
performed significantly less trophallaxis and less allo-grooming (nonsignificant trend). However, infected ants were groomed significantly more
by their nestmates, as well as increasing the time spent self-grooming. Data are presented as mean ± SE.
346 N. BOS ET AL.
ª2011 THE AUTHORS. J. EVOL. BIOL. 25 (2012) 342–351
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on receiving allo-grooming. In other words, the four
individual ants that remained uninfected upon treatment
with the pathogen were not those that spent most time
self-grooming or being groomed (GLM binomial, self-
grooming (Odds Ratio = 1.2, 95% confident interval
(CI) = (0.9,1.5), P= 0.3, being groomed OR = 1.3,
CI = (0.8,1.8), P= 0.5).
We found that treated ants spent significantly less time
in the nest than control ants (treated ants: 170.6 ± 14.9 s;
control ants 235.3 ± 15.1 s; GLMM, v
2
= 4.13, P= 0.04,
Fig. 3a). Control workers indeed spent almost all their
time in the nest whereas infected individuals spent an
increasing amount of time in the foraging arena as the
infection progressed (treatment by time interaction: LRT
v
2
= 30.6, P< 0.001, Fig. 3a). There was also a significant
effect of treatment on the time spent with the brood.
Treated individuals had almost no interaction with the
brood from day 1 post-infection until death (treated ants:
2.5 ± 0.23 s; control ants: 19.1 ± 1.2 s; LRT v
2
= 3.5,
P= 0.04, Fig. 3b). Control ants spent 8.13% ± 1.45 of
the time they spent in the nest with the brood. In contrast,
treated ants spent only 1.39% ± 0.99 of their time inside
the nest with the brood.
Overall, the analyses of location and behaviour fre-
quency and likelihood yielded similar conclusions (see
Table S1 for full model details of frequency and likeli-
hood analyses). In other words, if treated ants spent more
time performing a given behaviour than control ants,
they were also more likely to be engaged in the
behaviour (likelihood analysis) and performed it more
often (frequency analysis).
Experiment 2: Chemical analysis
The principal component analysis based on the normal-
ized area of the 36 identified peaks extracted six principal
components explaining 93.9% of the observed variation.
A plot of the first three PCs (explaining the 79.5% of the
total variance) shows that infected and uninfected ants
are grouping together within each subcolony (Fig. 4). No
significant difference was found between treated and
control ants (
MANOVAMANOVA
, Pillai V = 0.36, F
6,23
= 2.11,
P= 0.09, effect size f
2
V = 0.54), indicating that the
chemical profile did not change significantly after infec-
tion (Fig. 4, Fig. S1 and Table S2). Colonies were signif-
icantly different from each other (
MANOVAMANOVA
, Pillai
V = 1.98, F
12,48
= 318.23, P< 0.01); however, no signif-
icant interaction between colony and infection was
found (
MANOVAMANOVA
, colony x treatment, Pillai V = 0.59,
F
12,48
= 1.68, P= 0.10).
(a) (b)
Fig. 3 Changes in space use of infected and uninfected ants overtime. Infected ants spend less time than control ants both in
the nest (a) and with the brood (b) over the course of the experiment. Data are presented as mean ± SE.
Fig. 4 Chemical analysis of control and infected ants. Scatterplot of
the first three factors of the PCA (explaining 79.5% of the total
variation, Factor 1: 48.0%; Factor 2: 20.9%; Factor 3: 10.6%),
showing that treated ants were not separated from control ants.
Symbols denote different colonies. Black symbols are infected ants,
and white symbols are control ants.
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ª2011 THE AUTHORS. J. EVOL. BIOL. 25 (2012) 342–351
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Experiment 3: Aggression tests
Treated ants, tested on day 3 post-inoculation, showed a
higher level of aggression against non-nestmates than
uninfected control ants (GLM, F= 6.36, P= 0.01,
Fig. 5a). There was no significant effect of colony on
aggression level (GLM, F= 1.84, P=0.17); however, a
marginally significant effect on aggression was found in
the interaction between infected x colony (indicating
that the effect of infection on aggression was not
consistent across colonies, GLM, F= 3.26, P= 0.047).
Also, treated ants showed less avoidance behaviour when
compared to control individuals (GLM, F= 4.36,
P=0.04, Fig. 5b). No significant effect of colony was
found on avoidance (GLM, F= 1.80, P= 0.18); however,
a nonsignificant trend was found in the interaction
between infected x colony (GLM, F= 3.15, P=0.052.
Discussion
The main aim of this study was to investigate how
treatment with a pathogen affects behaviour and com-
munication in ants. We found that treatment with the
generalist entomopathogenic fungus M. brunneum pro-
foundly changes the behaviour of both infected ants and
their nestmates.
Treated ants spent significantly less time performing
trophallaxis than their uninfected counterparts. We also
found that the amount of trophallaxis decreased over the
course of the experiment in both treated and control
workers. A possible explanation for this is that almost no
foraging activity was observed in both groups, suggesting
that ants were satiated and thus newly provided food was
not brought into the nest, decreasing the need for
trophallaxis. Our results are different from those of De
Souza et al. (2008) and Hamilton et al. (2011), who found
an increase in trophallaxis after an immune challenge,
but in these studies ants were either socially isolated (for
24 or 8 h, De Souza et al., 2008) or the receiver was
starved (Hamilton et al., 2011), thus generally increasing
the need for trophallaxis. Moreover, in our case the ants
were exposed to a real parasite, whereas in the above
cited studies the ants were immunized and could share
their immunological factors with nestmates without the
risk of spreading the disease; however, whether the ants
can distinguish between an immune challenge and an
infection that will lead to its death remains to be
investigated.
We found that treated ants exhibited an increase in
self-grooming and received more allo-grooming in com-
parison with control nestmates on days 1 and 2 (i.e.
Rosengaus et al., 1998; Walker & Hughes, 2009).We
interpret this behaviour as a way to get rid of the conidia
(the infective units of the parasite) present on the cuticle
(Walker & Hughes, 2009). Indeed, salivary secretions
with antimicrobial properties (e.g. Bulmer et al., 2009)
could be spread through allo-grooming and thus poten-
tially decrease the microbial load. However, in this study
we did not find that the probability of becoming infected
upon inoculation with the pathogen depended on self-
grooming and or on receiving allo-grooming, possibly
because of the high concentration of conidia used.
Another possible function of grooming treated individu-
als could be for the uninfected nestmates to gain
resistance towards the pathogen (Ugelvig & Cremer,
2007; Walker & Hughes, 2009).
The fact that in the first 2 days the treated ants
received more grooming than control ants suggests that
nestmates can detect whether an ant is in need of
grooming. Several nonmutually exclusive explanations
(a) (b)
Fig. 5 Aggression tests. Aggression index (a) and avoidance index (b) of infected and control ants, measured on day 3 post-exposure
to Metarhizium brunneum. Infected ants were more aggressive and displayed less avoidance behaviour than control ants.
348 N. BOS ET AL.
ª2011 THE AUTHORS. J. EVOL. BIOL. 25 (2012) 342–351
JOURNAL OF EVOLUTIONARY BIOLOGY ª2011 EUROPEAN SOCIETY FOR EVOLUTIONARY BIOLOGY
may account for this observation. First, infected ants may
display an altered cuticular profile (as shown in immune
challenged Lasius niger queens, Holman et al., 2010),
inducing nestmates to groom them; however, we found
no evidence for this using GC-MS, as infection did not
affect the cuticular chemical profile; however, we con-
ducted chemical analysis on infected ants, and not on
infectious ants, so a possibility of ants changing their
profile after exposure in the first 2 days exists. Second,
ants might detect that another ant is grooming itself and
react by starting to groom this ant. We also did not find
support for this second hypothesis. Ants groomed them-
selves for a total of 816 times, and in only 17 of these
cases (2.1%), self-grooming was followed by being
groomed. Third, an ant may show other behavioural
abnormalities (e.g. vibration signals), which are difficult
for the observer to notice and the presence of conidia
might be detected by conspecifics using other cues (e.g.
tactile, visual). This hypothesis remains to be tested.
From day three on, no significant difference in groom-
ing behaviour was found between treated and control
individuals. However, at this point in time, striking
differences started to appear in other behaviours, which
is in accordance with the study of Gillespie et al. (2000),
showing that penetration of the conidia through the
cuticle (thus, actually infecting the ant) takes 2–4 days.
After penetration, removal of conidia will have no effect,
and only the insect’s physiological immune system can
save it, which might explain the decrease in self- and allo-
grooming after day two. Treated ants became less sociable
and spent more time outside the nest. Also, even when
inside the nest, the ants spent less time in contact with the
brood. Spending less time in the nest and with the brood
might be a way to increase the infected individual’s
inclusive fitness, by making sure related individuals are
not exposed to the pathogen. This altruistic self-removal
has recently been found in Temnothorax ants (Heinze &
Walter, 2010) and honey bees (Rueppell et al., 2010).
We found that treatment with the pathogen did not
alter the cuticular hydrocarbon profile of ants, suggesting
that they do not signal their physiological status with a
change in recognition cues. However, the results show
a nonsignificant trend, suggesting that infected indivi-
duals have a slightly different chemical profile than that
of uninfected individuals. The reason behind this slight
difference is currently unclear. Besides a direct effect of
infection on ants cuticular profile, we cannot rule out the
existence of an indirect effect through nest desertion, as
isolation has been shown to change the hydrocarbon
profile of ants (Boulay et al., 2000; Bos et al., 2011). In
our experiment, this self-isolation of infected individuals
might thus have affected their cuticular hydrocarbon
profile. We also cannot exclude the use of more volatile
compounds, which we did not investigate. However, the
fact that infected ants spend more time outside the nest,
and especially die outside, possibly makes signalling their
physiological state unnecessary.
We found for the first time that infected ants increase
their level of aggression towards non-nestmates. The
overall level of aggression was higher, and infected ants
expressed less avoidance behaviour than their uninfected
counterparts. We interpret this result as if the treated,
infected ant has ‘nothing to lose’. The ant will face
certain death, and when confronted with the choice of
deterring or avoiding possible threats to the colony, it will
act aggressively, instead of displaying the more usual
avoidance behaviour. The observed behaviours (self-
removal and increased aggression) are reminiscent of
what is also found in humans, where sick individuals
become reclusive and irritable, isolating themselves from
other individuals (Loehle, 1995).
In ants, age-dependant polyethism exists (O’donnell &
Jeanne, 1995), where older workers will take on more
risky tasks, such as foraging (Woyciechowski & Kozlow-
ski, 1998; Woyciechowski & Moron, 2009). Foraging
increases the time spent outside of the nest. Also, recent
evidence suggests that foragers are more aggressive
against non-nestmates than nurses (J. Larsen, unpub-
lished). It is possible that infection triggers the same
pathways that are involved in the process of ageing (e.g.
HIV in humans, Ances et al., 2010), and as a result
infected ants spend more time outside of the nest and
become more aggressive. One way in which both age and
health status could elicit the same behavioural changes in
ants, is if an ant cannot assess its age, but only its
longevity, which is affected both by age and by health
status (Tofilski, 2002).
Our results emphasize that parasite pressure has
shaped many behaviours in the evolution of social
insects. In particular, unsociable behaviours can be
selected for in social insects when benefits of preventing
disease outbreaks in the nest outweigh the potential cost
of losing workers (Rueppell et al., 2010).
It is easy to imagine how this is true in the case of
infection, where one infected individual could poten-
tially infect its entire colony, destroying it in the process.
Acknowledgments
We would like to thank all members of the Centre for
Social Evolution for providing a pleasant working envi-
ronment. Many thanks to Louise Lee Munck Larsen for
helping with rearing M. brunneum and to Jelle van
Zweden for comments and discussion. This work was
supported by the Danish National Research Foundation
and by a grant from the Faculty of Science, University of
Copenhagen to PdE.
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Supporting information
Additional Supporting Information may be found in the
online version of this article:
Table S1 Changes in behaviour and space use after
treatment.
Table S2 Chemical analysis of control and infected ants.
Figure S1 Chemical analysis of control and infected ants.
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Received 25 July 2011; revised 25 October 2011; accepted 26 October
2011
Sick ants become unsociable 351
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... avoiding interactions with O.F, potentially carrying more germs, in accordance with the fact that they have been shown in this study to have fewer antibiotic metabolites. It has also been shown that sick ants isolate themselves before dying (Heinze and Walter 2010;Bos et al. 2012). Both cases would lead to the social isolation of this group and therefore to a weaker synthesis of communication molecules. ...
Thesis
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My thesis analyses how the social environment influences longevity in various animal species (vertebrates and invertebrates) in a gradient of social complexity (gregariousness, cooperative breeding, eusociality). Thanks to complementary measurements (behavioural observations, oxidative stress, telomere length, proteomics, metabolomics), my work reflects contrasting ageing mechanisms depending on the species studied. For example, in birds, longer telomeres are associated with a favourable social environment and greater longevity. On the contrary, among the 10 species of ants studied, the longest-lived ants had shorter telomeres. Furthermore, the complete and combined study of the proteome and metabolome of ants has highlighted certain mechanisms (e.g., sirtuins, mTOR, anti-cancer mechanisms), the universality of which opens the way to a better understanding of the harmful effect of social stress on human health.
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