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

Abstract

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.

Full text

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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 speciation1–4. 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 group12–15.
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 niche17–20. 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 diet22–24. is could lead to an expansion in resource use or a shi
in diet to exploit a dierent dietary niche7,25–27. Group foraging can also provide benets by reducing variance in
prey capture rate and individual consumption rate, which buers the group against starvation28–30. 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: lubin@bgu.ac.il)
Received: 30 May 2017
Accepted: 6 July 2018
Published: xx xx xxxx
OPEN
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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 species32–36. 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 Anelosimus37–39. 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 Anelosimus20–22,
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 Roughgarden’s 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.
sarasinorum51
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
parentheses.
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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: =×
Vlwh
1
3
) 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,
R² = 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
collection.
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
1
2
.
Data analysis. Dietary niche width. e range of insect prey sizes utilized (niche width) was estimated
as the total niche width component of Roughgarden’s 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
TNW = WIC + BIC.
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 population’s 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
tests.
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.
Results
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 (R’s 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 R’s 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
Proportional
variance
Niche Breadth
TNW
environment TNW
web WIC Individual
specialization (R’s I) Wi
Proportional
variance
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
SOCIAL VS.
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 (R’s 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 ∗ 10−7).
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).
Discussion
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 ∗ cm−3) (mean ± SE) Web size (cm−1)
(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
section).
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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 living38,73–75. 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 reproduction12,28–30. 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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Acknowledgements
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 https://doi.org/10.1038/s41598-018-30199-x.
Competing Interests: e authors declare no competing interests.
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