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Cooperative foraging expands dietary niche but does not offset intra-group competition for resources in social spiders


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Group living animals invariably risk resource competition. Cooperation in foraging, however, may benefit individuals in groups by facilitating an increase in dietary niche. To test this, we performed a comparative study of social and solitary spider species. Three independently derived social species of Stegodyphus (Eresidae) occupy semi-arid savannas and overlap with three solitary congeners. We estimated potential prey availability in the environment and prey acquisition by spiders in their capture webs. We calculated dietary niche width (prey size) and breadth (taxonomic range) to compare resource use for these six species, and investigated the relationships between group size and average individual capture web production, prey biomass intake rate and variance in biomass intake. Cooperative foraging increased dietary niche width and breadth by foraging opportunistically, including both larger prey and a wider taxonomic range of prey in the diet. Individual capture web production decreased with increasing group size, indicating energetic benefits of cooperation, and variance in individual intake rate was reduced. However, individual biomass intake also decreased with increasing group size. While cooperative foraging did not completely offset resource competition among group members, it may contribute to sustaining larger groups by reducing costs of web production, increasing the dietary niche and reducing the variance in prey capture.
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ScIEntIfIc REPORTS | (2018) 8:11828 | DOI:10.1038/s41598-018-30199-x
Cooperative foraging expands
dietary niche but does not oset
intra-group competition for
resources in social spiders
Marija Majer1,2, Christina Holm2, Yael Lubin1 & Trine Bilde2
Group living animals invariably risk resource competition. Cooperation in foraging, however, may
benet individuals in groups by facilitating an increase in dietary niche. To test this, we performed a
comparative study of social and solitary spider species. Three independently derived social species
of Stegodyphus (Eresidae) occupy semi-arid savannas and overlap with three solitary congeners. We
estimated potential prey availability in the environment and prey acquisition by spiders in their capture
webs. We calculated dietary niche width (prey size) and breadth (taxonomic range) to compare resource
use for these six species, and investigated the relationships between group size and average individual
capture web production, prey biomass intake rate and variance in biomass intake. Cooperative foraging
increased dietary niche width and breadth by foraging opportunistically, including both larger prey
and a wider taxonomic range of prey in the diet. Individual capture web production decreased with
increasing group size, indicating energetic benets of cooperation, and variance in individual intake
rate was reduced. However, individual biomass intake also decreased with increasing group size. While
cooperative foraging did not completely oset resource competition among group members, it may
contribute to sustaining larger groups by reducing costs of web production, increasing the dietary niche
and reducing the variance in prey capture.
Inter-specic competition for resources plays an important role in the evolution of foraging adaptations and
the ecological niche of individuals and species, and may ultimately drive adaptive speciation14. Interspecic
competition exists at all trophic levels, but where herbivores are likely to be limited by their predators rather than
resources, predator populations are more likely to be limited by their prey5. Resource competition can be miti-
gated by the evolution of foraging strategies that modify the way resources are obtained6. Cooperative foraging
may function to change a species niche to exploit resources unavailable to individual foragers7, or facilitate more
ecient exploitation of resources8,9. e economics of social foraging depend on the dynamics of food discovery8,
and the means by which food is acquired, for example by the use of coordinated hunting strategies that enhance
prey capture (e.g. in lions)10, or the evolution of specialized food gathering castes as in the eusocial insects11. In
sedentary group foragers, such as social spiders, that are unable to forage outside the nest and associated capture
web, cooperative strategies that involve building larger communal webs and collective hunting of insect prey may
be particularly important for sustaining the group1215.
ere are several ways by which cooperative foraging may aord foraging benets. Group hunting can result
in increased prey capture rate12,16, or enable an increase of dietary niche1720. e latter may arise from inclusion
of larger prey to widen the dietary niche16,17,21, or through an increase in diet breadth by the inclusion of a broader
taxonomic range of food or prey types in the diet2224. is could lead to an expansion in resource use or a shi
in diet to exploit a dierent dietary niche7,2527. Group foraging can also provide benets by reducing variance in
prey capture rate and individual consumption rate, which buers the group against starvation2830. Finally, coop-
erative foraging can provide energetic benets by reducing the individual investment in resource acquisition31.
ese potential benets are not mutually exclusive.
1Blaustein Institutes for Desert Research, Mitrani Department of Desert Ecology, Ben-Gurion University of the
Negev, Midreshet Ben-Gurion, 8499000, Israel. 2Institute of Bioscience, Aarhus University, Ny Munkegade 114, 8000,
Aarhus, Denmark. Correspondence and requests for materials should be addressed to Y.L. (email:
Received: 30 May 2017
Accepted: 6 July 2018
Published: xx xx xxxx
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Cooperation in spiders has evolved independently multiple times, with approximately 20 origins in seven fam-
ilies out of the more than 46,000 known extant species3236. Social spiders share a communal nest, where they
cooperate in web building and prey capture, and breed cooperatively34. e permanently social species occur in
tropical and subtropical areas that are also characterized by higher productivity and prey biomass15,20,37, suggesting
that food availability or benets of cooperative foraging are involved in the evolution or maintenance of permanent
sociality. Prey size was shown to play an important role in predicting the elevational distribution of social spiders
of the new world spider genus Anelosimus3739. Social Anelosimus species were found to catch larger prey than
solitary congeners40,41, and prey size distributions diered between the lowland tropical habitat of social A. eximius
and higher elevation solitary species. Larger insects were found in the lowland habitat of social Anelosimus2022,
supporting the hypothesis that elevational dierences in the size of insect prey may explain the geographical dis-
tribution of these Anelosimus species. In addition, a study of the social A. eximius and subsocial (solitary) A. baeza
suggested that the social species exploit a wider range of insect types from their environment22. Collectively there
is good support for exploitation of larger prey, i.e. a wider dietary niche, in cooperatively foraging Anelosimus,
which is associated with their occurrence in lowland tropical areas with higher frequency of large prey (18, 29, and
references above). Whether this is a common pattern in other social spider species, however, remains unknown.
Group living invariably comes with competition for resources among group members that limits breeding
opportunities42,43, and competition for resources is expected to increase with group size9. Both the costs and
the benets of group living are oen positively correlated with group size, imposing opposing selective forces
on group size44. Social spiders build large capture webs, and individuals may acquire energetic benets from
reduced investment in silk production and web construction with increasing group size31,45. A study of the South
American A. eximius showed that the surface/volume ratio of the three-dimensional capture web decreased with
increasing group size, supporting this hypothesis13. However, prey capture per individual declined with increasing
group size, and although larger groups succeeded in capturing larger prey46, individual biomass intake was max-
imized at an intermediate group size13. In A. eximius and the African social spider Stegodyphus dumicola, colony
survival increased with increasing group size47,48, yet for both species, individual lifetime reproductive success
was highest at intermediate group sizes, suggesting that competition for resources within the colony limits group
size47,48. ese studies, as well as theoretical considerations49,50, suggest that the interaction between group size
and individual resource acquisition plays an important role in the maintenance of sociality.
Here we investigated the hypothesis that cooperative foraging enables groups to increase the range of resources
they can obtain from the environment, by comparing the foraging strategies of congeneric social and solitary spi-
ders in relation to resource availability in the habitat. We present results from a comprehensive, comparative study
of six species of the Old-World genus Stegodyphus (Eresidae), in which we investigated eects of cooperative forag-
ing on resource acquisition, as measured by the dietary niche width and breadth, and potential energetic benets of
silk saving in producing a group capture web. e genus Stegodyphus contains three independent origins of coop-
erative social species35,51. ese social species occur in areas characterized by higher productivity and insect abun-
dance compared with solitary congeners52, but unlike the New-World genus Anelosimus, there is no pronounced
elevational gradient, as both social and solitary species typically occur in low- to mid-elevation sub-tropical and
semi-arid regions. To test the hypothesis that social Stegodyphus avail themselves of a broader foraging niche, we
compared resource use in three social and three solitary Stegodyphus species. Using traps to estimate prey availabil-
ity in the habitat, and determining the prey captured and exploited by spiders, we obtained quantitative estimates of
dietary niche width (prey size) and breadth (taxonomic range) in relation to potential prey in the nearby habitat1,19.
To assess the potential collective benets arising from prey biomass acquisition in relation to web size at
the individual level, we determined capture web sizes in relation to group size and foraging type (social versus
solitary), and estimated per capita biomass consumption. We compared these data between social and solitary
species to assess benets of cooperative foraging, and within the three social Stegodyphus species to examine how
individual intake rate interacts with group size to oset intraspecic resource competition.
We addressed the following questions:
i. Does cooperative foraging facilitate an expansion in dietary niche width by opportunistic feeding on a
wider range of prey sizes, or by a shi in resource use through specialization on larger prey? Dietary niche
width was quantied based on the variation in resource use (prey size) within individuals or social groups
(WIC) and in the total population (TNW), corresponding to Roughgardens within-individual and total
population components of niche width, and compared between social and solitary foragers.
ii. Does cooperative foraging enable an expansion (or shi) in dietary niche breadth, i.e., do social species
forage on a wider taxonomic range of prey types than solitary foragers? Dietary niche breadth was deter-
mined as the taxonomic spectrum of insect prey included in the diet relative to potential prey in the local
environment and compared between social and solitary foragers.
iii. How do group living and group size inuence individual production of capture web? Capture web used for
interception of prey is costly to produce53; therefore, cooperative foraging may provide energetic benets
by reduced individual production of silk. We compared individual capture web size in social and solitary
foragers, and in relation to group size in social species.
iv. How do cooperative foraging and group size inuence individual consumption rate? We determined prey
capture in relation to group size, and estimated individual intake rate (mg biomass prey/hour/web cm2/
individual) using taxon-specic relationships between insect prey size and mass, comparing social and
solitary foraging mode.
v. How do potential benets of cooperative foraging interact with group size to oset intra-specic compe-
tition? We examined the relationship between group size, individual prey capture rate and web size within
the three social Stegodyphus species. Finally, we assessed the eect of group size on variance in individual
intake rate.
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Materials and Methods
Study species. We compared three solitary species and three social species in the genus Stegodyphus
(Eresidae) (Table1). Stegodyphus species construct nests that consist of silk and plant material on shrubs or trees,
either solitarily or in social groups54. All species construct aerial sheet-webs consisting of non-sticky threads radi-
ating from the nest and inter-connected with sticky, cribellate silk threads that serve to trap ying and jumping
insects51,55. Capture webs of both solitary and social species consist of one or more such planar webs. Females
produce a single egg-sac; the young are protected and fed by their mother by regurgitation feeding, and females
are eventually consumed by the ospring54,56. In the solitary species, the young disperse out of the maternal nest,
while in the social species, ospring remain in the maternal nest where they capture prey cooperatively, and mate
and breed within the group. Species of the genus have annual life cycles, but colonies of the social species may
persist for several generations57. Besides cooperation in brood care and prey capture, social spiders cooperate
in web building and nest maintenance. Solitary species reach on average 1.5–2 x larger adult body size than the
social species (Fig.1).
Study design. To compare dietary niche width and breadth, we calculated dietary niche from the fraction of
prey caught out of the amount of potential prey available at each individual nest. To do this, each nest was sam-
pled simultaneously for potentially available prey in the immediate vicinity of the nest using various traps, and
for actual prey captured by spider + web (or colony + web). Stegodyphus nests are stationary, and the spiders in
their webs capture insects selectively from the immediate surroundings. us, in this paired design, variation in
prey availability, assessed by trapping at the nest site, is directly comparable with the adjacent web captures. is
design therefore incorporates the local variation in potential prey at the same small scale as the prey captured in
an individual nest.
Available and captured prey. Field studies on all six species were conducted in the period when subdadult
or young adult females were present in the nest. is period is an important time for resource acquisition as
fecundity is related to body size in females48. In total, we studied 14 dierent populations, seven each of solitary
and social species (Table2). We refer to a population as an assembly of solitary individuals or of social nests at a
given location.
To obtain data on the availability and utilization of prey by spiders in their natural habitat, we quantied the
number of insect prey, insect size and the prey type dened by taxonomical order. Quantitative data on potential
prey abundance and prey type were collected using one window trap and two sticky traps near every solitary
or social nest selected for observations, placed at approximately the same height as the capture webs (Table2).
Window traps consisted of two Plexiglas sheets (2 mm thick, 40 × 30 cm) placed perpendicular to one another in
the form of a cross, and situated on top of a funnel (31 cm diameter) attached to a collection bottle (1L) containing
preservative liquid (20% ethylene-glycol (Namibia, India, South Africa) and 75% alcohol (Israel)). ese window
traps were designed to intercept ying and jumping prey58. Sticky traps, made of transparent 210 × 297 mm A4
plastic sheets with non-drying glue paste (Tree Tanglefoot (Tanglefoot company) (Namibia; India, South Africa)
and Rimifoot (Rimi Ltd., Petach Tikva) (Israel)), were used to trap small ying insects. e combination of these
two trapping methods was designed to catch available prey of a range of sizes and types. Trapping does not
provide an absolute measure of prey availability, and therefore the term ‘available prey’ should be understood
as potentially available prey trapped with these two sampling methods in the immediate vicinity of spider nests.
Concurrently, we recorded prey captured by spiders by observing their capture webs in the eld during day-
time, as previous studies showed that Stegodyphus capture most of their prey during the day59 (Y. Lubin unpub-
lished). Each nest was observed eight times over three-hour periods on consecutive days, for a total of 24 hours
in total, during which we noted the number and size of prey naturally captured by each individual or group. We
treated each prey item captured by the individual or group within the observation period as an independent
event. e surveys were done in consecutive days by the same two researchers across all the locations, and the
size, taxon and placement of each prey item was carefully recorded to avoid double counting. ree social nests
of S. sarasinorum in India were destroyed and disappeared during the study period, while the webs of eight nests
were damaged by the heavy rains at the end of November. Our observation eort for this species was therefore
18–21 hours for 11 nests, and 24 hours for 4 nests.
Species Social/solitary Distribution Habitat
S. africanus solitary Central and Southern Africa Arid and semiarid habitats; co-occurs with social
species, S. dumicola or S. mimosarum51,54
S. lineatus solitary Widespread through the Mediterranean
and North Africa, to Tajikistan Dry or seasonal watercourses, with clustered
distribution within habitats79
S. pacicus solitary India, Iran and Pakistan Arid and semiarid habitats; co-occurs with social S.
S. dumicola social Central and South Africa Arid, grazed areas. Nests occur in shrubs and bushes,
but also in tree tops in savanna areas51
S. mimosarum social Africa and Madagascar Same arid habitats as S. dumicola, but oen in trees
near water54
S. sarasinorum social India, Sri Lanka, Afghanistan and Nepal Semi-arid, grazed areas80
Table 1. Summary of the distribution and habitat of the six Stegodyphus species studied. References are in
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e number of prey, prey size measured as body length in mm, and taxonomic order were recorded for every
insect caught in webs or traps. Identication was done to order, rather than family or lower taxonomic level, due
to the diculty of identifying insects in the webs without disturbing the spiders. Body length of each prey item
was measured with callipers to the nearest mm from the tip of the head to the end of the abdomen. We never
removed prey items from the webs, and took care not to record a prey more than once by keeping track of prey
size and taxon noted on the previous observation. Since Stegodyphus webs are two-dimensional and positioned
vertically in relation to the ground, it was possible to measure the length of prey items without removing them.
Subsequently we used a set of taxon-specic power equations to estimate dry biomass [mg] from prey length60.
Prey length and mass, as well as wet mass and dry mass are highly correlated18,60,61.
Group size estimates. Nests of social spiders collected in the study areas (N = 48 for S. dumicola, and
N = 24 for S. sarasinorum), and in wooded savanna habitats across South Africa for S. mimosarum, (N = 23, d ata
kindly provided by M Greve), were measured by length, width and height, and nest volume was estimated using
the formula for the volume of two pyramids (volume of a pyramid:
) as an approximation48. Each
of these nests was opened to count the total number of spiders present. We used linear regressions to assess the
Figure 1. Boxplots of available (white) and consumed prey size (grey) in the diets of three solitary and three
social Stegodyphus species. Data are plotted for solitary S. africanus S. lineatus and S. pacicus, followed by social
S. dumicola, S. mimosarum and S. sarasinorum (social species marked with*). Numbers on the y-axis below the
species names show the average body size of adult female spiders. Abbreviations next to the boxplots represent
sites (see Table2), where species data collected at dierent sites are combined. Each boxplot shows the extremes,
the inter-quartile range, and the median.
Species Period (duration of trapping) Site Site name
(coordinates E, N) Number of
traps Number of
nests Total prey
in traps Total prey
in webs
S. africanus
18.3 mm South Africa Nov-Dec 2012
(35 days)
SA1 Witz (31.10, 24.55) 6 9 2830 37
SA2 Hspr (30.95, 24.35) 7 4 5961 26
S. lineatus
16.3 mm Israel April-May 2010 (17; 35;
35 and 21days)
Isr1 AF (34.78, 31.33) 8 37 1728 45
Isr2 Leh (34.83, 31.36) 9 47 6053 155
Isr3 ZA (34.78, 30.79) 9 14 6502 27
Isr4 SII (34.73, 30.83) 8 12 2552 31
S. pacicus
17 mm India Sep-Oct 2010 (24 days) Ind1 Agastya (78.25, 12.88) 3 7 299 7
S. dumicola*
10.1 mm Namibia Dec 2009- Jan 2010
(35 days)
Nmb1 H (17.23, 19.55) 9 11 3249 188
Nmb2 HH (17.20, 19.48) 9 11 2689 95
Nmb3 Usb (17.23, 19.55) 7 9 2802 82
S. dumicola*
10.1 mm South Africa Nov-Dec 2012
(35 days)
SA1 Witz (31.10, 24.55) 3 3 1815 35
SA2 Hspr (30.95, 24.35) 8 10 8166 160
S. mimosarum*
9.2 mm South Africa Nov-Dec 2012
(35 days) SA1 Witz (31.10, 24.55) 6 4 3840 38
S. sarasinorum*
10.2 mm India Sep-Oct 2010 (24 days) Ind1 Agastya (78.25, 12.88) 21 24 2323 166
Table 2. Summary of experimental design in the eld sites, showing the time period of sampling, geographic
locations where we studied three social (S. dumicola, S. mimosarum and S. sarasinorum) and the solitary (S.
africanus, S. lineatus and S. pacicus) species, number of nests used for observations and number of traps for
insect sampling in each site (=population). Location coordinates are given in decimal degrees. Number of nests
represent the number of individuals studied in each population of solitary species, and the number of colonies
studied in the populations of social spiders. Total prey is the total number of insect prey observed in webs and
traps for each population. Social species marked with*. Total body length is shown for each species.
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relationship between nest volume and group size as a linear model provided the best predictor of group size
among several approximations (S. dumicola, y = 0.2475x + 103.76, R² = 0.35; S. mimosarum, y = 0.1291x + 28.356,
= 0.35; S. sarasinorum, y = 0.4068x + 47.671, R² = 0.69; Fig.S2 in Supplementary Analyses B). Subsequently,
we used nest volume as a proxy for group size to avoid destruction of the natural nests involved in data
Estimated group sizes ranged from 109 to 1450 individuals per nest in the Namibian populations of S. dumi-
cola, and 115–256 individuals in South African ones. Nests of S. mimosarum in South Africa contained between
33 and 54 individuals, and Indian S. sarasinorum nests were 64 to 410 individuals per nest (all extrapolated from
the nest volumes measured in situ and the regressions given above). Nests of S. mimosarum at SA1 site were
smaller than those of the other two social species we studied (glmm coecient eects t (mimosarum) = 2.22,
p = 0.03).
Prey capture rate was calculated as the number of prey captured per capita per hour.
Web size. For social species, capture web size was measured by estimating web area of a rectangle
(height width). In case of more than one web sheet, each one was measured separately and summed to obtain the
total web area. To obtain the web area per capita, total web area was divided by the estimated number of spiders
in each nest of the relevant social species. Web size for solitary species was estimated by a rectangle if the web had
four sides (height width) or a triangle if the web had three sides ∗∗
( )
base height
Data analysis. Dietary niche width. e range of insect prey sizes utilized (niche width) was estimated
as the total niche width component of Roughgardens index1. Accordingly, total niche width (TNW) of a pop-
ulation can be broken down into two components: variation in resource use within individuals (or social
group) (WIC), and the variance between individuals (or group) (BIC). It follows that total niche width equals
Estimates of TNW and WIC are based on a matrix of a resource measure, i.e. size (xij) of jth prey item caught
by an individual or group (i), for all the individuals (or social groups) in a population. e ratio of WIC to
TNW, also known as Roughgarden’s index, measures the extent to which the total niche width of a population
is due to individuals (or groups) within the population being resource generalists. e index is the ratio of WIC
(within-individual component of size variance, WIC = E(Var(xj|i)) and TNW (total niche width, TNW = Var(xij)):
R’s I = WIC/TNW. Values approaching one indicate that all individuals/groups utilize the full range of
resources, whereas smaller values indicate decreasing inter-individual overlap and hence greater individual spe-
cialization. We included all the individuals of solitary species and colonies of social species that consumed more
than two prey items, and weighted the estimations by the number of items consumed. Since none of the S. paci-
cus individuals we observed caught more than two prey items during our observation bouts, we did not calculate
niche width measures for this species.
Monte Carlo simulation was implemented in the RInSp package62. We performed a nonparametric analysis
that creates replicate, null diet matrices from the population distribution. ese serve as null distributions and
sampled with replacement to calculate p-values. e exact procedure was a follows: First, the exact number of
prey captured by each individual (ni) in the population was determined. Subsequently, the resampling randomly
reassigned each individual ni prey item drawn from the population distribution of items. is was repeated 10000
times. e resampled population served as a null model corresponding to a population composed of generalists
that sample randomly from a populations diet, and have a sample size (number of prey items) equal to those of
the observed data set.
TNW and WIC of populations of social vs. solitary species were compared using one-sided Mann-Whitney
As an additional measure of prey utilization, we assessed the proportion of variance in prey size attributable
to nests (webs) out of the total variance in captures by webs and traps combined. Variance of prey sizes was cal-
culated for each nest and corresponding traps. e total variance was calculated by summing variances of webs
and traps and the proportion of prey size variance in webs out of the total variance was then computed for each
nest. e proportions were analyzed in relation to the foraging mode (group or solitary) and site. We arcsin trans-
formed proportional variance as a response variable, and tted a generalized linear mixed model with Gaussian
error distribution, using foraging mode as an independent variable, and sampling method (web/trap) nested
within site as a random eect. ANOVA statistics in the form of likelihood-ratio chi-squares are reported.
e population niche width and indices of individual/group specialization on prey size were calculated using
the RInSp R package62.
Dietary niche breadth. Measures of individual and population level niche breadth, respectively, were derived
as follows: A likelihood measure of diet breadth, the Petraitis index63, was derived for each nest of solitary and
social species, where λi is the likelihood ratio of the observed resource use of a given nest against the population
resource distributions (total niche breadth, calculated from pooled trap and web captures), and Di is the number
of diet items recorded in the diet of that individual (i). For a complete generalist, Wi = 1, and the value decreases
with greater specialization. e likelihood of the observed diet of individual i is: λi = j(qjpij)nij, where qj is the
population proportion of the resource j, pij is the proportion of the resource j in the diet of the individual i, and
nij is the number of items for individual i and resource j. is can be used to calculate a p-value to test the signi-
cance of the diet specialization using the generalized likelihood ratio test.
We calculated the proportion of variance in prey type attributable to nests (webs) out of the total captures by
webs and traps combined, using a modication of Petraitis’ index of niche breadth. Counts of dierent prey types
were computed for each nest and trap by pooling the two samples together. Based on these combined counts we
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calculated niche breadth for a population of a given species at a given site. e same was done for web samples
only, so that the niche breadth was calculated for a population based on web captures of nests of a given species
at a given site. e proportion of niche breadth of webs out of the total captures by webs and traps combined was
calculated by dividing the niche breadth in webs by the total niche breadth.
We examined whether there was taxon-specic prey size selection by comparing prey sizes of each insect
order found in Stegodyphus webs with prey sizes in their surrounding habitat (traps). Possible size dierences
of prey captured in traps and in webs were compared using penalized quasi-likelihood generalized linear mixed
models (GLMM), with quasi-Poisson family specication to correct for overdispersion. e models included
interaction eects of the capture method and prey order.
Individual biomass intake and capture web size of social and solitary foragers. Mean individual
biomass intake rate of social and solitary species was compared by tting a generalized linear mixed model with
Gamma error distribution and log-link, using cooperative or solitary foraging mode an independent variable, and
species nested within location as a random eect. is was done with the glmer function from the lme4 package
in R. Individual web production in relation to foraging mode was analysed in a similar way. Individual biomass
intake was divided by individual web production to obtain an estimate of biomass intake relative to web produc-
tion per capita. We ln-transformed this ratio as a response variable, and tted a generalized linear mixed model
with Gaussian error distribution, using foraging mode as an independent variable, and species nested within
location as a random eect. ANOVA statistics in the form of likelihood-ratio chi-squares are presented.
Group size eects. e eect of group size on the size of capture webs was analysed by generalized linear
mixed models t by REML, using lme4 package for R. Nest size and site were specied as xed eects, and species
nested within site as a random eect. We retained nest size as the single independent variable aer backwards
elimination of non-signicant factors in the initial model. Data were log or double square-root transformed
where necessary to satisfy assumptions of normality.
e eect of group size on prey size was tested by using a generalized linear model with quasipoisson distri-
bution for the response. Group size, prey order and their interaction were set as xed, and nest ID nested within
site as a random eect.
Mean prey biomass per capita consumed per nest, and total prey number caught per hour by groups of the
three social species were compared by generalized linear mixed models tted by REML, using lme4 package for R.
Nest size and site were specied as xed eects, and species nested within site as a random eect. We dropped the
random eect by backwards elimination of non-signicant factors in the initial model. Data were log or double
square root transformed where necessary to satisfy assumptions of normality.
e eect of group size on variance in per capita biomass intake, calculated as the standard deviation of bio-
mass of all the prey captured by each social nest and specied as Gamma family distribution, was analysed with
glmm, specifying nest volume as independent variable and species nested within location as random eect.
Variance ination factors of the models were checked using the vif function in Car package for R. All statistical
analyses were performed in R version 3.3.2. e data and analyses are available from the corresponding author
upon reasonable request.
Dietary niche width. Our data show that the social species foraged on a broader range of prey sizes and
therefore had a wider dietary niche than that of solitary species (Fig.1 and TNW web in Table3). Within-
individual niche variation (i.e. the variation in prey sizes captured by each social and solitary nest) was higher in
social compared to solitary species (WIC in Table3) and the proportional variance as well was higher for social
species (Table3). e variance of foraging mode nested within site as the random eect in the model was 0.128.
e available prey collected from traps in the vicinity of each observed nest showed similar prey size spectrum
and did not dier signicantly between micro-habitats occupied by nests of social and solitary spiders (TNW
environment, Table3; and additional results in the Supplementary analyses A).
We found limited evidence of individual specialization for prey size (Rs I, Table3). Bootstrapped values of
individual specialization within dierent populations in most cases ranged from 0.7–1 (with 1 indicating a gen-
eralist diet). Among solitary species, however, two populations of S. lineatus (sites Isr1, Isr2) showed signicant
specialization as indicated by low Rs I index. In these populations, spiders caught smaller sized prey, resulting in
narrower niche width. Specializing on smaller prey resulted in reduced within-individual variance components
of the niche indices in S. lineatus (Table3). Among social species, two populations of S. dumicola also showed
some specialization (sites Nmb3 and SA1, Table3), with total niche width and within-individual variation in prey
size for both of these populations being at the lower end of the range of values recorded among the social spiders.
A separate analysis of niche width using only nests of social and solitary species that co-occurred at a site
yielded no signicant dierence between solitary and social species (Fig.S1a in Supplementary analyses A). Low
sample sizes for solitary nests (S. africanus) at SA2 and solitary nests (S. pacicus) at Ind1 (see Table1 for sample
sizes), as well as the omission in the analysis of S. dumicola at the sites in Namibia and S. lineatus in Israel, may
explain the discrepancy (further discussion in Supplementary analyses A).
Dietary niche breadth. Social foragers showed an expanded dietary niche by including a broader range
of taxonomic prey orders in their diets compared with solitary foragers (Fig.2 and Wi in Table3). Furthermore,
likelihood estimates of niche breadth (Wi web) classied social spiders as more generalist (Wi approaches 1
for a complete generalist) than solitary foragers (Table3, glmm with sociality as xed eect and species nested
within site as random eect; sociality eect χ2 = 15.94, p < 0.0001). Wi values showed large standard deviations
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(Table3), and within species there was signicant variation in dietary niche breadth among dierent locations
(random eect χ2 = 13.5, df = 1, p < 0.0001, colony nested within location random eect variance 0.04). All of
the species used a large proportion of the prey types available from the overall sample of webs and traps com-
bined. Social species tended to use a larger proportion of prey types from the total samples than the solitary ones,
but the results were not statistically signicant (proportional variance, Table3). Analyses of the taxonomic range
of available prey from traps in the vicinity of each observed nest revealed signicant dierence in the composition
of prey taxa in micro-habitats of social vs. solitary species (TableS3 in the Supplementary analyses A).
Dierences in nest height did not contribute to the dierence in abundance of captured prey types. Nests of
social species were generally higher in the vegetation than those of solitary species (Wilcoxon rank sum test with
continuity correction, W = 30524, p < 0.0001), but the abundances of dierent prey orders trapped in spider webs
were not correlated with nest height (TableS3 in the Supplementary analyses A).
Opportunistic foraging or shift in resource use. We explored whether the increase in dietary niche
of social species reects opportunistic foraging on available insect prey, or indicates a shi in resource use to
specialise on larger prey or dierent prey taxa. Similar taxonomic orders were found in habitats (available prey)
and webs (captured prey) across study sites (Fig.2). No taxonomic order was captured exclusively in the webs of
social species, therefore there was no evidence for a shi in prey type use, although the frequency of capture of
dierent orders diered among social and solitary foragers (Fig.2 and TableS2 in the Supplementary analyses A).
e most abundant prey taxa trapped in the environment were Coleoptera, Diptera and Hymenoptera. Spiders
also caught other, less abundant prey taxa such as Isoptera, Lepidoptera, and Orthoptera (Fig.2 and TableS2 in
the Supplementary analyses A), however large orthopterans in particular may have been under-sampled in the
traps due to their ability to escape.
For each prey order, we compared insect size distributions between webs and traps, and although some spe-
cies caught larger sized prey in the webs relative to the available prey sizes, we found little consistent evidence for
size-specic specialization within each prey taxon (Fig.2). is result held also for two of the three sites where
solitary and social species overlapped (Table3 and Fig.S1 in the Supplementary analyses A). Our data collectively
suggest that social species expand dietary niche by opportunistic foraging rather than by specialization on large
insects or specic prey types.
Individual biomass intake and capture web size of social and solitary foragers. We calculated
individual consumption rate (biomass intake in mg per capita over the 24 h observation period) for social and sol-
itary species (Table4). Solitary spiders obtained approximately 100 times higher per capita biomass than individ-
uals of social species (glmm solitary species coecient estimate t = 9.122, p < 0.0001; sociality eect χ2 = 83.216,
df = 1, p < 0.0001; random eect variance of species nested within site in the model = 0.017). We also calculated
Species Site
Niche Width
Niche Breadth
environment TNW
web WIC Individual
specialization (R’s I) Wi
S. africanus SA1 63.055 35.580 27.654 0.777 (n.s.) 0.758 ± 0.121 0.399 ± 0.134 0.650
SA2 47.03 13.175 12 0.911 (n.s.) 0.705 ± 0.292 0.540 ± 0.215 0.732
S. lineatus
Isr1 21.559 10.25 2.865 0.280 (0.02) 0.406 ± 0.217 0.338 ± 0. 177 0.676
Isr2 62.482 12.941 4.800 0.371 (0.002) 0.365 ± 0.171 0.454 ± 0.237 0.726
Isr3 30.126 14.777 8.662 0.586 (n.s.) 0.357 ± 0.269 0.472 ± 0. 236 0.899
Isr4 24.887 10.21 8.007 0.784 (n.s.) 0.589 ± 0.180 0.398 ± 0.156 0.655
S. pacicus Ind1 NA NA NA NA 0.345 NA NA
S. dumicola*
Nmb1 70.213 44.649 43.105 0.965 (n.s.) 0.747 ± 0.053 0.793 ± 0.147 0.933
Nmb2 81.505 79.844 62. 957 0.789 (n.s.) 0.742 ± 0.123 0.556 ± 0.213 0.737
Nmb3 44.612 37.187 25.401 0.683 (0.004) 0.617 ± 0.207 0.601 ± 0.150 0.785
SA1 12.174 31.093 20.576 0.662 (0.005) 0.607 ± 0.237 0.762 ± 0.105 0.985
SA2 49.154 53.796 49.410 0.919 (n.s.) 0.777 ± 0.141 0.538 ± 0.223 0.756
S. mimosarum*SA1 30.371 21.356 17.848 0.836 (n.s.) 0.531 ± 0.344 0.760 ± 0.194 0.789
S. sarasinorum*Ind1 47.696 48.157 35.927 0.746 (n.s.) 0.608 ± 0.246 0.438 ± 0.242 0.608
ANOVA statistics
SOLITARY n.s. W = 2
(0.002) W = 3
(0.004) χ2 = 5.74
(0.002) χ2 = 15.94
(<0.0001) n.s.
Table 3. Summary statistics of indices of foraging niche width and breadth for Stegodyphus spiders. Measures
of niche width are based on prey size: total niche width of potential prey (TNW environment), total niche
width of spider prey (TNW web), within-individual (nest) component of size specialization (WIC), individual
specialization for prey size (Rs I, bootstrapped p-value), and proportion of variance of prey size in webs out of
total variance are shown. e likelihood measure of niche breadth based on prey type (Petraitis Wi, mean ± SD)
is shown, and the proportion of variance of prey type in webs out of the total variance. Signicant dierences
between population niche estimates of social (S. dumicola, S. mimosarum and S. sarasinorum) vs. solitary
species (S. africanus, and S. lineatus) are shown in the last row (Type II Wald χ2 test of sociality in glmm; one-
sided Mann-Whitney tests where parametric tests were not applicable). Social species marked with*.
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per capita capture web production in social and solitary species, and found that social spiders on average pro-
duced in the range of 3–22 fold less capture web per individual than solitary species, suggesting that cooperative
trap building confers energetic benets in terms of reduced silk production (Table4; glmm solitary species coef-
cient estimate t = 14.11, p < 0.0001; sociality eect χ2 = 198.98, df = 1, p < 0.0001; random eect variance of
species nested within site in the model = 0.003). Combining these two metrics, the mg prey biomass obtained per
unit capture web per individual was signicantly lower in social compared with solitary species (glmm solitary
species coecient estimate t = 4.176, p < 0.0001; sociality eect χ2 = 17.436, df = 1, p < 0.0001; random eect
variance of species nested within site in the model = 0.248). erefore, despite an energetic benet in terms of
reduced individual silk production, cooperation did not overall provide higher biomass intake.
Social spiders are smaller than solitary species. When correcting for the size dierence by dividing per capita
biomass by average body size of each species (from Fig.1), prey per capita estimates were on average 15 times
higher for solitary than for social species (sociality eect χ2 = 31.291, df = 1, p < 0.0001; random eect variance
of species nested within site in the model = 7.985 107).
Group size eects. Within the social species, web area increased allometrically with group size (group size
estimated as nest volume: mixed models group size eect t = 6.338; p < 0.0001; random eect of species nested
within location χ2 = 10.5, df = 1, p = 0.001). Per capita capture web area decreased with increasing group size,
suggesting that individuals in larger groups contributed relatively less silk to capture webs (Fig.3a; mixed mod-
els group size eect t = 4.087; p = 0.0002; random eect of species nested within location χ2 = 10.5, df = 1,
p = 0.001, variance = 0.111). Prey capture rates of social species increased with web size (generalized linear
Figure 2. Percentages of the most frequent prey taxa caught in the webs and traps in each site: (a) Solitary
species and (b) social species of Stegodyphus. Black bars represent their frequency in webs, while grey bars
represent their frequencies in traps. Plus symbols within brackets (+) above the bars indicate that the respective
prey order was of signicantly larger size in the webs than in traps; minus symbol () indicates the opposite
(rates of change in size were estimated from the exponents of coecient estimates in the models; full analysis
presented in TableS1). For details on the statistical analyses see Supplementary analyses A.
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models web area eect t = 4.158, p = 0.0001; with a signicant site eect χ2 = 21.968, df = 5, p = 0.0005; a weaker
eect was found in one S. dumicola population NMB3; t = 2.288, p = 0.03). However, there was a diminishing
rate of prey capture (number of prey/hour) with increasing group size (Fig.3b; generalized linear models, nest
size eect t = 2.753, p = 0.0009). is analysis showed a signicant interaction term between group size and site
(χ2 = 24.454, df = 5, p = 0.0002), suggesting that prey capture rate decreased more slowly in nests of S. dumicola at
NMB1 and NMB3 sites, while the decrease was steeper in nests of S. mimosarum and S. dumicola at SA1 site. is
eect was due to a narrower range of nest sizes and capture webs of the latter two populations.
Group size did not have any eect on prey sizes caught by social species (F1,38 = 0.2084; p = 0.65). e eect of
prey order on prey size was signicant (F10,448 = 20.431, p < 0.001), but there was no interaction between group
size and prey order. Mean prey biomass obtained per capita, however, decreased with group size (Fig.3c; general-
ized linear model, group size eect t = 3.121, p = 0.003). ere was a signicant interaction term of group size
and location (χ2 = 28.543, df = 5, p < 0.0001), as per capita biomass decreased less with group size in S. dumicola
at NMB3 site (t = 2.875, p = 0.006). Overall, prey-capture rate and estimated individual biomass intake declined
with increasing group size (Fig.3).
Finally, we assessed the potential for reduced intra-group variance in prey consumption (Fig.3d). Variance
in per capita biomass intake decreased with increasing group size (Wald chi square test of group size eect,
χ2 = 28.49, df = 1, p < 0.0001; random eect of species nested within location variation = 0.19).
Group living animals are faced with the challenge of acquiring sucient resources to meet the requirements of
maintenance and reproduction, thus allowing the group to persist. Cooperative foraging strategies may provide
a solution by expanding the dietary niche or facilitating exploitation of resources that are unavailable to solitary
foragers. We used a comparative approach to investigate whether cooperative foraging allows social Stegodyphus
spiders to expand or change their dietary niche. We compared the dietary niche of three social and three solitary
species, based on the range of prey captured by spiders in their webs relative to prey availability in their immediate
habitat. We found that there was no dierence in the sizes of trapped insects (potential prey) in the micro-habitats
of social and solitary spiders. Yet, the social species enlarged their dietary niche relative to solitary congeners by
including large prey relative to the average size of potential prey recorded in the adjacent micro-habitat, thereby
expanding dietary niche width, and also by including a wider taxonomic range of prey and thus broadening the
dietary niche. ese results support the hypothesis that cooperative foraging facilitates capture of larger prey sizes
and a wider variety of prey types in social Stegodyphus.
As social and solitary Stegodyphus overlap partially in their distribution ranges52, theory predicts the evolution
of dierentiated foraging strategies within the ecological niche2, either by expanding the dietary niche or facilitat-
ing a shi in resource use. Our analyses show that social Stegodyphus species forage opportunistically in relation
to available prey rather than selectively specializing on larger or specic prey types (Table3). us, social species
enlarged their foraging niche relative to solitary species, but they did not shi to a new niche. Even at sites where
social and solitary species occurred in the same habitat, social Stegodyphus species were not more specialized than
solitary congeners. us, the broader foraging niche of the social species appears to overlap and include within
it that of the solitary species. Opportunistic foraging on both large and small prey in social Stegodyphus may also
reect a low relative abundance of large prey in the sub-tropical semi-arid grasslands where these species occur.
us, while they are able to handle large insects by cooperating in prey capture, spiders in colonies do not ignore
small insects trapped in their webs. ese results contrast markedly with the New World Anelosimus. Social and
solitary Anelosimus are largely segregated by elevation: social species occupy lower elevation sites and feed on
larger prey that are scarce at higher elevations20. Where social and solitary species overlap, the social Anelosimus
captured on average signicantly larger prey relative to their less social counterparts14,41.
Geographic distribution and dierent life-history patterns may explain the distinct foraging niche responses
of these two genera. Social and solitary Stegodyphus occur across Africa and Asia, with several species overlap-
ping geographically; however, there is little elevational gradient and no separation between social and solitary
species by elevation52. On a large-scale, geographic range, the social species in both genera occur in habitats
characterized by higher productivity, and consequently high insect abundance, in comparison with habitats of
Species Prey biomass
(mg cm3) (mean ± SE) Web size (cm1)
(mean ± SE) Prey biomass/Web size
(mg cm2) (mean ± SE) Group size (range)
S. africanus 21.138 ± 4.785 574.900 ± 157.757 0.118 ± 0.065 1
S. lineatus 6.597 ± 1.058 339.323 ± 41.993 0.097 ± 0.032 1
S. pacicus 21.759 ± 9.692 492.071 ± 140.372 0.044 ± 0.020 1
S. dumicola* (site NMB1-3) 0.161 ± 0.075 23.479 ± 3.190 0.010 ± 0.005 109–1450
S. dumicola* (site SA1, SA2) 0.097 ± 0.031 21.450 ± 4.574 0.006 ± 0.002 115–256
S. mimosarum*0.114 ± 0.058 26.792 ± 11.647 0.007 ± 0.003 33–54
S. sarasinorum*0.185 ± 0.037 15.539 ± 2.954 0.021 ± 0.007 64–410
Table 4. Per capita estimates of prey biomass, web size scaled to nest size (web area divided by nest volume),
and their ratio, and the range of group sizes of all species studied. Prey per capita was estimated from mean prey
biomass obtained by each nest through an observation period totalling 24 hours (for most nests, see Methods),
which was divided by group size (for details on group size estimates of social species see the Methods). Web size
for S. pacicus was taken from unpublished work based on data from the same period and area (L. Grinsted,
pers. comm.). Social species marked with*.
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their solitary congeners15,37. In Anelosimus, these habitats are tropical with relatively little climatic seasonality.
e lack of strong seasonality in insect abundance enables Anelosimus colonies to remain active year-round, thus
increasing the potential for competition with co-occurring solitary species. Elevational separation and foraging
niche specialization may enable Anelosimus to overcome competition.
By contrast, both social and solitary Stegodyphus species typically occur in subtropical, semi-arid regions
with strong precipitation seasonality, and consequently face a dry or cold season with low insect activity. During
this period, Stegodyphus colonies and solitary species undergo a form of hibernation, reducing foraging activity
and ceasing to maintain a capture web (54,59; YL personal observation). Reduced foraging activity corresponds
to the period of maternal care, when newly emerged young are present in the nest. Females in the nest feed the
young with regurgitated liquid stored in the digestive system, and the young eventually kill and consume the
adults34,54,64, emerging to renew the colony capture web when insect abundance increases in spring. Climate and
insect availability impose a strongly seasonal activity pattern and developmental synchrony on social Stegodyphus
species54,59,65. us, both intraspecic and inter-specic competition may be avoided during the time of year when
prey abundance is low. We suggest that reduced competition in Stegodyphus allows social and solitary species to
overlap in their distributional ranges. Finally, additional mechanisms (e.g., dierential susceptibility to predators,
micro-habitat preferences) might enable overlap and coexistence of social and solitary Stegodyphus species.
Cooperative foraging is expected to yield energetic benets to group members through greater prey capture
eciency and reduced variance in resource acquisition9,66. We examined whether an increase in the dietary niche
Figure 3. Web area per capita (a) prey capture rate per capita (b) prey biomass per capita (c) and per capita
variance in biomass (d) in relation to group size for each observed nest of three cooperatively foraging species:
S. dumicola (N nests = 25 in Namibia, 10 in South Africa, lled and empty circles, respectively), S. mimosarum
(N nests = 4, lled triangles) and S. sarasinorum (N nests = 14, empty squares). Prey biomass is shown in mg;
web area in cm2. We used nest volume as a proxy for group size, as they are positively correlated (see Methods
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of cooperatively foraging species also translated into higher per capita biomass intake, but found that on average,
individual biomass intake of the social species was signicantly lower than that of solitary foragers (Table4).
Individuals of social species, however, contributed substantially less to capture web production, since per capita
unit web area was smaller in social compared with solitary species. Social species thereby gain energetic benets
from cooperative foraging, as silk is costly to produce53. Combining these two metrics, we found that individual
prey intake obtained per unit capture web was still lower in social compared with solitary species. is result
also holds when correcting for size dierences between social and solitary species. erefore, although gaining
an individual energetic benet by reduced capture web production, cooperative foraging did not provide social
species with a higher individual biomass intake. We did not take into account costs of web renewal, which might
be higher for solitary species with webs lower in the vegetation and therefore more prone to disturbance. If these
costs are substantial, the balance might be shied towards relatively greater energetic benet to social species over
solitary. is remains to be investigated.
As is the case in other social animals9,67, several lines of evidence suggest that competition for resources within
social spider colonies increases with group size. We showed that prey capture rate and estimated individual
biomass intake declined with increasing group size (Fig.3). Similar patterns were reported in the social spi-
der Agelena consociata from the equatorial African rainforests31, in A. eximius13, and in Anelosimus guacamayos
(E. Yip pers. communication). In social Stegodyphus species, female body size decreased with increasing nest
size53,54,68, and fecundity declined with increasing group size48. ese observations support the hypothesis that the
increase in competition with increasing group size negatively aects colony growth. e negative eects of group
size, however, may be balanced by increased cooperation within groups. Feeding eciency was greater in groups
of the social S. dumicola than in single spiders69, and an experimental study showed that when food was limited,
cooperative foraging allowed group members to exploit the available prey more eciently70. Nevertheless, the
food-limited spiders had a lower body mass.
Colony size may have important non-trophic benets that counteract the increased cost of foraging. Bilde et al.48
found that the survival of S. dumicola nests increased with group size, and larger groups were better able to defend
the nest against predators and parasites, for example predatory ant raids71,72. In Anelosimus, solitary individuals
and small colonies suered high predation rates, in particular by ants, in the lowland tropical rain forest, a fac-
tor that could favour group living38,7375. ese ndings suggest that improved predator defence with increasing
group size is an important factor in the maintenance of group living in spiders.
Food shortage and interference competition over prey during communal feeding results in asymmetric
rewards for cooperatively foraging individuals55,76. Under a scenario of contest competition50,77, some individ-
uals may not succeed in obtaining enough resources to breed42,43,70. In social spiders, allo-maternal care and
smaller clutch sizes may have evolved to mitigate these eects of resource competition and resulting reproductive
skew64,70,78. Cooperative foraging may also reduce the variance in prey capture and individual consumption rate,
which buers the group against starvation and increases the chance of successful reproduction12,2830. Our anal-
ysis showed that variance in individual biomass intake decreased with increasing group size (Fig.3d), an eect
that could represent an important benet of group living in spiders. Although per capita rate of biomass uptake is
low in large groups, smaller groups that experience a higher variance may drop below the threshold for survival.
Indeed, small nests of social spiders frequently experience high mortality rates46,48.
In conclusion, our study provides comparative evidence for the hypothesis that cooperative foraging increases
dietary niche in social spiders through opportunistic foraging. Social Stegodyphus species expand dietary niche
width by including prey of relatively larger size, and dietary breadth by including a broader taxonomic range of
prey types in the diet, compared with solitary congeners. is implies that opportunistic foraging rather than
resource specialization facilitates co-existence with solitary congeners. e social Stegodyphus species acquire
energetic benets from reduced individual capture web production, however, per capita prey capture rate and
biomass intake decrease with increasing group size, indicating that cooperation does not oset costs of competi-
tion. Given that social groups are sedentary and dependent on the stochastic arrival of insect prey in their capture
webs, a generalist and opportunistic foraging strategy may be the only way cooperative foragers can meet ener-
getic demands. Larger groups experienced reduced variance in individual intake rate, indicating an important
benet of group living that contributes to their persistence.
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We thank all the people who helped with carrying out eld work in all of our eld sites. Collection permits for
eld work in Namibia and South Africa were issued from the Ministry of Environment and Tourism of Namibia
(permit number: 1401/2009, to Y. Lubin), Ezemvelo KZN Wildlife (Permit #OP 1896/2012), SANPARKS
(Permit #BILDT1008) and the Limpopo Department of Economic Development, Environment and Tourism
(Permit #001-CMP402-00001). We are grateful to the Members of Spiderlab Aarhus University provided helpful
comments to previous versions of this manuscript. MM was supported by a grant from the Danish Council for
Independent Research to T.B. (Grant Number 09-065911). C.H. was supported by ERC StG-2011-282163 to T.B.
Field work was carried out with nancing from Drylands Research SSA grant (Ben-Gurion University) to M.M.
and C.H., (EC contract Number: 026064) and AGSoS Mobility grant from Aarhus University to M.M. is is
publication no. 980 of the Mitrani Department of Desert Ecology, Ben-Gurion University of the Negev.
Author Contributions
All authors contributed to conceiving the experiment and developing the experimental design. M.M. and C.H.
collected the data. M.M. performed statistical analyses, and all authors contributed to writing the manuscript.
Additional Information
Supplementary information accompanies this paper at
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Supplementary resource (1)

... In Stegodyphus, three species are social while the remaining species are subsocial (Kraus & Kraus 1988). This means that studies aimed at identifying costs, benefits and adaptations to sociality and cooperative behavior tend to view the social Stegodyphus species and their subsocial sister-species as three comparable replicas for the evolutionary transition to sociality (Grinsted et al. 2014;Settepani et al. 2016Settepani et al. , 2017Majer et al. 2018;Grinsted et al. 2020). All social spider species appear to share convergently evolved traits, such as cooperative brood care, cooperative prey capture, inbreeding, and a femalebiased sex ratio (Avilés 1997;, providing an excellent opportunity to perform independent comparative studies aimed at identifying different costs, benefits and adaptations to sociality and cooperative behavior (Grinsted et al. 2014;Settepani et al. 2016Settepani et al. , 2017Majer et al. 2018;Grinsted et al. 2020). ...
... This means that studies aimed at identifying costs, benefits and adaptations to sociality and cooperative behavior tend to view the social Stegodyphus species and their subsocial sister-species as three comparable replicas for the evolutionary transition to sociality (Grinsted et al. 2014;Settepani et al. 2016Settepani et al. , 2017Majer et al. 2018;Grinsted et al. 2020). All social spider species appear to share convergently evolved traits, such as cooperative brood care, cooperative prey capture, inbreeding, and a femalebiased sex ratio (Avilés 1997;, providing an excellent opportunity to perform independent comparative studies aimed at identifying different costs, benefits and adaptations to sociality and cooperative behavior (Grinsted et al. 2014;Settepani et al. 2016Settepani et al. , 2017Majer et al. 2018;Grinsted et al. 2020). In the context of foraging, studies have found that cooperative foraging increases dietary niche width (prey size) in some species, and dietary niche breadth (taxonomic groups) in others (Nentwig 1985;Powers & Aviles 2007;Majer et al. 2018). ...
... All social spider species appear to share convergently evolved traits, such as cooperative brood care, cooperative prey capture, inbreeding, and a femalebiased sex ratio (Avilés 1997;, providing an excellent opportunity to perform independent comparative studies aimed at identifying different costs, benefits and adaptations to sociality and cooperative behavior (Grinsted et al. 2014;Settepani et al. 2016Settepani et al. , 2017Majer et al. 2018;Grinsted et al. 2020). In the context of foraging, studies have found that cooperative foraging increases dietary niche width (prey size) in some species, and dietary niche breadth (taxonomic groups) in others (Nentwig 1985;Powers & Aviles 2007;Majer et al. 2018). These studies might suggest that different species employ different cooperative foraging strategies, for example depending on prey size or prey type availability, and the risks associated with hunting. ...
Sociality in spiders has evolved independently multiple times, resulting in convergently evolved cooperative breeding and prey capture. In all social spiders, prey is captured by only a subset of group members and then shared with other, non-attacking group members. However, spiders' propensity to attack prey may differ among species due to species-specific trade-offs between risks, costs and benefits of prey capture involvement. We explored whether engagement in prey attack differs among three social Stegodyphus species, using orthopteran prey, and found substantial differences. Stegodyphus mimosarum Pavesi, 1883 had a low prey acceptance rate, was slow to attack prey, and engaged very few spiders in prey attack. In S. sarasinorum Karsch, 1892, prey acceptance was high, independently of prey size, but more spiders attacked when prey was small. While medium-sized prey had higher acceptance rate in S. dumicola Pocock, 1898, indicating a preference, the number of attackers was not affected by prey size. Our results suggest that the three species may have different cooperative prey capture strategies. In S. mimosarum and S. dumicola, whose geographical ranges overlap, these strategies may represent niche specialization, depending on whether their respective cautious and choosy approaches extend to other prey types than orthopterans, while S. sarasinorum may have a more opportunistic approach. We discuss factors that can affect social spiders' foraging strategy, such as prey availability, predation pressure, and efficiency of the communal web to ensnare prey. Future studies are required to investigate to which extent species-specific cooperative foraging strategies are shaped by ontogeny, group size, and plastic responses to environmental factors.
... Cooperative construction has evolved several times independently across life (Schmitz & Moritz 1998;Armitage 1999;Danforth 2002;Lubin & Bilde 2007;While et al. 2009). For example, phylogenetic analyses show that sociality and cooperative construction evolved independently twice among vespid wasps (Schmitz & Moritz 1998;Hines et al. 2007) and marmots (Kruckenhauser et al. 2002), three times in halictid bees (Danforth 2002), and over 20 times among spiders (Agnarsson 2002;Agnarsson et al. 2006;Salomon & Lubin 2007;Majer et al. 2018). ...
... Among arachnids, cooperative construction and sociality are rare but widely interspersed across taxa; cooperatively constructing spiders have evolved independently at least 20 times across seven disparate families (Agnarsson et al. 2006;Salomon & Lubin 2007;Majer et al. 2018). Sociality in other arachnid orders is poorly understood, but has been observed among species of mite, scorpion, and pseudoscorpion. ...
Full-text available
Animals may build nests socially to minimize the energy required for nest construction. Paratemnoides spp. pseudoscorpions evolved sociality independently from all other social groups, and colonies create silken multi-chambered nests in which they molt and raise young, analogous in form to the nests of some wasps and bees. Here we describe these nests and examine pseudoscorpion construction efficiency. Silk is generally energetically expensive and as such, we hypothesized that P. elongatus build nests of a structure that minimizes silk use, thereby maximizing nest construction efficiency. We measured the number of nest chambers, their perimeter, and their area, for 31 nests, calculated several metrics of nest architecture, and developed five alternative mathematical models describing other possible nest geometries. We found that real social pseudoscorpion nests are constructed with high efficiency, measured as wall length per internal area, approaching that of mathematical optima. We also found that these nests use less silk per capita than if the same chambers were built separately, i.e., if they were solitary. This indicates a direct benefit to group members. We compared observed nest architecture with five mathematical models of nest geometry and found that pseudoscorpion construction efficiency outperformed all non-cooperative models and rivaled that of a cooperative one approximating the honeycomb conjecture - a mathematical proof describing the most efficient way possible to divide a 2-dimensional plane. In summary, social pseudoscorpions design group nests with multiple chambers in a way that minimizes wall length per internal area and approaches the efficiency of honey-bee-like hexagon constructcion.
... These individual preferences persist over several weeks and individuals can be considered to be more specialised than the population as a whole. This pattern appears common amongst generalist species [64] and is for example seen in a number of spider species [71]. These spiders showed similar individual specialisation levels (W = 0.32 − 0.79) to A. helena. ...
... It is possible that the pack hunting of coyote allows them to employ more different predation strategies than individuals. However, this broadening of niche breadth is not seen in social spiders [71], it thus seems likely that other predator traits underlie the level of individual specialisation within a species. Levels of A. helena individual specialisation showed a wide range of values (Fig 6). ...
Full-text available
Quantifying feeding behaviour of generalist predators at the population and individual levels is crucial for understanding the structure and functioning of food webs. Individual predator/consumer feeding niches can be significantly narrower than that of the population across animal taxa. In such species, the population of a generalist predator becomes essentially an ensemble of specialist individuals and this often highly affects the dynamics of the prey-predator interactions. Currently, few experimental systems exist that are both easily technically manipulated in a lab and are reliable to accurately assess effects of individual specialisation within generalist predators. Here we argue that a freshwater predaceous snail, Anentome helena (also known as an 'assassin snail'), is a convenient and reliable experimental system to study feeding of a generalist predator on multiple food types which exhibits well-pronounced specialisation of foraging individuals. Using A. helena we experimentally test: (i) how relative prey abundances in the environment affect the feeding patterns, (ii) whether the feeding patterns are consistent over the duration of the experimental period, and (iii) compare the feeding niche breadth of individuals to that of the laboratory population. By offering four different prey snail species, at a range of relative abundances, we show that there are consistent patterns in feeding. Importantly, the consumption of each prey was independent of the relative abundance at which they were present. Individual predators showed selectivity to a particular prey, i.e. the population of assassin snails seems to be formed of individuals that specialise on different prey. Our findings would contribute to the recent revision and the ongoing debate on the classification of predator species into generalists and specialists.
... Understanding how animals interact intra-and interspecifically in their environment during foraging and feeding processes provides insights into potential dominance hierarchies and competitive dynamics [4,8,14,50,51]. As resource exploitation and other anthropogenic pressures in the deep sea (e.g., habitat loss or fishery discards) are likely to increase in the future, understanding how currently understudied deep-water ecosystems function, and how the species within them interact, should allow us to better protect these unique areas and species [52][53][54][55]. ...
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Competitive interactions come in a variety of forms and may be modulated by the size and number of individuals involved, and/or the resources available. Here, intra-and interspecific competitive behaviours for food (i.e., foraging/food search and feeding/food ingestion) were experimentally characterized and quantified in four co-existing deep-sea benthic species. Three sea stars (Ceramaster granularis, Hippasteria phrygiana, and Henricia lisa) and one gastropod (Buccinum scalariforme) from the bathyal Northwest Atlantic were investigated using video trials in darkened laboratory conditions. A range of competitive or cooperative behaviours occurred, depending on species (conspecific or heterospecific), comparative body size, and the number of individuals involved. Contrary to expectations , small individuals (or smaller species) were not always outcompeted by larger individuals (or larger species) when foraging and feeding. Moreover, faster species did not always outcompete slower ones while scavenging. Overall, this study sheds new light on scavenging strategies of co-existing deep-sea benthic species in food-limited bathyal environments, based on complex behavioural inter-and intraspecific relationships.
... This includes social spiders that use collective web 10 This is the author's accepted manuscript without copyediting, formatting, or final corrections. It will be published in its final form in an upcoming issue of The American Naturalist, published by structures to capture larger prey (Majer et al. 2018), and some pathogenic bacteria that suppress their virulence until a quorum is reached to overcome a host's immune responses (Crespi 2001). ...
From biofilms to whale pods, organisms across taxa live in groups, thereby accruing numerous diverse benefits of sociality. All social organisms, however, pay the inherent cost of increased resource competition. One expects that when resources become scarce, this cost will increase, causing group sizes to decrease. Indeed, this occurs in some species, but there are also species for which group sizes remain stable or even increase under scarcity. What accounts for these opposing responses? We present a conceptual framework, literature review, and theoretical model demonstrating that differing responses to sudden resource shifts can be explained by which sociality benefit exerts the strongest selection pressure on a particular species. We categorize resource-related benefits of sociality into six functionally distinct classes and model their effect on the survival of individuals foraging in groups under different resource conditions. We find that whether, and to what degree, the optimal group size (or correlates thereof) increases, decreases, or remains constant when resource abundance declines depends strongly on the dominant sociality mechanism. Existing data, although limited, support our model predictions. Overall, we show that across a wide diversity of taxa, differences in how group size shifts in response to resource declines can be driven by differences in the primary benefits of sociality.
... Moreover, whether these demographic characteristics are shared with other eusocial organisms that exhibit various forms of social complexity and ecological success remains unclear. Although a few comparative studies have examined whether highly social populations and species have been able to expand their ecological niches (Brooks et al. 2017;Majer et al. 2018) and geographic ranges (Lin et al. 2019) relative to nonsocial ones, the specific life history and demographic characteristics that enable eusocial species to sustain their population densities over long periods of time remain largely unexplored. ...
Although eusocial animals often achieve ecological dominance in the ecosystems where they occur, many populations are unstable, resulting in local extinction. Both patterns may be linked to the characteristic demography of eusocial species—high reproductive skew and reproductive division of labor support stable effective population sizes that make eusocial groups more competitive in some species, but also lower effective population sizes that increase susceptibility to population collapse in others. Here, we examine the relationship between demography and social organization in Synalpheus snapping shrimps, a group in which eusociality has evolved recently and repeatedly. We show using coalescent demographic modelling that eusocial species have had lower but more stable effective population sizes across 100,000 generations. Our results are consistent with the idea that stable population sizes may enable competitive dominance in eusocial shrimps, but they also suggest that recent population declines are likely caused by eusocial shrimps’ heightened sensitivity to environmental changes, perhaps as a result of their low effective population sizes and localized dispersal. Thus, although the unique life histories and demography of eusocial shrimps have likely contributed to their persistence and ecological dominance over evolutionary timescales, these social traits may also make them vulnerable to contemporary environmental change.
... Foraging efficiency is considered one of the main benefits of group living and cooperation in spiders (Majer et al., 2018;Powers & Avil es, 2007). However, prey capture also involves costs including (1) the possible risks associated with attacking prey and (2) resource expenditure in terms of venom and digestive enzymes while killing prey. ...
Task specialization is a hallmark of social success as it can minimize among-individual conflict in task participation, and thereby optimize colony productivity. Task specialization is therefore predicted to manifest as a consequence of social evolution. While age or caste in some species explains task participation, a recent hypothesis states that personality type determines differential task participation and thereby facilitates specialization. The social spider Stegodyphus sarasinorum exhibits no castes or conspicuous dominance hierarchies or age polyethism yet shows consistent among-individual differences in prey capture. We tested whether one personality trait (boldness) determines task specialization by exploring the determinants causing consistent participation in prey attack. By integrating a personality type and individual hunger, we tested two mutually exclusive hypotheses. (1) Among-individual differences in personality determine consistent differences in participation in prey capture, leading to specialization. (2) Individuals are flexible, and individual hunger state determines the propensity to capture prey. We found that hunger state was the only significant determinant of attacking prey. By reversing individual hunger state, we were able to dramatically increase or decrease individual attack propensities. Spiders exhibited consistent among-individual differences in boldness, yet this personality type did not predict foraging participation. Therefore, we found no evidence for behavioural specialization in prey capture. Our study emphasizes the importance of internal state as a mechanism underlying variable task participation, suggesting caution in attributing behavioural specialization to personality without appropriate integration of state-dependent effects.
... In group-living spiders, group size is an important determinant of fitness. Studies have reported that transitioning from solitary to certain group size would lead to broader dietary niches (Majer et al., 2018), weaker individual hunting ability (Harwood and Avilés, 2018), and the capture of larger prey (Guevara et al., 2011;Dumke et al., 2018). However, the importance of kinship in foraging has rarely been tested in group-living spiders (but see Auletta and Rayor, 2011;Yip and Rayor, 2013). ...
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Evolutionary transitions from solitary to group-living are ubiquitous in animal systems. While the fitness consequences of group size changes are often investigated, the long-standing debate on whether kinship is a prerequisite of sociality is still ongoing. In the current study, we used kleptoparasitic spiders Argyrodes miniaceus (subfamily Argyrodinae, Theridiidae) as a model system to assess the role of group size on the foraging payoffs of kin and non-kin groups. We set up laboratory-manipulated kin and non-kin foraging groups and used feeding occurrence and duration as proxies for foraging benefits and feeding latency and the number of host attacks as estimates of foraging costs. Compared to solitary individuals, feeding durations of successfully fed individuals in groups was not significantly different from that of solitary foragers in both kin and non-kin groups. The occurrences of feeding decreased significantly in group sizes two and above, in non-kin groups, and in group sizes three and above, in kin groups. In kin groups, groups size two had significantly shorter feeding latencies compared to other group sizes, even though feeding duration did not change systematically with group size. Similarly, the number of attacks from the hosts were highest in non-kin groups with more than two individuals and in kin groups with more than three individuals. The juxtaposition of kin and non-kin group showed that A. miniaceus enjoyed the highest foraging payoffs when being solitary or in small groups (group size two). However, host attacks appeared to hamper feeding occurrences in kin groups, which was not observed in non-kin groups. Our results contrast sharply with the feeding benefits of kinship recorded in kin-based groups of sub-social species present in related subfamilies in the Theridiidae.
... If ballooning spiders exhibit any form of host plant choice, the presence or absence of an existing nest could further influence this choice. Given that nests depend on the arrival of insect prey into their capture webs, and that resource competition for prey increases with nest size [55], it seems reasonable to suggest that existing spider nests should deter dispersers from establishing a new nest in its immediate vicinity to avoid competition for prey. Indeed, natal nests are often found to go extinct while the new nest established by fission on the same host plant is active, perhaps due to competition for prey (T.L.B. and T.B., personal information). ...
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An animals’ habitat defines the resources that are available for its use, such as host plants or food sources, and the use of these resources are critical for optimizing fitness. Spiders are abundant in all terrestrial habitats and are often associated with vegetation, which may provide structure for anchoring capture webs, attract insect prey, or provide protective function. Social spiders construct sedentary communal silk nests on host plants, but we know little about whether and how they make nest-site decisions. We examined host plant use in relation to host plant availability in the social spider Stegodyphus dumicola Pocock, 1898 (Eresidae) across different arid biomes in Namibia and analysed the role of host plant characteristics (height, spines, scent, sturdiness) on nest occurrence. Host plant communities and densities differed between locations. Spider nests were relatively more abundant on Acacia spp., Boscia foetida, Combretum spp., Dichrostachys cinerea, Parkinsonia africana, Tarchonanthus camphoratus, and Ziziphus mucronatus, and nests survived longer on preferred plant genera Acacia, Boscia and Combretum. Spider nests were relatively more abundant on plants higher than 2 m, and on plants with thorns and with a rigid structure. Our results suggest that spiders display differential use of host plant species, and that characteristics such as rigidity and thorns confer benefits such as protection from browsing animals.
Cooperation involving shared-resource systems is prone to ‘the tragedy of the commons’, where individuals act in their own self-interest to exploit the resource in a manner that is detrimental to the common good of all group members. Directing cooperation towards kin provides a solution to this problem, and predicts the differential performance depending on relatedness of group members. In subsocial spiders, juveniles live in transient groups that cooperate in hunting and communal feeding. Prey capture is costly in terms of risk of injury and investment of venom and digestive enzymes, and therefore presents a situation where individuals may attempt to avoid costly interactions and exploit the resource acquired by other group members. We tested the prediction that individuals differentiate participation and/or investment in cooperative prey capture and extra-oral digestion (injection of digestive enzymes into prey prior to the initiation of extraction of nutrients) in response to relatedness of group members with whom they interact, in the subsocial spider Stegodyphus africanus. The performance of groups and interactions over prey attack in groups of either related or mixed kin spiderlings were determined over a period of four weeks. We show that kin groups attack the prey significantly faster, recruit individuals to form feeding groups faster, extract prey body mass more efficiently, and experience less antagonistic interactions than groups of mixed relatedness, which ultimately translates into an elevated growth rate. These results indicate that related individuals are more willing to take risks and invest in communal digestion when foraging with kin, as predicted by inclusive fitness theory as a solution to the tragedy of the commons.
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Group living carries a price: it inherently entails increased competition for resources and reproduction, and may also be associated with mating among relatives, which carries costs of inbreeding. Nonetheless, group living and sociality is found in many animals, and understanding the direct and indirect benefits of cooperation that override the inherent costs remains a challenge in evolutionary ecology. Individuals in groups may benefit from more efficient management of energy or water reserves, for example in the form of reduced water or heat loss from groups of animals huddling, or through reduced energy demands afforded by shared participation in tasks. We investigated the putative benefits of group living in the permanently social spider Stegodyphus dumicola by comparing the effect of group size on standard metabolic rate, lipid/protein content as a body condition measure, feeding efficiency, per capita web investment and weight/water loss and survival during desiccation. Because energetic expenditure is temperature sensitive, some assays were performed under varying temperature conditions. We found that feeding efficiency increased with group size, and the rate of weight loss was higher in solitary individuals than in animals in groups of various sizes during desiccation. Interestingly, this was not translated into differences in survival or in standard metabolic rate. We did not detect any group size effects for other parameters, and group size effects did not co-vary with experimental temperature in a predictive manner. Both feeding efficiency and mass loss during desiccation are relevant ecological factors as the former results in lowered predator exposure time, and the latter benefits social spiders which occupy arid, hot environments.
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The evolution of sociality in spiders is associated with female bias, reproductive skew and an inbreeding mating system, factors that cause a reduction in effective population size and increase effects of genetic drift. These factors act to decrease the effectiveness of selection, thereby increasing the fixation probability of deleterious mutations. Comparative studies of closely related species with contrasting social traits and mating systems provide the opportunity to test consequences of low effective population size on the effectiveness of selection empirically. We used phylogenetic analyses of three inbred social spider species and seven outcrossing subsocial species of the genus Stegodyphus, and compared dN/dS ratios and codon usage bias between social Inbreeding and subsocial outcrossing mating systems to assess the effectiveness of selection. The overall results do not differ significantly between the social inbreeding and outcrossing species, but suggest a tendency for lower codon usage bias and higher dN/dS ratios in the social inbreeding species compared with their outcrossing congeners. The differences in dN/dS ratio and codon usage bias between social and subsocial species are modest but consistent with theoretical expectations of reduced effectiveness of selection in species with relatively low effective population size. The modest differences are consistent with relatively recent evolution of social mating systems. Additionally, the short terminal branches and lack of speciation of the social lineages, together with low genetic diversity lend support for the transient state of permanent sociality in spiders.
Group-living organisms offer a unique perspective on how environmental gradients influence geographic distributions, as not only the properties of individuals, but also those of their groups interact with the environment to determine a species range. In turn, the ranges of group-living organisms should provide insights on the conditions that favor group versus solitary living. Here we show that rain intensity and predation by ants, factors postulated to exclude subsocial Anelosimus spiders from the lowland tropical rainforest, are greater in this habitat than at higher elevations. We further show that experimentally excluding these factors increases the survival of subsocial Anelosimus colonies when transplanted to the lowland rainforest, but not at their native higher elevation range. While providing a rare experimental test of the simultaneous importance of abiotic and biotic gradients on species range limits, these results provide direct evidence that adverse environmental factors may prevent solitary living and require group living in certain environments.
The evolution of resource specialism and generalism has attracted widespread interest. Evolutionary drivers affecting niche differentiation and resource specialization have focused on the role of trade-offs. Here, however, we explore how the role of cooperation, mediated through altruistic behaviors, and classic resource–consumer dynamics can influence the evolution of resource utilization. Using an evolutionary invasion approach, we investigate how critical thresholds in levels of altruism are needed for resource specialization to arise and be maintained. Differences between complementary (essential) and substitutable resources affect the evolution of resource generalists. The strength of resource preferences coupled with the levels of altruism are predicted to influence the evolution of generalism. Coupling appropriate evolutionary game and ecological dynamics lead to novel expectations in the feedbacks between social behaviors and population dynamics for understanding classic ecological problems.
There are many benefits of group living, but also substantial costs, one of which is competition for resources. How scarce food resources are distributed among different members of a population or social group – whether via scramble or contest competition – can influence not only the variance in individual fitness, but also the stability and therefore survival of the group or population. Attributes of the food resources themselves, such as their size, may influence the type of intraspecific competition that occurs and therefore the intrinsic stability of a group or population. By experimentally manipulating the size of prey fed to artificial colonies of the social spider Anelosimus eximius, we investigated whether prey size could alter the degree of scramble vs. contest competition that takes place and, thus, potentially influence colony population dynamics. We found that large prey were shared more evenly than small prey and that individuals in poor condition were more likely to feed when prey were large than when prey were small. Additionally, we show that individuals participating in prey capture are also more likely to feed on the captured prey. We developed a simple mathematical model to explore the prey sizes that would be energetically worth defending, i.e. prey that are ‘economically defendable’. The model shows that neither very small prey, nor prey above a certain size is worth monopolizing, with only intermediate size prey being ‘economically defendable’. We therefore suggest the small and large prey in our experiment corresponds to our model's intermediate and large prey categories, respectively. As the size of prey captured by social spider colonies increases with colony size, our findings suggest that scramble competition may predominate in large colonies. Scramble competition, combined with the fact that prey biomass per capita declines as colonies grow beyond a certain size, would then explain why extremely large colonies of this social spider may suddenly go extinct. Our project thus illustrates the potential triple link between characteristics of the resources, individual behaviour and population dynamics, a link rarely considered in an empirical setting.
Nature conservation authorities in southern Africa burn the grassland in nature reserves to reduce the incursion of woody plants and to encourage the growth of new grass for grazing. Studies have shown significant negative effects of burning on the abundance, species diversity and community composition of smaller vertebrates and invertebrates. Social arthropods are likely to suffer from repeated burning, as destruction of the colony means a permanent loss of the entire reproductive unit. Nests of social spiders of the genus Stegodyphus (Eresidae) are a prominent feature of African savannas and their size and visibility make them potentially good indicators of ecological consequences of fire. The colonies are sedentary and their nests may persist for several years, although individuals have an annual life cycle. We explored the mortality patterns and the effects of burning on survival of colonies of two species of social spider, Stegodyphus mimosarum and S. dumicola that live sympatrically. We compared colony mortality in burned and unburned areas to determine if colony mortality was due to burning or, alternatively, to natural senescence. Analysis of five years of nest mortality data together with information on the burning regime in Spioenkop Nature Reserve, KwaZulu-Natal province (South Africa), indicates that colony extinction in S. mimosarum is independent of the burning regime, while burning is a significant cause of colony mortality in S. dumicola. The different responses of the two species are likely a result of different colony dynamics and nesting sites.