Aggression in a western Amazonian colonial spider, Philoponella republicana (Araneae: Uloboridae)

Abstract

Group-living spiders are rare, and can be divided into multiple subcategories based on their tolerance of group mates. While social spiders are cooperative, colonial spiders are often antagonistic towards conspecifics. We examined colony dynamics in a colonial species, Philoponella republicana (Simon, 1891), focusing on aggressive behaviors to further understand this understudied species. We studied whether web region, sex ratio, web size, or spider size affected aggression. We also tested whether colony members discriminate against conspecific intruders, since this behavior, known as group closure, is prevalent in many other group-living animals but had not yet been tested in colonial spiders. Colony mates were often aggressive due to competition for limited resources, such as mates and orb webs, yet several characteristics of this species may reduce these competitive forces. First, female-biased secondary sex ratios appear to reduce male-male and female-male competition. Moreover, although some individuals defended orb webs, other areas in the communal web were not defended. Philoponella republicana also did not exhibit group closure. Our results further confirm that aggression between males decreases in colonies with more female-biased secondary sex ratios, and larger individuals correlate with a higher frequency of aggressive interactions. Moreover, we raise new questions concerning the evolutionary pressures that shape coloniality in spiders.

Full text

Aggression in a western Amazonian colonial spider,
Philoponella republicana (Araneae: Uloboridae)
Authors: Wu, Catherine, Bhagawat, Chaiti, Goldman, Modan R.,
Punjabi, Nihal A., Shier, Debra M., et al.
Source: The Journal of Arachnology, 50(3) : 277-287
Published By: American Arachnological Society
URL: https://doi.org/10.1636/JoA-S-20-093
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2022. Journal of Arachnology 50:277–287
Aggression in a western Amazonian colonial spider, Philoponella republicana (Araneae: Uloboridae)
Catherine Wu
1,*
,Chaiti Bhagawat
1,*
,Modan R. Goldman
1,*
,Nihal A. Punjabi
1,*
,Debra M. Shier
1,3
,Roxana P. Arauco-
Aliaga
2
, and Gregory F. Grether
1
:
1
Department of Ecology and Evolutionary Biology, University of California Los
Angeles, 612 Charles E. Young Drive E, Los Angeles, CA, 90095, USA; E-mail: catherinedwu@gmail.com;
2
Cocha
Cashu Biological Station, San Diego Zoo Global-Peru, Cusco, Peru;
3
San Diego Zoo Institute for Conservation
Research, Recovery Ecology, 15600 San Pasqual Valley Road, Escondido, CA, 92027, USA; * These authors
contributed equally to this work.
Abstract. Group-living spiders are rare, and can be divided into multiple subcategories based on their tolerance of group
mates. While social spiders are cooperative, colonial spiders are often antagonistic towards conspecifics. We examined
colony dynamics in a colonial species, Philoponella republicana (Simon, 1891),focusing on aggressive behaviors to further
understand this understudied species. We studied whether web region, sex ratio, web size, or spider size affected aggression.
We also tested whether colony members discriminate against conspecific intruders, since this behavior, known as group
closure, is prevalent in many other group-living animals but had not yet been tested in colonial spiders. Colony mates were
often aggressive due to competition for limited resources, such as mates and orb webs, yet several characteristics of this
species may reduce these competitive forces. First, female-biased secondary sex ratios appear to reduce male-male and
female-male competition. Moreover, although some individuals defended orb webs, other areas in the communal web were
not defended. Philoponella republicana also did not exhibit group closure. Our results further confirm that aggression
between males decreases in colonies with more female-biased secondary sex ratios, and larger individuals correlate with a
higher frequency of aggressive interactions. Moreover, we raise new questions concerning the evolutionary pressures that
shape coloniality in spiders.
Keywords: Uloboridae, orb-weaver, social evolution, territoriality, group closure
https://doi.org/10.1636/JoA-S-20-093
Of the over 50,000 spider species identified, fewer than 0.1%
display social behavior (Avil´
es & Guevara 2017; World Spider
Catalog 2022). Sociality is hypothesized to evolve when the
benefits of group-living outweigh the costs (Krause & Ruxton
2002). As compared to solitary web-building spiders, group-
living web-builders can occupy larger areas of open space,
which increases the likelihood that prey will intercept the
colony web; cooperate to secure larger prey; capture a greater
proportion of arriving prey due to insects getting caught in
surrounding webs while attempting escape (i.e., the ricochet
effect); and obtain earlier predator warnings (Uetz 1989).
Group-living spiders, however, have webs more conspicuous
to predators, are more susceptible to parasites and disease,
and are exposed to higher levels of within-group resource
competition (Alexander 1974; Nentwig 1985; Uetz & Hieber
1997).
Group-living spiders are commonly classified by two
criteria: territoriality, or the lack thereof, and duration of
sociality—permanent or periodic (Avil´
es & Guevara 2017).
Non-territorial permanent and periodic group-living spiders,
referred to as social or sub-social spiders, respectively,
cooperate in prey capture, brood care and web construction
(Schneider 1995; Avil´
es 1997; Avil´
es & Guevara 2017).
Territorial permanent and periodic group-living spiders,
referred to as colonial spiders, cooperate only to maintain
the shared structural framework of the colony web and often
display aggression towards colony mates (Lubin & Bilde
2007). The evolutionary pathways to territorial or non-
territorial group-living are thought to be distinct; that is,
non-territorial social spider species generally arise in lineages
with extended maternal care of offspring, and have subsocial
sister species, whereas territorial colonial behavior appears to
arise more sporadically, when favored by ecological conditions
(Agnarsson et al. 2006).
Aggression levels may vary within colonial spider colonies
in predictable ways because different regions within the
communal web offer different fitness payoffs (Rayor & Uetz
1990). The most common colonial spiders are orb weavers that
embed orb-shaped hunting webs guarded by an individual
(hereafter, orb webs) within a network of interconnecting webs
that are anchored to the surrounding vegetation (hereafter,
connecting webs) (Binford & Rypstra 1992; Avil´
es & Guevara
2017). Individuals move to communal retreats, which are
distinct sections of connecting web, for protection when they
are not hunting, especially at night (Lubin 1980; Binford &
Rypstra 1992). While colony members cooperate to build the
connecting webs, they work individually to either construct
their own orb webs or usurp another individual’s orb web
(Buskirk 1975a; Uetz & Hieber 1997). In one well-studied
tetragnathid orb weaver, Metabus ocellatus (Keyserling, 1864),
(formerly Metabus gravidus Pickard-Cambridge, 1899), many
individuals do not have orb webs and orb-web holders
respond aggressively to intruders (Buskirk 1975b). In another
colonial spider, Metepeira incrassata F. O.Pickard-Cam-
bridge, 1903 (Araneidae), the outermost layer or periphery
of a three-dimensional colony offers the greatest access to prey
but also the most exposure to predators (Rayor & Uetz 1990,
1993; Rayor 1996), and thus regions in the center or semi-
periphery may optimize the trade-off between prey capture
and predation risk. For the colonial spider Cyrtophora
citricola (Forsk˚
al, 1775) (Araneidae), there was a higher
frequency of prey captures, as well as conspecific aggressive
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interactions in the middle layers of the web as opposed to the
innermost and outermost regions (Rypstra 1979). Thus, orb
webs in the semi-periphery may be at the heart of territorial
disputes in the colony.
The frequency of aggressive behaviors may also be governed
by colony-level characteristics, such as the physical size of the
colony, the size and size distribution of the spiders, and the
colony sex ratio. Since larger colony webs generally capture a
greater number of prey due to the ricochet effect (Uetz 1988,
1989), we assume that larger colony webs may have greater
prey biomass per capita and consequently less within-colony
competition for prey. As a result, we expected to observe less
intra-colony aggression in larger colony webs while controlling
for the number of individuals (colony size), provided that
colony size and web size vary independently. Larger individ-
uals generally are more likely to win contests and thus have
more to gain by challenging conspecifics (Potter et al. 1976;
Riechert 1978; Christensen & Goist 1979; O’Neill 1983).
Therefore, the overall level of aggression may depend on
individual size or size distribution in a colony. As in other
taxa, variation in sex ratio is likely to affect the intensity of
competition among males for mates and the level of mating
harassment experienced by females (Clutton-Brock & Parker
1995; Kvarnemo & Ahnesj¨
o1996). Among social spider
species, those having a more female-biased secondary sex ratio
generally exhibit less male-male and female-male aggression
(Avil´
es 1997; Lubin & Bilde 2007; Avil´
es & Purcell 2012;
Avil´
es & Guevara 2017), but to the best of our knowledge, this
prediction has not been tested by comparing aggression levels
between colonies with different sex ratios within a colonial
species.
Conspecific familiarity is negatively correlated with aggres-
sion in diverse taxa (Ydenberg et al. 1988). In general,
individuals that are more familiar—because of genetic
similarity or spatial proximity—exhibit lower levels of
aggression towards each other compared to unfamiliar
conspecifics. This phenomenon, known as group closure, is
hypothesized to occur because an individual’s inclusive fitness
increases by cooperating with kin (Pasquet et al. 1997). Under
certain conditions, group closure can evolve when between-
group competition outpaces within-group competition (Leh-
mann et al. 2007; Gardner & Grafen 2009; Lion et al. 2011;
Marshall 2011). The way groups are formed may indicate the
presence or absence of group closure. Social spiders tend to
remain at their natal sites and inbreed, and therefore groups
can reach nearly clonal levels of relatedness (Avil´
es & Guevara
2017). Many colonial spider species tend to disperse as
competition in their natal colony intensifies, which reduces
within-group relatedness (Smith 1983). Unlike the many social
insects that exhibit both high levels of relatedness and group
closure, social spiders have not been found to discriminate
against unfamiliar conspecifics under normal conditions
(Pasquet et al. 1997). However, groups have been found to
discriminate against unrelated spiders during environmental
stress (e.g., starvation conditions) (Yip & Rayor 2014). In
general, social spiders are not known to be aggressive to
conspecifics. To the best of our knowledge, no study has
examined group closure in the more aggressive colonial
spiders.
We studied aggression in Philoponella republicana (Simon,
1891) (Uloboridae), a colonial, orb-weaving spider found in
secondary and mature forests of the Amazon Basin, and in a
geographic area ranging from Panama to Bolivia (Opell 1979).
Like other uloborids, P. republicana lack poison glands and
capture prey using extensive silk wrapping (Opell 1979). They
construct three-dimensional colonial webs close to the ground,
with support lines attached to vegetation, individual orb webs
interspersed throughout, and a centrally located communal
retreat (McCook 1889). Within the communal retreats,
individuals demonstrate limited forms of cooperation: some
large prey items are occasionally caught in an orb web but
wrapped in the communal retreat by more than one spider.
Once wrapping is complete, however, only one spider
consumes the prey (Masumoto 1998).
To assess whether P. republicana compete for specific spatial
positions within the colonies, we compared aggression
frequenciesbetweenspidersinwebtypes(i.e.,orbvs.
connecting webs which include communal retreats) and among
regions within the colony (i.e., periphery, semi-periphery, and
core). To directly examine how the spiders respond to
approaching colony members, and whether this depends on
the resident’s web type (i.e., orb vs. connecting), we measured
responses to staged intruders. Based on the hypothesis that
larger colonial web volumes result in increased prey capture
and reduced competition within colonies, we tested the
prediction that colony web size correlates negatively with
aggression, while controlling for the number of spiders in the
colony. We also examined whether female size class (i.e.,
small, medium, and large) affected the frequency of aggression
and kleptoparasitism (i.e., prey snatching). To test the
hypothesis that female-biased colony sex ratios reduce mating
competition among males and mating harassment between
females and males, we tested for effects of colony sex ratio on
levels of male-male and female-male aggression. Finally, we
conducted a translocation experiment to test for group
closure. All studies were completed in the field using naturally
occurring colonies.
METHODS
Study site.—We studied P. republicana in intact, lowland
Amazonian rainforest at Cocha Cashu Biological Station in
Manu National Park, Peru (11.88808S, 71.40788W, elevation
340 m, average annual temperature 24 8C) during the rainy
season from 26 January to 11 February 2019.
Colony census and behavior observations.—We recorded the
locations of 20 P. republicana colonies within 10 m of
established trails (Fig. 1) and observed each colony once
within three weeks of discovery in a randomized order,
between 0800 and 1800 hours. Fifteen colonies were in intact
forest, while the other 5 colonies were in treefall gaps, near the
edge of an oxbow lake, or near streams. To characterize
colony web size dimensions, we measured the longest
horizontal, the widest width perpendicular to the horizontal,
and the height from the lowest to the highest point (Alves-
Costa & Gonzaga 2001). Because some spiders moved in
response to our presence, we waited 10 min, which appeared to
be sufficient time for any disturbed spiders to return to their
original position, before collecting census data (sex, size class,
and web region). Sex was determined based on color markings
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and size: females are orange, black or brown with white
markings, while males are solid red-orange and smaller than
all but the smallest females (Opell 1979). We visually assessed
the size class of the female spiders (small: bottom 15%;
medium: middle 60%; and large: top 25%). Small and medium
size class females are likely to be juvenile and subadult
females. Few very small immatures were observed, and these
were not counted at all, as males and females at that age/size
are not easily distinguishable. We also did not attempt to
classify males by size due to the lack of variability in their size.
To avoid disrupting the web, we did not measure the length of
each spider, so we used relative size difference classification.
Finally, we counted the number of females within three
concentric web regions: the innermost core, the semi-periphery
and periphery. The periphery includes the outermost edges of
the web structure (e.g., colony web support lines attached to
vegetation, small orb webs imbedded between support lines or
not surrounded by other orb webs), the core contains the
communal retreat (as described in Lubin 1980; Smith 1983;
Breitwisch 1989; Binford & Rypstra 1992), and the semi-
periphery is comprised of the interstitial space between the
core and periphery (Fig, 2). Two observers counted the
spiders, categorized spiders into size classes, and determined
each spider’s web region independently and then compared
notes. When discrepancies between the observers’ counts
could not be reconciled, the average count was used in the data
analysis. Inter-observer reliability before reconciliation was
0.69 (Spearman rank correlation, P,0.001, n¼240 counts).
After each colony census, we observed and recorded
aggressive behaviors ad libitum for 80 minutes using the
ethogram in Table 1. For each behavior, we recorded the time
of occurrence, the web region, the sex and size class of initiator
and recipient, and the winner of the interaction. We classified
spiders as winners if they displaced the opponent. Repeated
behaviors, such as continuous web plucking, were recorded
once per minute.
Intra-colony intrusion experiment.—To examine how the
spiders respond to approaching colony members and to
examine how the type of web occupied by the resident (i.e.,
orb vs. connecting) affects the responses, we carried out
simulated intruder tests at 20 colonies in randomized order. In
a given trial, one female was captured and immediately
presented to another female of the same size class. We
excluded females of the smallest size class (because they rarely
occupy orb webs), females that were actively hunting or
consuming prey, and males. Otherwise, our choice of
‘‘intruders’’ and residents was based primarily on accessibility,
i.e., whether we could capture and release the spiders without
touching the web and disturbing the whole colony. The type of
web occupied by the resident and intruder varied throughout
each trial. We used an entomological aspirator to gently
suction a spider off the web. We then blew the intruder out to
within 15 cm of the resident, and in the same 2-dimensional
web plane as the resident. We recorded all interactions
between the resident and intruder for 5 min., using the
ethogram in Table 1, and recorded whether the resident had
returned to its original position by the end of the trial. We
used each colony for two trials and conducted a total of 21 orb
web tests and 19 connecting web tests.
Translocation experiment.—We performed inter-colony
translocations to determine whether P. republicana treat
colony mates and conspecifics from other colonies differently,
and whether the distance (and possibly relatedness) between
colonies affects the occurrence of aggression. We used a
repeated-measures design in which medium-sized females from
each of 10 source colonies were assigned to three translocation
Figure 1.—A map of the 34 colony sites, created using the 2004 map of the Cocha Cashu trail system, courtesy of the Cocha Cashu website,
accessed 29 April, 2020, https://cochacashu.sandiegozooglobal.org/maps/
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treatments: same colony, ,300 m from the source colony
(proximal), and .800 m from the source colony (distal).
Distances were selected based on logistical feasibility because
the dispersal abilities of P. republicana have not been well
studied. All colonies used in this experiment as either source or
receiving colonies contained 15 females. Source colonies
were selected at random from a sample of 20 such colonies and
visited in randomized order. Proximal and distal receiving
colonies for each source colony were selected at random from
the colonies available, with the restriction that no receiving
colony could be used more than once for a given translocation
treatment. We captured focal spiders by placing a transparent
plastic cup near the spider and gently closing the lid with the
spider inside. Regardless of the translocation treatment, we
marked the focal spider with a drop of acrylic paint on the
abdomen and held the focal spider in the container for 20
minutes, to account for the travel time required for distal-
colony translocations. After placing the focal spider on the
web of the receiving colony, we recorded all interactions
involving the focal spider for 1 hour, using the ethogram in
Table 1. The focal spider was then recaptured and immediately
returned to its original colony.
Statistical analysis.—To compare the frequency of aggres-
sive interactions among female size classes and between
regions within the webs, we used a Friedman test with trials
matched by source colony, followed by Wilcoxon signed-rank
tests with p-values adjusted for multiple comparisons based on
an extension of the method of Holm (1979). The Friedman test
is the standard non-parametric equivalent of a repeated
measures ANOVA for making comparisons among three or
more groups (Sprent & Smeeton 2007). Holm’s (1979) method,
also known as the sequential Bonferroni method (Rice 1989) is
more powerful than the original Bonferroni method but more
conservative than most other methods, including the false-
discovery rate method (Hochberg & Benjamini 1990; Verho-
even et al. 2005). To obtain a measure of heterogeneity of size
classes of females in each colony, we calculated female size
class variability using the measure ‘unalikeability’ from Kader
& Perry (2007), which is a measure between zero and one that
indicates how unalike or different observations are from each
other, with zero meaning all are identical, and one meaning all
are different. To determine whether females of different size
classes were distributed non-randomly across regions within
webs, we compared observed frequencies to expected frequen-
cies based on the proportion of all females found in each
region, using a goodness-of-fit G test. To determine whether
aggression occurred non-randomly with respect to female size
class, we compared the observed frequency of aggressive
interactions within and among size classes to expected
frequencies based on the expansion of (sþmþl)
2
where s,
mand lare the proportions of females in each size class.
Expected frequencies were generated for each colony by
multiplying the expected proportion of aggressive interactions
of each type by the total number of aggressive interactions
observed in that colony, and then summing observed and
expected frequencies across colonies for an overall goodness-
of-fit G test.
We carried out a Principal Component Analysis (PCA) of
the three web-size dimensions (length, width and height) to
find the primary axis of variation in web size (PC1). To test
predictions about how colony metrics (web size, sex ratio,
female size, colony size, web region, and web type) affect
different types of aggression (total aggression, prey snatch
events, female-female aggression, female-male aggression and
male-male aggression), we constructed the five linear models
shown in Table 2 (n¼20 colonies). To properly account for
the dispersion of these count-based dependent variables, we
used General Linear Models in the negative binomial family
(glm.nb in the R package Stats).
For the intra-colony intrusion experiment, we tested if the
total number of vibrations, shakes, and leg taps differed
between web types using a Wilcoxon rank sum test with
continuity correction. To test for an association between web
type and the resident’s final position, we used Fisher’s exact
test (Fisher 1930).
For the inter-colony translocation experiments, behaviors
were grouped together as aggressive interactions, non-aggres-
sive interactions, as indicated in the ethogram (Table 1). We
tested for treatment effects on the number of these grouped
behaviors using a Friedman test with trials matched by source
colony. We used the R base package, and the packages
ggplot2, ggpubr, reshape, ragree, MASS and cowplot (Ven-
Table 1.—Ethogram of Philoponella republicana behaviors. Aggressive behaviors are marked with an asterisk.
Behavior Definition
Approach Individual walks towards another individual
Chase* Focal individual runs towards and follows another individual, and the other individual runs away
Displace* Focal individual approaches another individual, and the other individual moves away while the focal
individual stays, taking the original place of the other individual
Prey Ball Snatch* Focal individual takes a food ball of crushed prey from another individual
Leg Attack* Focal individual quickly forces leg onto another individual’s body, physically moving them away
Leg Tap Focal individual slowly touches another individual with one of their front legs, without moving the
other individual
Prey Snatch* Focal individual takes prey item away from another individual
Throw* Focal individual grabs and tosses another individual away
Web Cut* Focal individual breaks web between itself and another individual
Shake* Focal individual quickly pulls on web between itself and another individual, often performed in a series
of plucks
Web Pull* Focal individual pulls on web between itself and another individual, decreasing the distance between
the focal and the other individual
Web Vibrate Individual vibrates web by tapping the strands directly at another individual
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ables and Ripley 2002; Wickham 2007, 2016; Wilke 2019;
Kassambra 2020; R 2020; Redd 2021).
RESULTS
Colony web characteristics.—We found that colony webs
were amorphous structures with a median height of 173 m,
median length of 1.77 m, and median width of 1.53 m. The
first principal component (PC1) of colony web size dimensions
accounted for 62%of the variance and had positive loadings
for all three colony web size variables, and thus provided a
suitable measure of variation in overall web size (Table 3).
Colony Web size (PC1) was not a significant predictor of
intra-colony aggression while controlling for the number or
sex ratio of spiders in the colony (Table 2). Fig. 2 is a
representative diagram of a colony web.
Colony composition.—Philoponella republicana colonies
ranged from 8–88 individuals (mean 6sd, 411 620.24; n¼
20 colonies), including 3 to 16 large females (median ¼8.5), 0
to 64 medium females (median 16.5), 0 to 19 small females
Table 2.—Effects of colony characteristics on the frequency of
aggressive interactions (negative binomial GLMs) in Philoponella
republicana colonies. N ¼20 colonies.
Model Estimate Pdf
Total aggression
Web size (PC1) -0.03 0.6 16
Colony size (number of spiders) 0.02 0.02
Female size class variability 2.35 0.02
Female-female aggression
Web size (PC1) 0.02 0.39 13
Number of large females 0.11 0.01
Number of medium females 0.29 0.03
Number of small females 0.22 0.19
Female-male ratio 0.28 0.35
Female size class variability -0.15 0.92
Prey Snatch Frequency
Web size (PC1) 0.01 0.77 14
Number of large females 0.17 0.02
Number of medium females 0.34 0.08
Number of small females 0.28 0.32
Female size class variability -0.10 0.97
Female-male aggression
Web size (PC1) 0.24 0.44 16
Female-male ratio -0.61 0.04
Female size class variability 2.09 0.10
Male-male aggression
Web size (PC1) 0.03 0.92 16
Female-male ratio -1.36 0.01
Female size class variability 4.68 0.01
Table 3.—Web size PCA loading matrix.
Web dimension PC1 PC2 PC3
Length 0.177 -0.793 -0.584
Width 0.258 -0.535 0.804
Height 0.950 0.292 -0.110
Proportion of Variance 0.623 0.301 0.076
Figure 2.—A composite diagram of a colony web with the three web regions delineated. The dotted orange line encompasses the core, and the
dashed blue line minus the dotted orange line equals the volume of the semi-periphery. The periphery covers the remainder of the colony web,
including the support lines secured to vegetation.
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(median ¼3), and 3 to 21 males (median ¼6) (Fig. 3a). Female
size class variability ranged from of 0.00 to 0.66 (mean 6SE ¼
0.48 60.03, median ¼0.49, n¼20 colonies), where 0 would be
totally homogeneous and 1 would be maximally heteroge-
neous.
The number of spiders varied among regions in the web
(Friedman test, v2¼16.13, df ¼2, P¼0.0003), with fewer
spiders in the periphery (median ¼9, n¼20) than in the semi-
periphery (median ¼16, n¼20, Wilcoxon signed rank test with
Holm’s adjustment for multiple comparisons P¼0.0003) but
no significant difference between the number of spiders in the
core (median ¼13) and semi-periphery (P¼0.16) or core and
periphery (P¼0.08) (Fig. 3a). Median percentages for spiders
in the three web regions were 31.70%in the core, 40.45%in the
semi periphery, and 20.64%in the periphery.
The distribution of female size classes across web regions
differed significantly from chance expectations (G-test, P¼
0.0004). Small females were found in the periphery more often
than expected (44.0 observed and 27.0 expected) and in the
core less often than expected (16.0 observed and 29.2
expected), and large females were found in the periphery less
often than expected (27.0 observed and 40.2 expected) (Table
4).
Females outnumbered males in every colony (Wilcoxon
signed rank test, P¼0.0001, n¼20), and the female to male
sex ratio ranged from 1.05 to 14 (median ¼2.98, n¼20).
Colony aggression.—The number of aggressive interactions
per individual (total number of aggressive interactions
initiated by the female class divided by the total number of
females of the size class) varied among the three female size
classes (Friedman test, v2¼2226, df ¼3, P,0.0001, n¼20
Figure 3.— (a) Distribution of size and sex classes of spiders by region within the colony webs. For each web region, size and sex classes are in
the same order, from left to right, as shown in (b). (b) Variation in the frequency of aggression per individual by size and sex class (n¼14
colonies). Six colonies were excluded from this analysis because one or two female size classes were absent. (c) Observed rates of aggression per
capita in relation to colony web region. Box plots show the following statistical values: the thick horizontal line indicates the median, the lower
and upper boundaries of the box indicate the 25
th
and 75
th
percentile values (Q1 and Q3) respectively, the box represents the interquartile range
(IQR), the whiskers represent the minimum (Q1 – 1.5(IQR)) and maximum (Q3 þ1.5(IQR)) values without outliers, and the circles represent
outlier values.
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colonies) (Fig. 3b). Large females were involved in more
aggressive interactions (median ¼1.33, n¼20 colonies) than
medium-sized females (median 0.59, n¼19, Wilcoxon signed-
rank tests with Holm’s adjustment for multiple comparisons,
P¼0.05) and small females (median ¼0, n¼15, P¼0.003),
but medium-sized and small females did not differ in
aggression levels (P¼0.07, n¼15). Males were involved in
more aggressive interactions (median ¼0.52, n¼20) than small
females (Wilcoxon signed rank test with Holm’s adjustment
for multiple comparisons, P¼0.007, n¼15), and fewer
aggressive interactions than large females (P¼0.05, n¼20)
but there was no significant difference between males and
medium-sized females (P¼0.98, n¼19). Sample sizes vary
among tests because five colonies had no small females and
one colony had no medium-sized females. Female size class
variability was a significant positive predictor of total
aggression and male-male aggression in the colony (Table 2).
Observed female-female aggression totals differed signifi-
cantly from the expected (G-test, P,0.0001). The largest
differences in expected and observed totals were in large
female-large female aggressive interactions (124 observed and
33.6 expected), medium female-medium female aggressive
interactions (11 observed and 47.44 expected), and large
female-medium female aggressive interactions (53 observed
and 86.65 expected, Table 5).
The number of aggressive interactions per individual varied
among regions within the web (Friedman test, v2¼16.57, df ¼
2, P¼0.0003) and tended to be highest in the semi-periphery,
where most orb webs are located, and lowest in periphery (Fig.
3c). The number of aggressive interactions was lower in the
periphery (median ¼0.14, n¼19) than in the semi-periphery
(median ¼0.82, n¼20, Wilcoxon signed rank test with Holm’s
adjustment for multiple comparisons, P¼0.004) and core
(median ¼0.54, n¼20, P¼0.004), but did not differ
significantly between the core and semi-periphery (P¼0.28).
Sample sizes vary among tests because there were no spiders in
the periphery of one colony.
While controlling for web size (PC1), the total number of
aggressive interactions increased with colony size and female
size class variability. The frequency of female-female aggres-
sion increased with the number of medium-sized and large
females, and the number of prey snatch events also tended to
increase with the number of large females in the colony (Table
2).
As predicted, the frequencies of male-male aggression and
female-male aggression both decreased with the female-to-
male sex ratio (Table 2). Female-to-male sex ratio had no
effect on female-female aggression (Table 2). Male-male
aggression also increased with an increase in female size class
variability.
Intra-colony intrusion experiment.—We consistently ob-
served high levels of aggression from resident females in orb
webs and almost no aggression from resident females in
connecting webs toward the simulated intruder females. In 17
trials, the resident stayed in the center of the orb web and
shook and vibrated the web until the intruder withdrew. In the
remaining 4 trials, the resident left the center and aggressively
fought with the intruder until she moved off the orb web. The
connecting web trials typically unfolded in one of the
following ways: the two females sat very close together but
did not interact, moving either very little or not at all (2 trials);
the females did not interact overtly and slowly moved apart
over the course of the 5-minute trial (8 trials); or the females
came into very close contact, even physically touching through
leg taps but did not display aggression (5 trials). In the four
connecting-web trials in which aggression was observed, the
duration of the behaviors was shorter than in the orb web
trials. Resident females from orb webs performed more shakes
(Wilcoxon test, W ¼0, P,0.01, n
1
¼21 orb webs, n
2
¼19
connecting webs; Fig. 4a) and vibrates (W ¼7, P,0.01, Fig.
4b) than resident females from the connecting web. The
number of leg taps performed by the two groups did not differ
significantly (W ¼214, P¼0.57). The orb-web residents were
also more likely than the connecting-web residents to be in the
same position at the end of the trial with respect to their
position at the beginning of the trial (Fisher’s exact test, P,
0.01, Fig. 4c).
Translocation experiment.—We found no differences among
translocation treatments in any of the behaviors recorded
(Friedman tests, P0.05, n¼10 colonies; Table S1, online at
https://doi.org/10.1636/JoA-S-20-093.s1). Whether the trans-
located spiders were taken from the same web, a proximal
web, or a distal web, members of the receiving colonies in
general did not interact with the translocated females very
often. Across treatments, the median number of interactions
ranged from 1 to 2.5 (Fig. 5a; for specific interactions, see
Table S2, online at https://doi.org/10.1636/JoA-S-20-093.s2).
Table 4.—Observed frequencies and expected frequencies based on
proportions of Philoponella republicana females of different size
classes in relation to regions within webs. N¼20 colonies.
Region Small Females Medium Females Large Females
Core
Observed 16 129 54
Expected 29.17 113.47 56.37
Semi-periphery
Observed 38 149 87
Expected 44.09 158.52 71.39
Periphery
Observed 44 81 27
Expected 27 87.01 40.24
Table 5.—Frequency of female-female aggression within and
between size classes. Observed frequencies are below the diagonal
and expected frequencies are above the diagonal and in boldface. Data
from all 20 colonies are combined here.
Small Females Medium Females Large Females
Small 10.51 47.44 18.11
Females 7
Medium 109.67 86.65
Females 11 93
Large 33.6
Females 16 53 124
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Most interactions that did occur were aggressive (Fig. 5b,
Table S2).
DISCUSSION
Conspecific aggression can be an indicator of cooperation
and competition within a group, so examining antagonistic
behaviors can help elucidate the balance between these two
types of behaviors in colonial spiders (Opell 1979; Binford &
Rypstra 1992; Uetz & Hieber 1997; Avil´
es & Guevara 2017).
We studied P. republicana, reported to display both tolerance
and discrimination towards conspecifics (Opell 1979; Binford
& Rypstra 1992), to identify and quantify the impact of
aggression on colony dynamics. We studied 34 colonies in
total and performed colony censuses and translocation
experiments to understand aggression. As in other colonial
spiders, aggression between colony members was common
(Buskirk 1975a, b; Rayor & Uetz 2000; Lubin & Bilde 2007;
Avil´
es & Guevara 2017). The frequency of aggressive
interactions was higher in optimal hunting areas (i.e., orb
webs throughout the colony, both web types in the semi-
periphery), colonies with less female-biased sex-ratios, and
colonies with a greater number of large and medium females.
However, P. republicana also demonstrated some characteris-
tics that may indicate greater levels of cooperation. All
colonies had female-biased secondary sex ratios, and a larger
bias was associated with both reduced male-male and female-
male competition. Even though spiders individually defended
orb webs, they moved freely in the connecting web, which may
provide a setting for spiders to jointly accomplish tasks such as
colony web construction (Uetz & Hieber 1997). We found no
evidence for group closure in P. republicana (i.e., the spiders
did not discriminate between outsiders and colony mates), but
to the best of our knowledge, group closure has not been
reported in social spiders either. Overall, the patterns of
aggression in P. republicana reveal a balance between
aggression and tolerance towards conspecifics.
Aggression was concentrated in orb webs and the semi-
periphery.—Our intra-colony intrusion experiment uncovered
elevated levels of aggression among orb-web holders towards
individuals found within their territories. As in the colonial
species M. ocellatus (Buskirk 1975a, b), P. republicana orb web
Figure 4.— Effects of web type on the responses of resident females to simulated intruders. In (c) the height of the bars represents the total
number of resident females observed in each ending position across all trials. Box plot definitions as in Fig. 3.
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holders shook and vibrated their webs in response to
approaching conspecifics. However, residents in connecting
webs usually only displayed non-aggressive behavior (e.g., leg
tapping) toward intruders, if they responded at all.
Our census data showed that the highest levels of aggression
occurred in the semi-periphery, suggesting increased compe-
tition for this optimal space (Rayor & Uetz 1990). The highest
density of orb webs was also found in this region. Although
the periphery also contained orb webs, we observed less
aggression and more small females in that region (Figure 3).
This may be indicative of an adaptive spacing pattern in which
smaller individuals avoid competition by building their orb
webs in less-contested areas of the web, similar to the colonial
spiders Cyrtophora citricola and Metepeira incrassata (Pick-
ard-Cambridge, 1903) (Rayor & Uetz 2000; Yip et al. 2017).
Alternatively, this pattern could be the result of exclusion of
smaller spiders from preferred regions by larger spiders.
The lack of aggression in the core and in the connecting
webs provides quantitative evidence for the use of communal
retreats, consistent with observations from previous studies
(Lubin 1980; Binford & Rypstra 1992). Philoponella repub-
licana individuals sat in very close proximity to one another in
communal retreats without displaying any strong signs of
aversion to their nearby neighbors, as they did in orb webs.
Philoponella oweni (Chamberlin, 1924) (Smith 1983) and an
unidentified species of Philoponella from southern Cameroon
(Breitwisch 1989) have also been observed to use communal
retreats.
Aggression was not dependent on colony web size.—Our
analysis of colony web size also revealed interesting colony
dynamics in P. republicana. While we hypothesized that large
colony webs would reduce competition for prey, the total
number of aggressive interactions per colony did not co-vary
with colony web size, even though colony web size and colony
size varied independently. This result could be explained by
environmental factors. For instance, high prey availability in
the wet season might mask differences in prey capture
efficiency between colony web sizes. Moreover, if larger
colony webs have more access to prey, spiders in large colony
webs could have more energy to expend on aggression to
retain captured prey, negating our prediction that larger
colony webs would have less competition. Alternatively,
aggression related to prey might be expressed more gradually
over time; the number of immature spiders that eventually
remain in a colony as subadults and adults might be dependent
on food resources. Colony populations might grow in size
until they reach the limit that local food supplies can support
(e.g., Smith 1983). In that case the amount of prey per capita
and level of aggression over prey might stay relatively constant
across a range of colony sizes.
Colonies with more large and medium females had more
aggression.—Female-female aggression positively correlated
with the number of large females and medium females in the
colony (Table 2). Medium and large females participated the
most in prey snatching and fighting over captured insects. This
finding is consistent with the widespread pattern that larger
individuals have an advantage in aggressive interactions
because they are more likely to win contests (Potter et al.
1976; Riechert 1978; Christensen & Goist 1979; O’Neill 1983).
The increased aggression observed in larger females could
make colonies more resilient. Previous studies in both colonial
and social spiders have found that dominant, large individuals
consistently have greater access to prey (Hodge & Uetz 1995;
Ulbrich & Henschel 1999). Ulbrich & Henschel (1999) suggest
that in conditions of low prey density, large, dominant
individuals may still be able to acquire enough food, which
ensures the survival of the colony. In contrast, if individuals in
a colony had equal access to prey, no single spider would be
able to eat enough to reproduce, and the entire colony would
die out.
Colonies with more female-biased sex ratios had lower
aggression.—Lubin (1980) reported finding a primary 1:1 sex
ratio in P. republicana. However, the secondary sex ratios we
measured in P. republicana were female-biased. A potential
explanation for this is that males are more susceptible to
predation or there is male-biased dispersal in this species
Figure 5.— Translocation experiment results. (a) Variation in the total number of interactions between colony members and translocated
individuals. (b) Variation in the total number of aggressive interactions between colony members and translocated individuals. Box plot
definitions as in Fig. 3.
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(Lubin & Bilde 2007). Nevertheless, the levels of both male-
male and female-male aggression decreased as the secondary
sex ratio became more female-biased, which could be
explained by reduced mating competition (Binford & Rypstra
1992). Secondary sex ratio did not affect female-female
aggression, suggesting that this type of aggression arises from
factors other than mating competition.
Colonies did not exhibit group closure.—Regardless of the
translocation treatment, colony members did not display
elevated levels of aggression towards translocated females
compared to the same-colony controls, meaning P. repub-
licana did not exhibit group closure. These results are similar
to those found in the social spider Anelosimus eximius
(Keyserling, 1884) (Pasquet et al. 1997). This does not support
our hypothesis that aggressive behaviors would be more
frequent towards outsiders than towards members of the same
colony. A previous study suggests P. republicana disperses in
sibling groups and thus within-group relatedness should be
higher than between-group relatedness (Lubin 1980). Yet,
colony members still responded similarly to all translocated
females. Shared natal sites among individuals from different
colonies may explain this result, since spatial or genetic
relatedness decreases aggression. However, because of the
relatively large distances between colonies in the distal
treatment of our study, this explanation seems unlikely.
Pasquet et al. (1997) hypothesized that the social spider
species they studied (A. eximius) does not exhibit group
closure because (a) it is not part of the spiders’ natural
behavior to move into existing colonies, so there was no
selection on the spiders to evolve a discriminatory response to
invaders; (b) spiders lack the variation in chemical compounds
between colonies needed to recognize that individuals are
invaders or not; and (c) group closure is required in social
insects that compete with other colonies for resources and
since all of the spiders’ feeding is limited to their own colonies,
there is no competition with other colonies. These hypotheses
may also apply to P. republicana. This experiment was limited
in that we did not examine how spiders respond to intruders in
varying conditions. While group-living spiders have not been
found to exhibit group closure under normal conditions, other
studies have demonstrated that extenuating circumstances can
cause colonies to preferentially discriminate against outsiders
(Yip & Rayor 2014). For example, the social spider
Australomisidia ergandros (Evans, 1995) (Thomisidae) was
more likely to cannibalize unfamiliar conspecifics, but only
under conditions of starvation (Evans 1995, 1999). Future
research on P. republicana should focus on the chemical
compounds expressed by individuals from different colonies,
dispersal distances, dispersal group composition, genetic
relatedness between colony mates, and behaviors towards
immigrants in varied circumstances and extended periods of
time to better understand the apparent lack of group closure in
this species.
ACKNOWLEDGMENTS
We thank the Cocha Cashu staff members for introducing
us to P. republicana and for their generous hospitality. We are
also grateful for the guidance of our teaching assistants,
Marcel Vaz and Maddi Cowen. Brent Opell kindly identified
the spiders to species from photographs. This research was
carried out under Servicio Nacional de ´
Areas Naturales
Protegida (SERNANP) research permit RJ N 025, with
funding from the UCLA Department of Ecology and
Evolutionary Biology Field Biology Quarter program and
the UCLA Office of Instructional Development.
SUPPLEMENTAL MATERIALS
Table S1.—Effects of inter-colony translocation treatment
on responses of Philoponella republicana. Online at https://doi.
org/10.1636/JoA-S-20-093.s1
Table S2.—Results of inter-colony translocation experi-
ment. Online at https://doi.org/10.1636/JoA-S-20-093.s2
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