On status badges and quality signals in the paper wasp Polistes dominulus: Body size, facial colour patterns and hierarchical rank

Abstract

To establish a dominance order, social animals often rely on indicators of fighting to avoid costly aggressive encounters. In some species, individuals use colour patterns to signal their social status. Recent studies claimed that facial markings in the eusocial paper wasp Polistes dominulus are status badges that allow co-foundresses to form a linear hierarchy based on individual quality. Here, we evaluated facial patterns in natural populations of P. dominulus, in its native range, to observe whether the marks reflect overall wasp quality in different contexts. We used the same measures of clypeus patterns used by earlier studies, but did not find that they functioned as status badges. Our analyses showed no evidence that visual markers are related to: (i) size, (ii) probability of surviving winter, (iii) social rank in spring associations, or (iv) health status (assessed by the presence of strepsipteran endoparasites). Size, however, is important. Larger wasps are more likely to survive the winter and to acquire the dominant position in spring associations. Larvae infected with endoparasites become smaller adult wasps. These findings suggest that body size is a reliable quality indicator on which wasps build their social networks, and that clypeus patterning is not involved.

Full text

On status badges and quality signals in
the paper wasp Polistes dominulus: body size,
facial colour patterns and hierarchical rank
R. Cervo
1,
*, L. Dapporto
1
, L. Beani
1
, J. E. Strassmann
2
and S. Turillazzi
1,3
1
Dipartimento di Biologia Animale e Genetica, Universita
`degli Studi di Firenze, via Romana n. 17,
50125 Florence, Italy
2
Department of Ecology and Evolutionar y Biology, Rice University, Houston, TX 77005, USA
3
Centro Interdipartimentale di Spettrometria di Massa dell’Universita
`di Firenze, Viale G. Pieraccini n. 6,
50100 Firenze, Italy
To establish a dominance order, social animals often rely on indicators of fighting to avoid costly aggressive
encounters. In some species, individuals use colour patterns to signal their social status. Recent studies
claimed that facial markings in the eusocial paper wasp Polistes dominulus are status badges that allow
co-foundresses to form a linear hierarchy based on individual quality. Here, we evaluated facial patterns in
natural populations of P. dominulus, in its native range, to observe whether the marks reflect overall wasp
quality in different contexts. We used the same measures of clypeus patterns used by earlier studies, but did
not find that they functioned as status badges. Our analyses showed no evidence that visual markers are
related to: (i) size, (ii) probability of surviving winter, (iii) social rank in spring associations, or (iv) health
status (assessed by the presence of strepsipteran endoparasites). Size, however, is important. Larger wasps
are more likely to survive the winter and to acquire the dominant position in spring associations. Larvae
infected with endoparasites become smaller adult wasps. These findings suggest that body size is a reliable
quality indicator on which wasps build their social networks, and that clypeus patterning is not involved.
Keywords: dominance hierarchy; Polistes wasps; status badges; body size; winter survival; Strepsiptera
1. INTRODUCTION
Dominance hierarchies are widespread in group-living
animals (see Theraulaz 2001). Stable rank order reduces
the costs of group living by substituting inter-individual
overt conflicts with a network of non-aggressive dom-
inance–submission relationships among group members
(Chase 1974). However, in the initial phase of hierarchy
establishment, potentially costly fights may arise among
group contenders. Usually, individuals with better fighting
abilities have higher probabilities of reaching the dominant
position and several characters directly linked to strength
(including body size, weapons, etc.) are often evaluated by
opponents before contests.
According to the ‘status-signalling hypothesis’ (Rohwer
1975), contestants may be advantaged if they can rely on
additional signals of fighting ability to predict the outcome
of the agonistic contests and thereby avoid costly
interactions. Conspicuous colour asymmetry between
contestants has been proposed as a ‘badge of status’
evolved to communicate differences in competitive ability.
Several studies have demonstrated a positive correlation
between colour patches, size and dominance status in
many vertebrates, especially in birds and lizards but also in
mammals (for reviews, see Senar 1999;Whiting et al.
2003;Setchell & Wickings 2005), as well as in some
invertebrates (e.g. butterflies, Shreeve 1987;wasps,
Tibbetts & Dale 2004). In some avian species, individuals
displaying the largest melanin-based badges are
behaviourally dominant to those having smaller colour
patches (see Senar 1999). However, other studies on birds
failed to find a clear correlation between colour patch and
status (see Whitfield 1986;Belthoff et al. 1994;McGraw &
Hill 2000). A further controversial topic is the honesty
of status badges. When the production of these badges
is too expensive for low-quality individuals, they should
be considered to be honest signals (Zahavi 1975;
Grafen 1990). In some cases, such badges are cheap to
produce and are then vulnerable to use by dishonest
individuals that lie about their status (Rohwer 1975;
Maynard Smith & Harper 1988). However, their mainten-
ance in the social network could betoo costly for low-quality
individuals (‘social control hypothesis’; Rohwer 1977;
Rohwer & Rohwer 1978). Recent studies (for a review, see
Jawor & Breitwisch 2003) suggest that rank markings
must be costly to their bearer, and only high-quality
individuals can support the social cost of aggressive
interactions directed towards high-ranking individuals.
Social wasps of the genus Polistes are the first
invertebrate taxon in which a linear dominance hierarchy
was reported (Pardi 1942). Future queens of various
species of Polistes living in temperate areas (the so-called
‘gynes’ or future foundresses) hibernate in aggregations to
increase winter survival. Winter represents a bottleneck
selecting the individuals that will reproduce in the
following year, as about half of future queens die during
hibernation (see Dapporto & Palagi (2006) and references
therein). In early spring, surviving wasps exit from
diapause and start a new colony either alone or associated
Proc. R. Soc. B (2008) 275, 1189–1196
doi:10.1098/rspb.2007.1779
Published online 19 February 2008
*Author for correspondence (rita.cervo@unifi.it).
Received 30 December 2007
Accepted 31 January 2008
1189 This journal is q2008 The Royal Society

with other females. Foundresses, before nest foundation,
engage in violent fights to establish hierarchical relation-
ships; with time, the aggression diminishes and is replaced
with low-cost ritualized postures between dominant and
subordinate individuals (Pardi 1942). There is some
evidence that the order of arrival at the nest site is also
important in establishing dominance; the first to arrive is
dominant (Field et al. 1998;Seppa¨ et al. 2002). Once an
individual’s rank in the queue is established, it may be
maintained for a prolonged period without additional
aggression. Moreover, according to recent findings
(Dapporto et al. 2006), dominance interactions even
occur in Polistes dominulus in autumn pre-hibernating
clusters of wasps, where they are predictive of spring rank
though not having an immediate link with direct
reproduction.
Pardi (1942), describing this spring social organization
in associative foundations of P. dominulus (formerly Polistes
gallicus), observed that the dominance order mirrors a
reproductive skew as well as a division of labour among
co-foundresses: the dominant female (afemale) assumes
the queen position, becoming the principal colony egg-
layer, while the other foundresses (subordinates) perform
different tasks linked with colonial development (nest
building, foraging behaviour, brood care and colony
defence) and progressively relinquish opportunities of
direct reproduction. As all overwintered females are mated
and are potentially egg-layers, the competition for the
high-rank position is expected to be strong (Hart &
Monnin 2005). Moreover, in P. dominulus, it seems crucial
to become the dominant individual in a group of
foundresses, because relatedness among co-foundresses
can sometimes be low and reproductive skew is high (the
dominant foundress lays more than 90% of the eggs;
Queller et al. 2000). As a consequence, when co-
foundresses are unrelated, subordinates obtain neither
direct nor indirect fitness.
Much work has been done on the determinants of
dominance in colonies of Polistes wasps to understand
which factors are important for nest initiation and for
acquiring a dominant rather than a subordinate position
in a group. Body size (see Reeve 1991) as well as
neuroendocrine activity and oogenesis (see Ro¨seler
1991) play a role in establishing the outcome of
dominance order in founding associations. Dominant
individuals are usually larger, have better developed
ovaries and have more active corpora allata (endocrine
glands that produce gonadotropic juvenile hormone) than
their subordinates. The larger size clearly facilitates
fighting ability, allowing individuals to reach the dominant
position among co-foundresses. The chemical signature of
cuticular hydrocarbons is the principal cue used in nest-
mate recognition in Polistes wasps (Gamboa 2004) and
may function as a dominance signal among co-foundresses
of P. dominulus (Dapporto et al. 2007). However,
chemical differences between dominant and subordinate
individuals are lacking at the beginning of an association
(Sledge et al. 2001). The appearance of distinctive
chemical patterns among co-foundresses occurs only
after the hierarchy has been established, suggesting that
it is the result and not the cause of dominance hierarchy
position. Moreover, direct experimental evidence is still
lacking on whether or not cuticular odour plays a role in
the maintenance of dominance order.
In line with status-signalling and social control
hypotheses (sensu Rohwer), a visual facial pattern has
recently been proposed as a rank indicator in P. dominulus
(Tibbetts & Dale 2004). In fact, foundresses of North
American populations show a wide range of variation in
the black spots on their yellow clypeus in terms of spot
number, proportion of black area and irregularity in spot
shape (reported as ‘the brokenness index’; see Tibbetts &
Dale 2004). Tibbetts & Dale (2004) explored the signal
value of clypeus patterns measuring the variation in
number, size and shape of the black spots on the clypeus
in relation to rank in the group and to overall size.
According to these authors, the facial pattern of the
foundresses strongly predicts both overall body size and
social dominance. First, larger females have a higher
proportion of black on their clypeus as well as more broken
up black spots than smaller ones. Second, in short-term
contests between pre-nesting unfamiliar gynes matched
for body size, the winners had significantly more black and
more broken spots than losers. Third, Tibbetts & Dale
(2004) manipulated the status signals of wasps by masking
their presumed badges of status with black and yellow
paint on their clypeus, to furnish an experimental support
for the social control hypothesis. As they predicted, wasps
painted to look more dominant received more aggression,
but only after hierarchy establishment was complete, not
before; furthermore, the same pattern of increased
aggression was directed towards wasps marked to look
less dominant, against all predictions. Honest signalling
requires that subordinates with false dominant signals be
punished, not females with false subordinate signals (see
also Strassmann 2004).
More recently, Tibbetts (2006), using scores to
categorize both wasp size and ‘brokenness’ of black
clypeus spots, reported a difference in the distribution of
facial patterns in P. dominulus between workers and gynes
emerging at mid-summer. The facial pattern of wasps
seems to be related to either their emergence time or
body size, and gynes show more broken clypeus patterns
than do workers. Moreover, Tibbetts & Curtis (2007)
showed that P. d o m i n u l u s females that were food
supplemented during the larval stage had more disrupted
black spots than unsupplemented females. However, in
contrast to a previous study (Tibbetts & Dale 2004), they
found a negative correlation between body size and
clypeus spot brokenness. They interpreted these results
to mean that nutritional factors could influence clypeus
black spot development in P. d o m i n u l u s wasps, so that this
trait—ratherthanbodysize—isacostlyandreliable
quality indicator.
In the present work, we repeated the study carried out
by Tibbetts and co-workers to determine whether their
results could be replicated using a more long-term
measure of dominance in the native range of P. dominulus.
We measured clypeus marks on the faces of P. dominulus
foundresses collected in the field. If facial patterns reflect
overall wasp quality in a natural context, the following
predictions should be true. First, if facial markers are
reliable indicators of caste (Tibbetts 2006) and nutritional
intake at the larval stage (Tibbetts & Curtis 2007), we
expect that wasp winter survival—and its physical/
behavioural predictors—should be related to the clypeus
pattern. Second, if the clypeus pattern does function as a
pivotal badge of status, then we predict a conspicuous
1190 R. Cervo et al. On status badges in Polistes dominulus
Proc. R. Soc. B (2008)

asymmetry in the number, size and shape of black spots
among spring co-foundresses in relation to high/low social
dominance on the nests, in agreement with the pre-nesting
dyadic contests in small arenas carried out by Tibbetts &
Dale (2004). Third, we predict that facial patterns of
healthy overwintered females differ from those of ‘stylo-
pized’ ones (i.e. parasitized by Xenos vesparum, Strepsip-
tera, Stylopidae). This endoparasite infects wasp larvae
and grows at the host’s expense until the wasp reaches
adulthood (Giusti et al. 2007). Parasitized wasps are
castrated, have no active role in colony life and desert their
nest early to form aberrant summer extra-nidal aggre-
gations, where the wasps are inactive (Beani 2006).
We also checked for the predictive importance of body
size as a more reliable quality indicator than facial pattern.
As for facial marks, we predict that body size is correlated
with winter survival, rank in spring associations and
health status.
2. MATERIAL AND METHODS
(a)Study on overwintering clusters
In September, we collected four clusters of P. dominulus
females (i.e. groups of females preparing to overwinter) from
Reggello (Florence, Italy; C1, nZ40; C2, nZ48; C3, nZ51;
C4, nZ43). The wasps were individually marked, then caged
in four containers (50!50!50 cm) with water and sugar ad
libitum and maintained under natural light and temperature
conditions. We recorded from 10.00 to 16.00 hours between
5 September and 1 November (85 hours in all) the frequency
of six behavioural acts using the ‘all occurrences sampling
method’ (Altmann 1974): dominance performed and
received; attacks performed and received; and trophallaxis
transferred and received. Moreover, we recorded individuals
foraging on the sugar by the ‘scan sampling method’
(Altmann 1974) at 5 min intervals. We calculated the hourly
frequencies of each behaviour, and we added a variable to
quantify the total food intake calculated for each individual as
(as trophallaxis was measured by all occurrences and sugar
foraging by scan),
food intake Zsugar foraging Ctrophallaxis obtained
Ktrophallaxis given:ð2:1Þ
Frequencies were normalized. The four cages were then
transferred to an unheated room with natural outdoor
temperature and photoperiod. At the beginning of March,
i.e. the end of hibernation, we recorded which individuals
survived the winter and we evaluated body size and clypeus
patterns on saved individuals (see below).
Wasp aggregations are characterized by the presence of
some individuals that perform most of the foraging activity
and do not survive until spring (Dapporto et al. 2005). These
individuals should probably be classified as ‘late workers’
(Hunt & Amdam 2005). In order to determine which females
of overwintering clusters should have been considered late
workers and not future foundresses, we used a K-means test
on the hourly frequencies of all the behaviours imposing the
formation of two clusters. K-means divided the wasps of each
aggregation into two clusters based on trophallaxis given and
sugar foraging. This allowed us to separate future foundresses
from late workers. By cluster membership, we identified 28
late workers (seven in C1 and C2, six in C3 and eight in C4;
K-means, trophallaxis given and sugar foraging p!0.001 for
each cluster) and they were removed from the following
analyses. We removed them because we are interested in the
differential dominance and winter survival of future foun-
dresses, not late workers. Thereafter, we performed a
multiple regression using a dominance index as the
dependent variable. Dominance index (D
i
) was calculated
for each wasp as
DiZ
DpKDr
DpCDr
;ð2:2Þ
where D
p
is the dominance performed and D
r
is the
dominance received. Head width, the number of black
spots, the percentage of black area on the clypeus and the
brokenness index were used as independent predictors.
Finally, we performed LogRA to discover the influences of
the 10 covariates on the binary-dependent variable, wasp
survival. The covariates were entered into the model after
selection by the forward stepwise method. We used SPSS v.
13.0 for all statistical analyses.
(b)Study on foundresses of established
spring colonies
In April and May, we collected 74 nests of P. dominulus (26
solitary and 48 associative foundations, 14 with more than
two foundresses) from the field. Colonies were collected from
five different sites: four located in Central Italy (Tuscany) and
one in northeast Italy (Veneto). All the colonies were in the
late pre-worker stage, i.e. when dominance hierarchies were
well established. Colonies were collected early in the morning
to assure the capture of all the foundresses. Solitary
foundresses were frozen at K208C upon arrival at the
laboratory. Multiple colonies were transferred to the labora-
tory and housed in glass boxes (15!15!15 cm). Each box
was provided with food (sugar and fly maggots), water and
paper material for nest building. Before housing the wasps in
the boxes, we individually marked the foundresses of each
association with coloured marks on the thorax. After some
days of acclimatization to the laboratory conditions, beha-
vioural observation sessions were performed independently
by more than one observer to establish the dominance
hierarchy among co-foundresses of each colony. Hierarchical
ranks in each colony were checked for stability in two
subsequent observations, spaced by a few days, and in the
warmest hours of the day to assure a high activity level. The
following behavioural patterns were considered to be
indicators of rank: dominance–submission posture; aggres-
sive behaviour; presence–absence on the nest; foraging
behaviour; egg-laying; and nest-building activity (Pardi
1942). Once the linear dominance hierarchy was established
for every colony, all the individuals were killed at K208C for
body size and clypeus measurements (see below).
(c)Study on healthy and stylopized wasps
Between February and March, we collected mixed hibernat-
ing clusters of healthy and parasitized wasps from two areas in
Tuscany (Italy) characterized by high parasitism levels:
Impruneta (Florence) and S. Gimignanello (Siena). Wasps
were caged in laboratory conditions until April, when the first
aggressive interactions occurred among unparasitized wasps.
All individuals were then killed at K208C. We evaluated body
size and clypeus patterns (see below) of 38 female wasps that
were infected by one X. vesparum female (with the exception
of two infected by two Xenos females) and 74 healthy (i.e. not
stylopized) female wasps as controls.
On status badges in Polistes dominulus R. Cervo et al. 1191
Proc. R. Soc. B (2008)

(d)Morphological measurement of wasps
The head of each dead female wasp in the three studies was
separated from the body and a picture was taken of its front
view. Each picture was imported into ADOBE PHOTOSHOP.
Body size estimates were obtained by measuring the
maximum width of the head (Eickwort 1969). To compare
co-foundresses differing in social rank, as well as healthy/
stylopized wasps, we counted the number of black spots and
measured the percentage of black pigmented area on the
clypeus using the software IMAGEJ. Finally, for overwintering
gynes and for each alpha and beta female of the 48 associative
foundations, we calculated the ‘badge brokenness index’ as
defined by Tibbetts & Dale (2004): ‘the standard deviation of
amount black pigment deposited along the horizontal
gradient located between two peaks’. We calculated the
brokenness index using the software (named ‘black spot’) we
developed for this purpose.
To compare clypeus pattern characteristics (number of
spots, percentage of black spots and brokenness index)
between different groups (dominants versus subordinates
and healthy versus parasitized wasps), we used non-
parametric tests for both independent and paired data
(Mann–Whitney U-test and Wilcoxon test). For body size
data (head width), we used Student’s t-test for both
independent and paired samples after testing for normality
and equality of variance. LogRA was performed to search for
influences of clypeus pattern characteristics and body size on
the rank of each wasp (binary dependent). The covariates
were entered into the model after selection by the forward
stepwise method. Finally, we tested for correlations between
clypeus pattern characteristics and head width using the
Pearson correlation test. All analyses were performed using
the statistical program SPSS v. 13.0.
3. RESULTS
(a)Study on overwintering clusters
Multiple regression analysis showed that the autumnal
dominance index (D
i
), which measures the relative number
of dominance actions performed and received, is not related
to any predictors (head width, bZ0.061, pZ0.448;
percentage of black area, bZK0.059, pZ0.527; spot
number, bZK0.067, pZ0.651; brokenness, bZK0.061,
pZ0.681). Wasp survival differed among the four aggrega-
tions: C1Z60.6; C2Z29.3; C3Z82.2; and C4Z64.4%
(G-test, GZ28.24, p!0.001). The overall survival
percentage was 59.8%, in line with previous studies (see
Dapporto & Palagi (2006) for references). Finally, logistic
regression analysis showed that the only variables correlated
with winter survival were head width (BZ4.21, WaldZ
13.59, p!0.001) and, to a lesser extent, trophallaxis given
(BZK0.91, WaldZ5.74, pZ0.017). However, LogRA
assigned only 68.52% of the cases to their correct group.
Particularly, the analysis correctly attributed to their own
group 81.25% of surviving wasps, but 50% of the wasps that
LogRA predicted to die actually survived winter.
(b)Study on foundresses in spring colonies
Overall, the average head width of alpha females (3.42G
0.11 mm, nZ48) is larger than that of subordinate females
(3.31G0.11 mm, nZ69; t-test, tZ5.01, p!0.001). More-
over, the head width of each alpha was larger than that of
her own beta female (paired t-test, tZ5.61, p!0.001,
3.43G0.12 and 3.33G0.12 mm, respectively). In 35 out
of the 48 colonies, the dominant female was larger than
the beta female. In 11 out of the 48 cases, the alpha and
beta females were of the same size, and the beta female was
larger than the alpha female on only 2 out of the 48 nests
(c
2
-test, with Yates’ correction c
2
Z34.05, p!0.001).
The clypeus pattern in P. dominulus foundresses is
variable, as previously reported by Tibbetts & Dale (2004)
for a North American population of this species. However,
in our populations, approximately 60% of 157 foundresses
(131 from foundress groups and 26 from single foundress
nests) have a completely yellow clypeus (figure 1). The
average number of spots on the clypeus did not
significantly differ between alpha (0.56G0.71, nZ48)
and subordinate females (0.55G0.79, nZ69; U-test,
UZ1599, p!0.72). Moreover, the mean number of
spots of each alpha was not higher than that of the
beta female on the same nest (Wilcoxon test, ZZK0.241,
pZ0.81). In 24 out of the 48 colonies, the dominant
female had the same number of spots as her own beta
female; in the remaining 13 and 11 cases, alpha
foundresses had a higher and a lower number of spots
than their own beta ones, respectively (c
2
Z25.17, n.s.).
These results support a random distribution of black spots
among the foundresses of different ranks (figure 2a).
0
20
40
60
80
100
0123
no. of foundresses
Figure 1. Number of P. dominulus spring foundresses with
0, 1, 2 and 3 black spots on the clypeus (nZ157).
0
5
10
15
20
25
30(a)
(b)
(c)
no. of colonies
0
5
10
15
20
25
30
no. of colonies
0
5
10
15
20
25
30
>al
p
ha >beta
no. of colonies
Figure 2. Number of associative foundations (nZ48) where
the dominant female had a higher, a lower or the same (a)
number of spots, (b) brokenness index and (c) percentage of
black on the clypeus, than or as her own subordinate female.
1192 R. Cervo et al. On status badges in Polistes dominulus
Proc. R. Soc. B (2008)

Overall, the badge brokenness index was not different
comparing all the alpha females (2.1G2.7, nZ48) versus
all the beta females (1.6G2.4, nZ48; UZ1078, p!0.55),
even with a test where alpha and beta were paired by nest
(ZZK1.04, pZ0.29, nZ48). In 18 out of the 48 colonies,
the dominant female had the same brokenness index as her
own beta female (both had a solid yellow clypeus), while in
17 and 13 colonies, respectively, the alpha had a higher
and a lower brokenness index than her own beta female
(c
2
Z0.547, n.s.; figure 2b).
The black pigmented area was not different between
alpha and subordinate females: this is true comparing
either all alpha females (1.9G4.7, nZ42) versus all sub-
ordinate females (2.9G6.5, nZ43; UZ899, pZ0.97), or
alpha and beta females from the same nest (ZZK0.24,
pZ0.81, nZ41). In 18 out of the 41 colonies, the dominant
female had the same proportion of black clypeus as her own
beta female(both had a solid yellowclypeus), while in 13 and
10 associative foundations, respectively, the alpha had a
higher and a lower proportion of black than her own beta
female (c
2
Z1.81, n.s.; figure 2c). Moreover, logistic
regression analysis showed that the only predictor of rank
was head width (BZK6.707, WaldZ9.028, p!0.001).
According to Tibbetts & Dale (2004), the brokenness
index is a reliable general predictor of dominance in
P. dominulus and was positively correlated with body size in
their sample. Our analysis of 131 associated and 26
solitary foundresses of the same species (figure 3) showed
that this index was not correlated with body size (nZ157,
r
2
Z0.006, FZ0.563, pZ0.45). Moreover, we found no
correlation between spot number and body size (nZ157,
r
2
Z0.001, FZ0.01, pZ0.92).
(c)Study on healthy and stylopized wasps
Figure 4 shows the distribution of both healthy and
stylopized overwintering females with different numbers
of spots on their clypeus. The average number of black
spots was not different between 38 stylopized and 74
healthy wasps (UZ1261, pZ0.3). Moreover, healthy
wasps did not have a higher proportion of black area on
the clypeus than stylopized wasps (UZ1130, pZ0.6).
Even though healthy wasps had no more pigmented area
on their clypeus than stylopized wasps, they were larger
overall than stylopized wasps (tZK3.12, d.f.Z100,
p!0.005, 3.36G0.16 and 3.26G0.1 mm, respectively).
No correlation was found between body size and
proportion of black on the clypeus of overwintering
wasps (nZ101, r
2
Z0.019, FZ1.922, pZ0.169).
4. DISCUSSION
(a)Clypeus patterns as a signal of rank: unlikely
We did not find support for the hypothesis that the
clypeus markers proposed by Tibbetts & Dale (2004) and
Tibbetts & Curtis (2007) work as status badges or quality
signals in foundresses of P. dominulus in its native area.
Our analyses showed no evidence that visual markers on
the clypeus of P. dominulus foundresses are related to:
(i) size, (ii) probability of surviving the winter, (iii) social
rank in spring associations collected in the field, after a
dominance hierarchy had been established, and (iv)
health status assessed by the presence of strepsipteran
endoparasites. Conversely, all these characteristics are
largely predicted by body size. Indeed, larger wasps have a
higher probability of surviving the winter and of acquiring
the dominant position in a colony in the subsequent
spring. Moreover, larvae suffering from the endoparasite
presence emerged as smaller adult wasps.
Food intake in the adult stage and facial patterns do not
predict future foundress survival during winter. In fact,
our results suggest that autumnal feeding (both by
foraging and trophallaxis) does not increase the prob-
ability of surviving the winter, but perhaps meets
immediate needs. Conversely, P. dominulus survival was
positively related to body size (large wasps have a higher
probability of surviving) and negatively with trophallaxis
given to others. However, 50% of the wasps predicted to
die by logistic regression analysis actually overwintered,
thus demonstrating that a large number of future
foundresses of small size giving food to cluster mates,
not recognized as late workers (helpers), survived and
were present in spring colonies probably with a very low
hierarchical rank.
The large number of cases incorrectly assigned by
logistic regression analysis also suggests that other
variables probably play a role in wasp survival. One of
these is the quantity and the quality of fat bodies the
foundresses have at their emergence as adults, which are
determined during the larval stage by feeding (Hunt &
Amdam 2005). This parameter was not measured in this
study because measuring it requires that we kill the wasps
0 2.00 4.00 6.00
brokenness index
8.00 10.00
head width (mm)
3.00
3.20
3.40
3.60
3.80
Figure 3. Relationship between head width (mm) and
brokenness index (nZ157, r
2
Z0.006, FZ0.563, pZ0.45).
0
10
20
30
40
50
60
70
0123
percentage of wasps
Figure 4. Percentage of stylopized (nZ39, grey bars) and
healthy (nZ74, white bars) wasps with 0, 1, 2 and 3 black
spots on the clypeus.
On status badges in Polistes dominulus R. Cervo et al. 1193
Proc. R. Soc. B (2008)

before their winter fate could be recorded, something that
was incompatible with the other goals of the study.
However, the lack of a correlation between winter
survival and facial pattern, as well as the lack of a
difference in visual markers between individuals in good
condition and those carrying parasites, was unexpected.
Indeed, Tibbetts & Curtis (2007) stated that P. dominulus
females reared with supplemental food during the larval
stage have more disrupted facial patterns, suggesting that
this parameter may convey information about quality
linked to early nutrition. In our study, both winter-
survived and non-stylopized wasps (from areas with high
parasitism levels) can be considered to be high-quality
individuals. Clearly, most of their good condition was
acquired during the larval stage, when the host defence
mechanisms play a role against parasites (Manfredini et al.
2007). Recent data demonstrate that food depletion by
Xenos parasites stops when the parasite protrudes from the
host abdomen, i.e. a few days after the wasp becomes an
adult (Giusti et al. 2007). As a whole, in hibernating
wasps, clypeus patterns do not seem to represent any kind
of quality signal.
In line with these data, at the beginning of the annual
colonial cycle, when foundresses interact frequently to
assess their position in the linear hierarchy, we found that
none of the visual indicators of status and quality
previously proposed by Tibbetts & Dale (2004) were
significantly correlated with social dominance order for
wasps in their native range. In our sample of well-
established spring associations, dominant individuals
have neither a higher number of black spots on the
clypeus, nor a higher proportion of black, nor a higher
‘brokeness index’ when compared with their subordinates,
suggesting that these facial visual markers are not
indicators of rank.
There are two main differences between our study and
those of Tibbetts and collaborators. First, in our samples,
approximately 60% of foundresses had a completely yellow
clypeus, whereas only 20% of American P. dominulus
foundresses had a completely yellow clypeus (Tibbetts &
Dale 2004). In our test sites, clypeus marks are not variable
enough to make them very good candidates for dominance
decisions. In half of our 48 natural co-foundresses pairs,
both individuals had the same number of spots and, in 18
pairs, the clypeus was entirely yellow. The recent invasion of
the European P. dominulus into North America (Cervo et al.
2000) could be the reason for this difference in clypeus
marks (bottleneck effect). However, the relatively high
genetic variability in these introduced populations suggests
multiple independent introductions of P. d o m i n u l u s into
North America (Liebert et al. 2006). Moreover, as we have
studied P. dominulus foundresses in their natal range, what
we found there is the most likely to be biologically real; it is
very unlikely that they would have evolved such sophis-
ticated new abilities in the short time since the 1970s when
they came to America (Cervo et al.2000).
Second, the experimental design of two-player contests
(Tibbetts & Dale 2004) seems too brief to establish stable
dominance interactions. At the beginning of interactions
for dominance order establishment, the wasps compo-
sition in the associations is very unstable and could change
rapidly (Strassmann et al. 2004). Our dominant and
subordinate individuals, collected in the field and when
their rank positions were well established, were not bearers
of high and low status badges, respectively, on their
clypeus. Moreover, the trials carried out for 2 hours by
Tibbetts & Dale (2004) furnished no clear-cut results: in
dominance trials between foundresses paired for similar
size, the dominant individuals have, on average, more
spots and higher brokenness index but a lower percentage
of black on their clypeus than the subordinate ones
(Tibbetts & Dale 2004). These dominance trials were
performed during pre-nesting period, when it has been
suggested that physiological traits could be more import-
ant than body size (Ro¨seler et al. 1984). Even if Tibbetts &
Dale’s wasp pairs were controlled for body size, other
uncontrolled physiological factors might have influenced
the outcome of the contests.
Moreover, black spots on the clypeus are permanent
patches of colour, while the status of a female cannot be
so; whenever a queen dies, she is replaced by a
co-foundress or by the oldest worker (Pardi 1946)
that, according to Tibbetts (2006), should bear a low
status badge.
Interesting future work would include examining
recognition and colour marks, and repeating this study
on the same population that Tibbetts and colleagues used.
(b)The role of body size and other hypotheses
Body size is a morphologically significant though not
absolute predictor of rank in Polistes,andprobably
confers competitive ability (Reeve 1991). Our study
confirmed this.
Through direct conflicts, an individual could estimate
its own competitive ability from previous wins and losses
and ‘build’ an expectation of the outcome of future
competitions, regardless of the opponent (Barnard & Burk
1979). Even if a mechanism based on ‘loser and winner
effects’ could be sufficient to explain rank-order formation
in Polistes wasps (Markiewicz & O’Donnell 2001), the
evolution of signals for status recognition could be
advantageous for both the opponents. However, we did
not find any correlations between body size of foundresses
and facial pattern. Moreover, the correlation between
visual markers and head width found by Tibbetts & Dale
(2004) is very weak (accounting for a small percentage of
the variance), as was noted by Strassmann (2004).
Although intraspecific colour variability is very com-
mon in many animal species, it does not always have a
communicative function. Face colour polymorphism in
Polistes wasps was documented by various authors in the
past (see Enteman 1904;Guiglia 1972) for different
species, but it was never found to have a functional
explanation. However, colour patterns might serve as
individual identity markers rather than status badges
(Whitfield 1987) with hierarchy then maintained through
individual recognition. In small colonies, co-foundresses
might be able to recognize each other based on facial
markings and memorize the fighting ability of each
foundress. Although only few cases of rank-order
maintenance based on individual recognition are reported
in the arthropod literature (Hazlett 1969;Caldwell 1979;
Karavanich & Atema 1998;Gherardi & Atema 2005), this
could be a way that Polistes co-foundresses assess
hierarchical relationships. As visual recognition has
been reported for Polistes fuscatus (Tibbetts 2002),
co-foundresses could use visual patterns for individual
recognition rather than for rank recognition. However,
1194 R. Cervo et al. On status badges in Polistes dominulus
Proc. R. Soc. B (2008)

individual recognition based on visual cues is likely to work
only in the very beginning phase of the colony cycle when
few individuals are in the colony; it is highly unlikely that
many wasps in a mature colony would be able to or benefit
from individual recognition based on visual cues. Defini-
tive studies are necessary to demonstrate that Polistes
wasps are capable of individual recognition, i.e. the
capacity of discrimination of multiple individuals accor-
ding to their unique features (Thom & Hurst 2004).
The authors wish to thank Dr D. Croci, Dr C. Bruschini and
Dr I. Ortolani for their support with wasp collection and
Dr S. Nappini who helped us to measure some specimens. We
also thank Dr L. Rossi Geocoste SRL for developing the
image processing software. This research was supported by
the University of Florence and the US National Science
Foundation under grant IBN-9975351.
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