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All content in this area was uploaded by Yael Lubin
Content may be subject to copyright.
The functions of societies and the evolution
of group living: spider societies as a test case
Mary E. A. Whitehouse
1
,
2
* and Yael Lubin
1
1
Mitrani Department of Desert Ecology, Blaustein Institute for Desert Research, Ben Gurion University, Sede Boker, Israel 84990
2
Department of Zoology and Entomology, The University of Queensland, Brisbane, QLD 4072, Australia
(Received 5 September 2003 ; revised 8 November 2004; accepted 16 November 2004)
ABSTRACT
Many models have been advanced to suggest how different expressions of sociality have evolved and are main-
tained. However these models ignore the function of groups for the particular species in question. Here we
present a new perspective on sociality where the function of the group takes a central role. We argue that sociality
may have primarily a reproductive, protective, or foraging function, depending on whether it enhances the
reproductive, protective or foraging aspect of the animal’s life (sociality may serve a mixture of these functions).
Different functions can potentially cause the development of the same social behaviour. By identifying which
function influences a pa rticular social behaviour we can determine how that social behaviour will chang e with
changing conditions, and which models are most pertinent. To test our approach we examined spider sociality,
which has often been seen as the poor cousin to insect sociality. By using our approach we found that the group
characteristics of eusocial insects is largely governed by the reproductive function of their groups, while the group
characteristics of social spiders is largely governed by the foraging function of the group. This means that models
relevant to insects may not be relevant to spiders. It also explains why eusocial insects have developed a strict caste
system while spider societies are more egalitarian. We also used our approach to explain the differences between
different types of spider groups. For example, differences in the characteristics of colonial and kleptoparasitic
groups can be explained by differences in foraging methods, while differences between colonial and cooperative
spiders can be explained by the role of the reproductive function in the formation of cooperative spider groups.
Although the interactions within cooperative spider colonies are largely those of a foraging society, demographic
traits and colony dynamics are strongly influenced by the reproductive function. We argue that functional
explanations help to unders tand the social structure of spider groups and therefore the evolutionar y potential for
speciation in social spiders.
Key words: sociality, colonial spiders, social spiders, kleptoparasites, cooperation, reproductive skew theory,
maternal care, foraging society.
CONTENTS
I. Introduction ....... ......... ...... ......... ......... .......... ......... ...... ......... ......... .......... ...... ......... ......... .......... ......... ...... ... 000
II. Defining cooperation ............................................... ......... ...... .......... ......... ......... ...... ......... .......... ......... ...... 000
III. Advantages of living togeth er and characteristics of social groups ........................................ ...... ......... 000
IV. Spider groups as foraging , protective and reproductive societies .......................................... ......... ...... 000
V. Reproductive asymmetry in spiders as a consequence of competition .............................. ......... ......... 000
VI. Phylogenetic constraints and group characteristics .................................. ......... ...... ......... .......... ......... ... 000
VII. Has group function influenced speciation in group-living spiders ? ............................................... ...... 000
(1) Cooperative and colonial spiders ............... ......... ...... ......... .......... ......... ...... ......... ......... .......... ...... ...... 000
(2) Cooperative spiders and kleptoparasitic Argyrodes ......... .......... ......... ......... ...... ......... .......... ......... ...... 000
VIII. Conclusions ................. ...... ......... ......... .......... ...... ......... ......... ......... ....... ......... ......... ......... ...... ............. ...... ... 000
IX. Acknowlegements ............................................ ......... ......... ...... .......... ......... ......... ...... ......... .......... ......... ...... 000
X. References .......... ...... ......... ......... ......... ....... ......... ......... ......... ...... .......... ......... ......... ...... ......... .......... ......... ... 000
* Present address : CSIRO Cotton Research Unit, Locked Bag 59, Narrabri, NSW 2390, Australia
Biol. Rev. (2005), 80, pp. 1–15. f 2005 Cambridge Philosophical Society 1
doi:10.1017/S1464793104006694 Printed in the United Kingdom
I. INTRODUCTION
One of the most perplexing and enduring questions in
animal behaviour is how sociality evolved and what main-
tains it (e.g. Darwin, 1859; Fisher, 1930; Haldane, 1953;
Hamilton, 1964; Wilson, 1971; West Eberhard, 1975 ;
Costa & Fitzgerald, 1996). To answer this question, the costs
and benefits of grou p living and of cooperation have been
explored in different organisms (see Emlen, 1991, 1997;
Brockmann, 1997; Avile
´
s, 1997; Dugatkin, 1997 for
reviews). Studies show that many of the costs and benefits
are similar in widely differing taxa (Rypstra, 1989 ; Harvey
& Pagel, 1991 ; Crespi & Choe, 1997) and have led to the
development of general models of the evolution of sociality
which are not taxon specific. Although there are problems
when comparing across taxa (Cres pi & Yanega, 1995) the
advantage of this approach is that it highlights the common
conditions and factors that facilitate the development of
sociality regardless of phylogenetic lineage.
Models that have been used widely to explain the evol-
ution and maintenance of sociality in different species and
social systems inclu de kin selection, ecological constraints
on independent breeding and more recently, re productive
skew (Vehrencamp, 1983; Keller & Reeve, 1994 ; Reeve
& Keller, 1995 ; Bourke, 1997; Emle n, 1997 ; Johnstone &
Cant, 1999). These models have contributed greatly to our
understanding of sociality. However, all of these models
are based on cooperatively breeding species. They explain
the benefits of cooperative breeding and the asymmetr y in
reproduction within groups in terms of ecological factors
influencing reproduction, genetic relatedness and the
reproductive payoffs to dominant and subordinate indi-
viduals in the group. Some of these models may be less
appropriate for societies based largely on cooperative group
hunting, as reproduction is not the dominant function of
these groups.
It is unlikely that any one model can explain all facets and
expressions of sociality. One reason for a lack of generality is
that different models focus on different group functions.
Individuals may live in groups to provide shelter, protection
or food for offspring, or to obtain resources for themselves.
Social groups can perfo rm several of these functions simul-
taneously, but often one predominates and thus determines
the nature of group interactions at a given stage in the life
cycle (Whitehouse & Lubin, 1999). Thus the immediate
or proximate benefits of living in a cooperative society
determine the nature of interactions within the society. We
propose here that groups may be classified as primarily
reproductive, protective or foraging societies, and that this
distinction allows one to select an appropriate model, i.e. a
model with the greatest explanatory power. Here we use
spiders, a group in which sociality is re latively rare but has
evolved independently in different fa milies, to demonstrate
this approach.
Of more tha n 38 000 species of spiders, only approxia tely
60 are considered to be group living (Uetz & Hieber, 1997;
Avile
´
s, 1997; Table 1 : colonial and cooperatively social
categories). Group-living spiders have been divided into
those living colonially, on the one hand, and the cooper-
ative, social species on the other. The colonial spiders, which
form a continuum with facultatively aggregating spiders,
include species that form more-or-less permanent aggre-
gations in which each spider has its own web where it catches
its own prey. Spiders defend their individual webs against
other individuals and each spider breeds independently
(Lubin, 1974; Buskirk, 1975a; Smith, 1982; D’Andrea,
1987; Uetz & Hieber, 1997). Furthermore, the groups are
regarded as open to immigrants and relatively fluid in
composition. Colonial species have little opportunity for
direct cooperation, and their aggregations have been like-
ned to foraging flocks in birds (Rypstra, 1979, 1989 ; Rayor
& Uetz, 1990). Most of these colonial species belong to
the orb-weaving families Araneidae, Tetragnathidae and
Uloboridae. The structural difficulty of creating a commu-
nal orb web may have constrained the evolution of cooper-
ation in these groups (Lubin, 1974; Buskirk, 1975 b; Uetz &
Hieber, 1997).
The group of social (‘cooperative ’ or ‘ non-territorial
permanently social’) spiders includes 23 species in seven
different families, representing at least 12 independent
evolutionary transitions to social ity (Avile
´
s, 1997; Table 1).
Cooperative spiders live throughout their lives in communal
webs and nests. The colonies or nests may contain a few to
thousands of individuals (Avile
´
s, 1997) and there is no
evidence of individually defended territories. Individuals in
these colonies cooperate in prey capture, feed together on
prey items and may have communal brood care (Kullmann,
1972; Kraft, 1979; Buskirk, 1981). Coopera tive species are
considered to have evolved from species showing subsocial
maternal care. The extended period of maternal care of
the young in subsocial species results in groups consisting of
one mother and her offspring. Once the mother dies, the
offspring stay together for some time (Evans, 1998) but dis-
perse before m aturing to the adult stage. The transition
from subsocial to cooperative social groups involved a
transition from an outbreeding breeding system to one with
regular inbreeding (Bilde et al., in press).
Another type of group-living spider, not usually discussed
in the context of sociality (but see Whitehouse & Jackson,
1993; Agnarsson, 2002) is found in the genus Argyrodes
(Theridiidae). Most species of Argyrodes that have been stud-
ied are kleptoparasitic, i.e. the spiders congregate around
the edges of the webs of spiders of other (usually larger)
species, from which they steal food. The re are 226 known
species of Argyrodes worldwide (Platnick, 2004), and they
are regarded as the sister group to clades containing social
spiders in the fa mily Theridiidae (Agnarsson, 2002). The
group-living kleptoparasitic Argyrodes species (Table 1)
defend territories within the framework of the host web,
but they may also feed together on the same prey item
(Whitehouse, 1997). Consequently, the degree to which they
tolerate each other an d interact with each other is very
similar to the ‘social ’ characteristics of colonial spiders.
The diverse forms of sociality found in spiders provide the
opportunity to investigate key issues relevant to the proxi-
mate mechanisms maintaining sociality : (1) what are the
factors that encourage spiders to live socially, and (2) what
factors influence the character of social groups ? The aim of
this paper is not to discuss the costs and benefits of sociality
in spiders. Rather we discuss how the above questions may
2
Mary E. A. Whitehouse and Yael Lubin
Table 1. A list of species classified as colonial, facultatively aggregating, cooperative, sub-social and kleptoparasitic, and
references in which their social status is described. Spider names follow the Spider Catalog (Platnick, 2004). Species not found
in the Spider Catalog were omitted from the table. In cases where the species name has changed, we included the original
name next to the reference in which it appeared.
Family Genus and species References
Colonial spiders
Araneidae Arachnura higginsi (L. Koch) Coleman (1932)
Austracantha minax (Thorell) Lloyd & Elgar (1997)
(Gastercantha minax)
Cyclosa caroli (Hentz) Buskirk (1981)
Cyclosa trilobata (Urquhart) Main (1964)
Cyrtophora cicatrosa (Stoliczka) Lubin (1974)
Cyrtophora citricola Forskal Blanke (1972)
Cyrtophora moluccensis (Doleschall) Lubin (1974)
Cyrtophora monulfi Chrysanthus Lubin (1974)
Parawixia bistriata (Rengger) Fowler & Diehl (1978)
(Eriophora bistriata)
Metepeira atascadero Piel Uetz & Hieber (1997)
Metepeira incrassata O. P. Cambridge Rayor & Uetz (2000)
Metepeira labrynthea (Hentz) Burgess & Uetz (1982)
Metepeira spinipes O. P. Cambridge Uetz & Burgess (1979)
Dictinynidae Dictyna albopilosa Franganillo Jackson (1977)
Dictyna calcarata Banks Jackson (1977)
Mallos bryantae Gertsch Chamberlin & Gertsch (1958)
(in Buskirk, 1981)
Mallos trivittaus Banks Jackson (1977)
Diguetidae Diguetia canities (Mc Cook) Buskirk (1981)
Linyphiidae Drapetisca socialis (Sundevall) Kullmann (1961)
Oecobiidae Oecobius annulipes Lucus Buskirk (1981)
Oecobius civitas Shear Shear (1970)
Salticidae Semorina meachelyne Crane Crane (1949)
(in Buskirk, 1981)
Tetragnathidae Metabus gravidus O. P. Cambridge Buskirk (1975 b)
Leucauge venusta (Walckenaer) Valerio (1976) (in Buskirk, 1981)
Theraphosidae Heterothele darcheni (Benoit) Darchen (1967) (in Buskirk, 1981)
(Macrothele darcheni)
Uloboridae Philoponella arizonia (Gertsch) Buskirk (1981)
Philoponella congregabilis (Rainbow) Clyne (1969) (Uloborus congregabilis)
Philoponella oweni (Chamberlin) Smith (1983)
Philoponella raffrayi (Simon) Buskirk (1981) (Uloborus raffrayi)
Philoponella republicana (Simon) Lubin (1980) (Uloborus mundior in
Struhsaker 1969)
Philoponella semiplumosa (Simon) Spiller (1992 b)
Zosis geniculata (Olivier) Buskirk (1981)
Facultatively aggregating spiders
Araneidae Araneus bandelieri* (Simon) Buskirk (1981) (Epeira bandelieri)
Larinioides sclopetarius (Clerck) Burgess & Uetz (1982) (Nuctenea sclopetaria ;
Epeira sclopetaria in Buskirk, 1981)
Metepeira datona Chamberlin & Ivie Schoener & Toft (1983)
Metepeira labyrinthea (Hentz) Uetz & Hieber (1997)
Zygiella x-notata (Clerck) Spiller (1992 a)
Dictynidae Dictyna foliicola Bosenberg & Strand Honjo (1977)
Gnaphosidae Drassodes neglectus (Keyserling) Kaston (1948) (in Buskirk, 1981)
Herpyllus cockerelli (Banks) Jennings (1972)
Herpyllus ecclesiasticus Hentz Kaston (1948) (in Buskirk, 1981)
(Herpyllus vasifer)
Miturgidae Cheiracanthium erraticum (Walckenaer) Kajak & Luczak (1961) (in Buskirk, 1981)
Pholcidae Holocnemus plucheii (Scopoli) Jakob (1991)
Salticidae Phidippus audax (Hentz) Jennings (1972)
Platycryptus undatus (De Geer) Kaston (1948) (in Buskirk, 1981)
(Marpissa undata)
Tetragnathidae Nephila clavipes (Linnaeus) Uetz & Hodge (1990)
Nephila plumipes (Latreille) Mascord 1970 (Nephila ornata); Elgar 1989 (Nephila edulis)
Nephila pilipes (Fabricius) Mascord 1970 (Nephila maculata)
Function of spider societies 3
Table 1 (cont.)
Family Genus and species References
Nephila tetragnatoicles (Walckenaer) M. A. Elgar (personal communication)
Tetragnatha elongata Walckenaer Gillespie (1987)
Bassaniana versicolor (Keyserling) Kaston (1948) (in Buskirk, 1981)
Thomisidae (Coriarachne versicolor)
Philodromus spp Buskirk (1981)
Xysticus ulmi (Hahn) Kajak & Luczak (1961)
(in Buskirk, 1981)
Social (Cooperative) spiders
Agelenidae Agelena consociata Denis Roeloffs & Riechert (1988)
Agelena republicana Darchen Kullmann (1972)
Desidae Phryganoporus candidus (L. Koch) Kim (2000) (Amaurobius socialis)
(Ixeuticus candidus in Mascord, 1970)
Dictinynidae Aebutina binotata Simon Avile
´
s (1993 b)
Mallos gregalis Simon Jackson (1979)
Eresidae Stegodyphus dumicola Pocock Kraus & Kraus (1988)
Stegodyphus mimosarum Pavesi Crouch & Lubin (2001)
Stegodyphus sarasinorum Karsh Smith & Engel (1994)
Oxyopidae Tapinillus sp. Avile
´
s (1994)
Sparassidae Delena cancerides Walckenaer Rowell & Avile
´
s (1995)
Theridiidae Achaearanea disparata Denis Darchen (1968) (in Buskirk, 1981)
Achaearanea vervortii Chrysanthus Levi et al. (1982)
Achaearanea wau Levi Lubin (1995)
Anelosimus domingo Levi Avile
´
s (1997)
Anelosimus dubiosus (Keyserling) Marques et al. (1998)
Anelosimus eximius Keyserling Avile
´
s & Tufin
˜
o (1998)
Anelosimus jabaquara Levi Gonzaga & Vasconcellos-Neto (2001)
Anelosimus lorenzo Levi Avile
´
s (1997)
Anelosimus rupununi Levi Avile
´
s (1997)
Argyrodes flavipes Rainbow Whitehouse & Jackson (1998)
Theridion cf. nigroannulatum Keyserling Avile
´
s (1997)
Thomisidae Diaea ergandros Evans Evans & Goodisman (2002)
Diaea socialis Main Main (1988)
Subsocial spiders (show maternal care)
Amaurobiidae Coelotes terrestris (Wider) Gundermann et al. (1991)
Amaurobius fenestralis (Strom) Kim (2000)
Amaurobius ferox (Walckenaer) Kim (2000)
Amaurobius similis (Blackwall) Kim (2000)
Eresidae Eresus cinnaberinus (Olivier) Kullmann & Zimmermann (1975)
(Eresus niger in Buskirk, 1981)
Stegodyphus lineatus (Latreille) Kraus & Kraus (1988)
Stegodyphus pacificus Pocock Buskirk (1981)
Lycosidae All species ?
Aulonia albimana (Walckenaer) Job (1974) (in Buskirk, 1981)
Schizocosa avida (Walckenaer) Rovner et al. (1973)
Sosippus floridanus Simon Buskirk (1981)
Sosippus janus Brady Burgess & Uetz (1982)
Pisauridae Dolomedes aquaticus Goyen Forster & Forster (1973)
Dolomedes minor Koch Forster & Forster (1973)
Salticidae Pellenes nigrociliatus var bilunulata (Simon) Mikulska (1961)
Theridiidae Achaearanea riparia (Blackwall) Buskirk (1981) (Theridion saxatile)
Achaearanea tepidariorum (C. L. Koch) Clyne (1969) (Theridion tepidariorum)
Anelosimus studiosus (Hentz) Jones & Parker (2002)
Theridion impressum (L. Koch) Kullmann (1972)
Theridion pictum (Walckenaer) Nielsen (1932) (in Buskirk 1981)
Theridion sisyphium (Clerck) Kullmann (1972) (Theridion notatum in
Kullmann, 1969)
Thomisidae Diaea megagyna Evans Evans (1998)
Kleptoparasitic group-living spiders
Theridiidae Argryodes miniaceus (Doleschall) Grostal (1999)
Argyrodes alannae Grostal Grostal (1999)
Argyrodes antipodianus O. P. Cambridge Whitehouse (1986)
Argyrodes argentatus O. P. Cambridge Robinson & Robinson (1973)
4 Mary E. A. Whitehouse and Yael Lubin
be answered by examining the reproductive, foraging, and
protective functions of group living. We consider all types
of group-living spiders – colonial, social (or cooperative),
subsocial and kleptoparasitic – and refer to current models
of the evolution and maintenance of sociality in order to
compare the factors that influence the wide array of societies
seen among spiders. Finally, we suggest how the functional
approach developed below can help to understand the
mechanisms that maintain sociality.
II. DEFINING COOPERATION
Cooperation is often regarded either implicitly (Boesch,
1994a) or explicitly (Kullmann, 1972) as the basic charac-
teristic necessary for the evolution of social behaviour.
However, there is no generally accepted definition of co-
operation (Downes, 1995). Dugatkin (1997) defined it as
‘an outcome that – despite potential relative costs to the
individual – is ‘‘good ’’ in some appropriate sense for the
members of a group, and whose achievement requires
collective action ’. This definition implies the existence of
purpose o r intent and thus is goal-directed. In the literature
on sociality in spiders, the foraging behaviour of social
spiders is often described as ‘cooperative’ (Kullmann, 1972;
Buskirk, 1981; Ward & Enders, 1985 ; Downes, 1995) or
‘passively cooperative’ (Uetz, 1988b). However, it is some-
times unclear whether social spiders actually need to co-
operate in order to catch prey on the web. In some social
species, the web is so sticky that it effectively holds the prey
without the help of the spiders ( Jackson, 1979 ), while in
colonial species the prey is captured by individual spiders
(Uetz & Hieber, 1997).
In the literature on social mammals there is also discrep-
ancy in the use of the term cooperation. Boesch (1994a, b),
examining chimpanzees hunting red colobus monkeys,
implied that some form of intentional collabo ration belongs
in the definition of cooperation. He defined hunting in
the presence of a companion without collaboration as
‘simultaneous solitary hunts’. Packer and Ruttan (1988),
investigating cooperative hunting in large predatory mam-
mals, provided a simpler definition of cooperation as
‘hunting in the presence of a companion ’. This definition is
very similar to that originally proposed by Kullmann (1972)
for cooperation in spiders (‘showing collective activities ’).
Cooperation is further cha racterised by long-term interac-
tions among individuals, where individual fitness benefits
accrue to cooperating individuals (Puse y & Packer, 1997).
A simple and more general definition of cooperation may
be more useful than one that is more complicated. The
simple definition based on proximity and long-term inter-
actions of individuals removes the need to demonstrate
psychological processes such as intent or purpose in the
actions of the animals for their behaviour to be called
cooperative. Consequently, in this review, we will use Packer
and Ruttan’s (1988) definition of cooperation in hunting
groups, namely, ‘ hunting in the presence of a companion’.
We can extend the definition by analogy to other social in-
teractions, e.g. defence and reproduction. Finally, the term
‘social spider’ can mean all the social spiders (including
colonial) or just the non-territorial or ‘ cooperative ’ social
spiders. To avoid confusion we will refer to all group-living
spiders as ‘social spiders ’ and to non-territorial social spiders
as ‘ cooperative’.
III. ADVANTAGES OF LIVING TOGETHER AND
CHARACTERISTICS OF SOCIAL GROUPS
The first step in understanding sociality is to understand
why animals form groups . The fitness of an animal depends
on its ability to forage, reproduce, and defend or protect
itself from detrimental biotic and abiotic factors (e.g. pred-
ators, parasites, adverse climate). Therefore, a successful
social group (in terms of the average fitness of its grou p
members) is expected to fulfil the foraging, reproductive and
protective functions of its members more successfully than
individuals could on their own.
Table 1 (cont.)
Family Genus and species References
Argyrodes argyrodes (Walckenaer) Kullmann (1959)
Argyrodes bonadea (Karsch) Miyashita (2002)
Argyrodes caudatus (Taczanowski) Vollrath (1984)
Argyrodes elevatus Taczanowski Vollrath (1979)
Argyrodes fissifrons O. P. Cambridge Grostal (1999)
Argyrodes flavescens O. P. Cambridge Miyashita (2002)
Argyrodes fuscatus (O. P.Cambridge) Y. Henaut (unpublished data)
Argyrodes globosus Keyserling Henaut (2000)
Argyrodes kulczynskii (Roewer) M. E. A.Whitehouse (unpublished data)
Argyrodes lanyuensis Yoshida,
Tso & Severinghaus
Yoshida et al. (1998)
Argyrodes mariae Gonzalez & Carmen M. E. A. Whitehouse (personal observations)
Argyrodes rainbowi (Roewer) Grostal (1999)
Argyrodes syriacus O. P. Cambridge M. E. A. Whitehouse (personal observations)
Argyrodes ululans O. P. Cambridge Cangialosi (1990)
* Females aggregate when constructing egg sacs.
Function of spider societies 5
In societies where the foraging function dominates (for-
aging societies), group living may be seen as a means to
maximise food intake for the benefit of the individual for-
ager. Foraging societies may have simple or complex group
structure. In some caterpillar societies, individuals recruit
others to a feeding site (Costa & Pierce, 1997) although the
presence of conspecifics is not required to feed. Individuals
in some foraging societies coordinate their activities and
appear to show intentional collaboration in order to obtain
their food , as occurs for example in cooperatively hunting
raptors and social carnivores (Bednarz, 1988 ; Moehlman,
1989; Scheel & Packer, 1991 ; Heinsohn & Packer, 1995).
Where the reproductive function dominates (reproductive
societies), the primary benefits of group living are enhanced
production and survival of offsprin g. Such societies in-
clude cooperatively breeding birds and mammals (e.g. Stacey
& Koenig, 1990; Emlen, 1991), eusocial molerats (e.g.
Lovegrove, 1991) and eusocial insects (e.g. Ho
¨
lldobler &
Wilson, 1990). In reproductive societies, individuals collect
food mainly for the brood, although clearly some resources
will go to individua l maintenance. Helping to raise young
produced by a few individuals, including nest building,
feeding and protection, are the predominant forms of
cooperation in these groups. As a consequence, there is an
inherent asymmetry in the costs and benefits to the different
group members (Emlen, 1997).
In protective societies, the survival of individuals depends
on group defences. The protective funct ion of a group is the
sum of individual behaviours in the group that enhance the
survival of group members under adverse environmental
conditions or in the presence of predators or parasites.
Communal roosting in some birds (Pulliam & Caraco, 1984)
are examples of groups benefiting mainly from a protective
function. In the thornbug treehopper (Umbonia crassicornis)
maternal defence enhances the anti-predatory function of
sibling groups (Cocroft, 2002).
For sociality to be advantageous, the average fitness ben-
efit to individuals living in a group (v
G
) must be greater than
the fitness of those living solitarily (v
S
). Once v
G
<v
S
, all
things being equal, the group will disband or individuals will
disperse from the group. v
G
is dependent on the additive
and interactive effects of the foraging, reproductive, and
defence functions of the group. The advantage of group
living will hold even if the benefit of one function is actually
at the cost of another, as long as v
G
>v
S
. Therefore, we
suggest that it is necessary first to identify the main function
of group living, i.e. the function that provides the strongest
selective advantage to group living. Once this function is
identified, appropriate models can be used to make predic-
tions about the specific factors that influence group living.
The degree to which foraging, reproductive, and protec-
tive functions are advantageous to the group as a w hole, or
just to some members of the group, will affect the nature of
the society, namely whether the group is egalitarian and
lacking division of labour, or divided into a caste system with
reproductive skew, dominance hierarchy, or pecking order.
In some primate societies, for example, individuals compete
for access to food, and the competition results in a domi-
nance hierarchy (Saito, 1996 ; Hall & Fedigan, 1997). Isbell
(1991) examined the effect of resource distribution (clumped
or dispersed food sources) on the presence of female domi-
nance hierarchies in 20 species of primates. She found that
when the food was clumped, and thus more easily monop-
olised by a few individuals, societies were more structured
and had well-developed dominance hierarchies.
Dominance hierarchies are often associated with access to
mates as well as food, and therefore often serve a repro-
ductive function. In the primate dominance hierarchies
reported by Isbell (1991), changes in foraging behaviour
have a major influence on the social structure indicating that
characteristics of these groups are best understood in the
context of foraging societies. Thus different factors may
affect the same group trait (e.g. dominance hierarchy),
depending on whether it serves a foraging, protective or
reproductive function. Without taking function into con-
sideration, it is not possible to predict how a group trait will
change in response to selective forces.
The proximate function that drives group traits is not
necessarily the function that selected for group formation in
the first place. For example, groups may form to provide
protection, but if the effectiveness of the protection does not
depend on the structure of the group, then another function
(or functions) may determine group traits. In the primate
case, the animals may form groups for protection against
predators. However, the individuals spend a large pro-
portion of their time feeding in the group, and therefore the
foraging function of the group may be the dominant influ-
ence on its group traits.
Finally, the function of a society influences group struc-
ture within the co nstraints of phylogeny. For example, long-
term alliances between individuals (which can influence
social dynamics, Packer, 1977 ; Noe, 1990) are unlikely to
occur in anima ls that do not have the ability to recognise
each other individually.
IV. SPIDER GROUPS AS FORAGING,
PROTECTIVE AND REPRODUCTIVE SOCIET IES
The evolu tion of group living in spiders is thought to follow
two alternative pa thways (Shear, 1970), both of which were
originally developed from the hymenopteran model of social
evolution (Evans, 1958 ; Michener, 1958 ; Wilson, 1971).
One route is from a sub social precursor, where group living
evolved from extended maternal care. The other route is
from a parasocial precursor, where group living evolved
from aggregations around a resource. Cooperative spiders
are likely to have evolved via the sub-social route (Kraft,
1979; Uetz, 1988 b; Avile
´
s, 1997 ; Schneider, 2002) while
colonial spiders are thought to have evolved via the para-
social route (Smith, 1983 ; Uetz, 1986 ; Wickler & Seibt,
1993).
Colonial spiders form aggregations around resources,
such as sites rich in prey (Shear, 1970; Lubin, 1974; Uetz,
1988a). Colonies may persist for many gene rations (Lubin,
1974) and at any time may contain individuals of a range of
ages. Spiders in these societies show little or no m aternal
care and usually produce many young, some of which dis-
perse away from the colony to establish new colonies (Fig. 1).
6
Mary E. A. Whitehouse and Yael Lubin
The spiders usually hunt individually on their own webs (e.g.
Cyrtophora citricola, Blanke, 1972 ; Metabus gravidus, Buskirk,
1975b), but may steal food from one another (Buskirk,
1975a) or form small groups to catch large prey relative to
their body size [Parawixia (=Eriophora) bistriata, Fowler &
Diehl, 1978]. Although living in a colony may not result in
greater prey capture succes s (Gastercantha minax, Lloyd &
Elgar, 1997), colony structure can provide additional
foraging benefits by means of the ‘ricochet effect’, where
flying insects bounce off one web to land in the neighbour-
ing one (Uetz, 1989) or by enabling spiders to take advan-
tage of prey-rich web sites that are unavailable to solitary
spiders (e.g. gaps between trees, Lubin, 1974). The presence
of common web attachments saves on silk production and
lowers the foraging costs of web building and web main-
tenance (Philoponella oweni, Smith, 1982; Holocnemus plucheii,
S olita ry
adults
Solitary
adults
All
spiderlings
disperse
All adults
disperse before
mating
All adults
disperse after
mating
Adults don't
disperse
S olita ry
fe m a le c a re s
fo r yo u ng
Solitary
female cares
for young
Group-living
fe m a le s c a re
young
Spiderlings
grow
s o lita rily
Spiderlings
grow
solitarily
All juveniles
disperse
Juveniles move
between groups
Adults move
between groups
F e m a le s la y
egg sacs
Females lay
egg sacs
Juveniles
form groups
Juveniles
form groups
Adults
form groups
Adults
form groups
Juveniles
grow
s o lita rily
Juveniles
grow
solitarily
Group-living
fe m a le s c a re
only for their
own young
Group-living
females care
only for their
own young
Spiderlingsstay
as a group
Spiderlings stay
as a group
Cyrtophora moluccensis
Cyrtophora citricola
Metepeira spinipes
Metabus gravidus
Eriophora bistriata
cooperative
kleptoparasitic
colonial
Argyrodes antipodianus
Argyrodes argyrodes
Stegodyphus dumicola
Stegodyphus mimosarum
Stegodyphus sarasinorum
Achaearanea wau
Achaearanea vervortii
Agelena consociata
Agelena domingo
Anelosimus eximius
Anelosimus lorenzo
Diaea megagyna
subsocial
Usual pathways:
Possible pathways:
Group-living
females care
for each other's
young
Fig. 1. Diagrammatic representation of all possible life-history pathways for spiders. The most common pathways of some species of
subsocial, cooperative, kleptoparasitic and colonial spiders are provided as examples.
Function of spider societies 7
Jakob, 1991; Gastercantha minax, Lloyd & Elgar, 1997).
Colonial spiders are thus foraging societies whose function is
to enhance individual growth by means of group hunting.
Group characteristics are dominated by interactions during
foraging (such as during web building and prey capture;
Rayor & Uetz , 1990, 2000). Colonies provide protection
from some predators (Lubin, 1974 ; Smith, 1982; Uetz,
1988a, b) and may thus have a protective function as well.
Groups of kleptoparasitic Argyrodes spp. are also foraging
societies (Fig. 1). Many individuals may inhabit a single host
web. Although fights ensue over access to food (Whitehouse,
1997), the spiders can also feed side by side on a single, large
prey item. Spiders readily move between host webs.
Parental care is limited to defending egg sacs that are usually
produced away from the colony. Relatively few young
(10–40) are produced in each egg sac, but there is no
maternal care beyond guarding the egg sac.
In both sub-social and cooperative spiders, the repro-
ductive function may explain the initial formation of
the group. In sub-social groups females provide food and
protection for their offspring, thus from the mother’s per-
spective, the society is a reproductive society. But from the
offspring’s perspective, the group is a foraging society as the
young use both the mother and the shared web to obtai n
food for themselves. The influence of the reproductive
function on the character of sub-social groups is limited
to the period of maternal care. Similarly, in cooperative
spiders, the maternal-care period is short relative to the
lifetime of the colony. During the maternal care stage,
females may feed only their own young or each other’s
young (Whitehouse & Jackson, 1998 ; Fig. 1). Care by other
females may be limited to females with young of their own
(Schneider, 2002). In some spe cies, maternal care extends to
matriphagy when the juveniles consume their mother (e .g.
Seibt & Wickler, 1987 ; Gundermann Horel & Roland,
1991) and possibly other females as well (M. Salomon, per-
sonal communication). For the majority of the colony’s life,
however, it consists of juveniles whose interactions are
mainly in the context of foraging. As the juveniles become
more independent of the mother and begin to captur e prey
on their own, the importance of the group foraging function
in determining group behaviour will increase.
The protective function of group living may also be
important in the evolution of subsocial and cooperative
groups. One of the main arguments for the evolution of both
extended parental care and cooperative breeding is that it
enhances offspring survival and reduces the risks to juveniles
of dispersal away from the maternal nest (Lambin, Aars &
Piertney, 2001). Offspring survival may be enhanced by the
structure of the nest, which provides protection from pred-
ators. For example, in Diaea megagyna, groups of young
construct nests that, at the beginning of the season, include
living, green leaves that provi de protection from heavy
rains. As the season progresses and the leaves dry out, the
protection from the nest diminishes, so that by the time
the spiders disperse, the protection provided is minimal
(Evans, 1998).
Low survival associated with dispersal seems to be a
major problem for social spiders (Gonzaga & Vasconcellos-
Neto, 2001 ; Bodasing, Crouch & Slotow, 2002). In three
social species, Stegodyphus dumicola (Eresidae), Achaearanea wau
and Anelosimus eximius (Theridiidae), small colonies and
especially newly founded groups (incipient colonies) have
low survival (Vollrath, 1982; Christenson, 1984; Lubin,
1991; Avile
´
s & Tufin
˜
o, 1998; Henschel, 1998). Large
colonies have higher survival, owing to passive protection of
the large web and nest as well as active protection against
predators of large numbers of spiders (Uetz & Heiber, 1994 ;
Henschel, 1998). A large nest also may provide better pro-
tection from physical elements such as rain, solar radiation
and even fire (Seibt & Wickler, 1988; Lubin & Crouch,
2003). Consequently, the protective function enhances both
colony growth and the formation and maintenance of social
spider colonies.
V. REPRODUCTIVE ASYMMETRY IN SPIDERS
AS A CONS EQUENCE OF COMPETITION
We have argued above that in spite of having different life
histories, colonial spiders, kleptoparasitic spiders and coop-
erative spiders all form societies whose group structure is
determined to a large extent by the foraging function of
the group, where group living enhances individual for-
aging success. In such societies, cooperation would be a by-
product of behaviours that promote individual foraging
advantage. Understanding the foraging role of these societies
helps to explain the lack of reproductive division of labour
among group members and the apparently egalitarian
nature of the group (Buskirk, 1981 ; D’Andrea, 1987; Lubin,
1995; Ainsworth et al., 2002). In such societies, reproductive
skew, where some individuals forego reproduction in
favour of help ing others, is unlikely to develop.
Nevertheless Ebert (1998) challenged the egalitarian
nature of spider societies and suggested that a caste system
based on size, which could result in a reproductive skew ,
may occur in some spider societies. Ebert reported that in
Anelosimus eximius, smaller females with low body mass did
most of the building, cleaning and repairing of the commu-
nal web. They stayed outside the nest (where it is more
dangerous) and were more likely to attack prey than were
larger individuals. The larger females took care of egg sacs,
stayed inside the nest and fed by displacing smaller spiders
from captured prey. Rypstra (1993) also showed that in
colonies fed with a limited supply of small insects that could
be monopolised by a single spider, some individuals received
less food, grew slowly and did not reproduce. She suggested
that these spiders became, effectively, a non-reproducing
worker caste.
The apparent reproductive skew in A. eximius invites
analysis using reproductive skew theory, which was dev el-
oped to explain why some animals forego their reproductive
potential in order to help others reproduce (Vehrencamp,
1983; Keller & Reeve, 1994; Reeve & Keller, 1995). In
A. eximius it appears that the larger, reproductive females can
manipulate the behaviour of the smaller, non-reproductive
foragers. A prediction derived from skew theory would be
that the smaller spiders give up their chance to reproduce,
8
Mary E. A. Whitehouse and Yael Lubin
either due to the manipulation of the larger females, or
voluntarily, in order to stay in the group.
An alternative hypothesis is that the variation in repro-
ductive success is an outcome of group foraging dynamics.
Several authors have demonstrated co mpetition for food in
cooperative spider colonies (A. eximius : Rypstra, 1993;
Stegodyphus mimosarum: Ward & Enders, 1985; S. dumicola:
Whitehouse & Lubin, 1999; Amir, Whitehouse & Lubin,
2000). Limited prey supply (partic ularly if the prey can be
monopolised) will le ad to contest competition, whereby lar-
ger individuals displace smaller ones from the prey (Ulbrich
& Henschel, 1999). Even collective feeding on large prey
can increase the variation in body mass promoting an un-
equal distribution of food (Gonzaga & Vasconcellos-Neto,
2001) The best option for small spiders that are too small to
reproduce and can obtain more food only if they capture it
themselves, is to cooperate in foraging until they are large
enough to reproduce (Ebert, 1998). Consequently, large
reproductive females do not necessarily manipulate the
behaviour of small foragers ; rather, the behavioural differ-
ences between large and small individuals of A. eximius are
a consequence of differences in the amount of food
obtained during growth. Competition for food in this case
determines the structure of the society and apparent repro-
ductive skew is a consequence of competition for limited
resources.
VI. PHYLOGENETIC CONSTRAINTS AND
GROUP CHARACTERISTICS
Sociality evolved many times in spiders along both para-
social and subsocial pathways, and arose independently
in different families. These two pathways exert different
phylogenetic constraints on the evolution of group living.
The same environmental factors promoting group living can
give rise to different expressions of sociality. One such
phylogenetic constraint is the degree of maternal care. All
cooperative spiders, for example, evolved via the subsocial
route and have a phylogenetic history of feeding commu-
nally with closely related siblings in the maternal nest.
Group hunting and communal feeding among adults in
these societies are extensions of behaviours already present
in the juvenile stages (Burgess, 1976 ; Buskirk, 1981; Uetz,
1988b).
Colonial spiders that originated via the paras ocial route
have no phylogenetic history of feeding together as juveniles,
and individuals are territorial on their own webs. The
groups are formed in part by aggregation and therefore, the
individuals are not necessarily closely related. Colonial
groups are foraging and protective societies, but the lack
of extended maternal care in these spider families and the
defence of individual web-territories leads to a different
group character to that of the cooperative spiders. The
defence of individual webs within a colony has two conse-
quences for cooperation. First, insects landing in one web
will be defended by the web owner (but see below), and
second, communication of prey-induced web vibrations is
less effective across a colony consisting of individual webs
than across a colony that constitutes a single, large capture
web (Lubin, 1974). Int erestingly, the colonial species in
which spiders do move among different webs to capture
prey and exhibit the greatest degree of cooperation are those
whose webs are joined to form continuous sheets (e.g.
Parawixia bistriata, Fowler & Gobbi, 1988).
Once spiders form groups, whether by the subsocial or
parasocial route, their behaviour will be influenced by the
foraging and protective function of the group. Within a
group the arrival of food is unpredictable in that one spider
may capture a large prey while others may catch none.
Large prey are difficult to defend and will attract other spi-
ders that will attempt to feed on it. Furthermore, uneaten
prey in the web or nest can attract both spider predators and
scavengers such as ants (Y. Lubin, personal observation).
Thus it is advantageous to allow a neighbour to share large
prey before it attracts predators. In cooperative spiders
and in some colonial spide rs, large prey are attacked and
fed upon by more than one individua l (Fowler & Diehl,
1978; Pasquet & Krafft, 1992). The evolution of communal
feeding in group-living spiders will depend on the average
size of prey caught, the cost of sharing prey, the ease of
monopolizing prey and the risk associated with uneaten
prey on the web, as well as the phylogenetic constraints
mentioned above.
Kleptoparasitic Argyrodes species form strictly foraging
societies. Even though kleptoparasitic Argyrodes spp. produce
few offspring per clutch, females do not provide any
parental care of the young. However, parental care is not
constrained phylogenetically in Argyrodes because a non-
kleptoparasitic species, Argyrodes flavipes, does care for its
young (Whitehou se & Jackson, 1998). In ad dition, the genus
Argyrodes forms a monophyletic clade with Theridiidae gen-
era containing social species which do show parental care,
and thus the web-sharing behaviour of Argyrodes may have its
roots in maternal care (Agnarsson, 2002). Group foraging
in these spiders is possible because at least some prey are
very large relative to the kleptoparasite and may be shared
with the host spider and with other Argyrodes individuals
(Whitehouse, 1997).
Once spiders feed communally on the same prey item,
the possibilities for both cooperation and cheating are in-
creased. This is where the models of co operative hunting
may be especially relevant to social spiders. Cooperation
enables social spiders to subdue prey that is too large for a
single individual (Pasquet & Krafft, 1992). The cooperative
hunting model (Packer & Ruttan, 1988) investigates the
conditions in which cooperation, cheating or scavenging
would be expected in foraging groups. Packer and Ruttan
(1988) argue that a population of pure cooperators could be
an evolutionarily stable strategy (ESS) only if the typical prey
is small enough to be monopolised by one hunter. If large
animals are the usual prey, cheaters and scavengers could
invade, as their strategy will yield larger individual benefits.
Because social spiders often catch large prey that cannot be
monopolized by one animal, opportunities arise for cheat-
ing. Spiders could cheat by not capturing a prey item but
nevertheless feeding on it, even to the extent of allowing
others to pre-digest the prey for them (spiders digest their
prey externally before it is ingested). Cooperation can be
Function of spider societies 9
stable only if it provides higher rewards on average than
cheating. Kleptoparasitic Argyrodes spp. have taken cheating
to an extreme by using another species to capture and digest
large prey for them (Kullmann, 1959; Legendre, 1960;
Vollrath, 1976, 1984; Smith Trail, 1980; Whitehouse,
1986, 1997 ; Elgar, 1989, 1993; Cangialosi, 1990, 1991).
However, in agreement with predictions of the cooperative
hunting model, kleptoparasitic Argyrodes spp. compete for
small prey that they can monopolise individually
(Whitehouse, 1997).
VII. HAS GROUP FUNCTION INFLUENCED
SPECIATION IN GROUP-LIVING SPIDERS ?
Out of over 38 000 species, cooperative sociality has evolved
12 times, resulting in some 23 cooperative species (Table 1).
Why are there so few cooperative spider species ? To
examine this question we will first compare the group
function of cooperative and colonial spiders, then that of
cooperative and kleptoparasitic spiders.
(1) Cooperative and colonial spiders
Cooperative spiders carry as a legacy the reproductive func-
tion typical of subsocial species. The reproductive function
takes the form of maternal care, an increase in invest-
ment per individ ual young (Stearns, 1992) , which is often
associated with a decrease in the number of offspring pro-
duced (Clutton-Brock, 1991 ; Schneider, 1996). If only a few
offspring are produced then the low probability of succes sful
dispersal, which can be very risky under some conditions
(Avile
´
s & Tufin
˜
o, 1998 ; Samu, Sunderland & Szineta
´
r,
1999), may result in a delay in dispersal until spiders are
adults and already mated, if they disperse at all. A conse-
quence of this mating system is highly inbred colonies with
low genetic variation and high relatedness, as has been
recorded in several social spider species (Stegodyphus dumicola :
Wickler & Seibt, 1993 ; Johannesen et al., 2002; Stegodyphus
sarasinorum: Smith & Engel, 1994; Agelena consociata : Roeloffs
& Riechert, 1988; Achaearanea wau : Lubin & Crozier, 1985;
Anelosimus eximius : Smith, 1986; Diaea ergandros : Evans &
Goodisman, 2002) along with a female-biased sex ratio
(Avile
´
s, 1986, 1993a, 2000 ; Lubin, 1991 ; Avile
´
s, Varas &
Dyreson, 1999 ; Evans, 2000). Individual fitness increases
with colony size, at least up to some optimum (Zemel &
Lubin, 1995). Thus, larger colonies have bett er survival, are
more effective at capturing prey, and are also more likely to
produce successful dispersers (Avile
´
s & Tufin
˜
o, 1998).
Therefore both the colony size advantage and female-biased
sex ratio provide the potential for rapid increase in colony
population size.
However, colonies may increase in size too quickly so that
they overshoot their optimum size, which can lead to wild
fluctuations in colony size (Zemel & Lubin, 1995 ; A vile
´
s,
1999; Crouch & Lubin, 2001). The resulting instability
means tha t colonies are vulnerable to stochastic effects,
which could cause their extin ction. In general, colonies of
cooperative spiders are relatively short-lived (e.g., 3–6
generations for Stegodyphus dumicola, Lubin & Crouch, 200 3 ;
5–6 generations for Achaearanea wau, Y. Lubin, unpublished
data). The short expected colony lifespan and low survival of
dispersing propagules and of small groups result in selection
that should favour rapid colony growth as well as early
dispersal (in terms of the colony’s lifespan). High colony
turnover rates and the establishment of new colonies by
single or small numbers of females that mated in the par-
ental colony (by inbreeding) could lead to low genetic vari-
ation overall and strong genetic similarity across the species’
range (Avile
´
s, 1986). Thus, once the suite of demographic
characteristics typical of cooperative spider colonies has
become established, the genetic variation necessary for
speciation is lost and adaptive radiation may be impossible.
The low number of species of cooperative spiders (23) and
the evidence that cooperative sociality evolved indepen-
dently at least 12 times, support the argument that co-
operative spiders show little adaptive radiation. In the genus
Stegodyphus, for example, where there are three coopera tive
species, each species evolved sociality independently (Kraus
& Kraus, 1988 ; Wickler & Seibt, 1993). Cooperative spiders
show no evidence of radiation, even though at least one
species (Stegodyphus dumicola) contains old lineages, suggesting
that this social spid er is not recently evolved ( Johannesen
et al ., 2002).
Do colonial spiders also show little adaptive radiation?
This is less likely. Maternal care is not well developed in
colonial species, nor do colonies provide a strong repro-
ductive funct ion. Colonial spiders usually produce hundreds
of offspring and their survival is not a function of maternal
reproductive effort. Some exceptions occur: Metepeira in-
crassata, has small clutch sizes of about 20–40 young (Rayor
& Uetz, 1990), but it also has reduced dispersal rates com-
pared to sibling species (Uetz & Hieber, 1997), and no
maternal care. The protective function of the group, such as
protection from predators, is important for colonial spiders
and increases with group size (Uetz & Hieber, 1994 ; Uetz
et al., 2002). However, this benefit may be outweighed by
disadvantages such as cannibalism by larger group mem-
bers, or competition for prey within the group. Colonial
spiders of all age classes have greater flexibility in deciding
whether to stay or leave a group. Colonial groups are more
responsive to current conditions and lack the characteristic
boom-and-bust life cycle of the social spiders. Extremely
long-lived colonies of colonial spiders (e.g. Cyrtophora
moluccensis, Lubin, 1974) may be evidence of this greater
flexibility of life history in the colonial species. Social evol-
ution in the colonial spiders shows greater flexibility than in
the social spiders: in the species-rich genus Cyrtophora, for
example, the degree of coloniality ranges from obligate to
largely solitary, and some species exhibit all levels from
solitary to colonial in different habitats.
(2) Cooperative spiders and kleptoparasitic
Argyrodes
Argyrodes appear to have followed a similar phylogenetic
pathway to cooperative spiders in the development of
cooperative group foraging and brood reduction. To date,
the behav iour of approximately 30 species has been studied,
and of these 18 are kleptoparasitic. With 226 species in the
10
Mary E. A. Whitehouse and Yael Lubin
genus Argyrodes, the final count of kleptoparasitic species is
likely to be high. How has Argyrod es taken the benefits of
group foraging without paying the costs of loss of variation
for speciation?
In kleptoparasitic Argyrodes species, maternal care does not
occur within the confi nes of the group. Females move away
from the foraging group to build an egg sac web
(Whitehouse, 1986) and stay with the egg sac until the time
of hatching, at which point the female returns to the group
(M. E. A. Whitehouse, personal observations). The group of
kleptoparasitic Argyrodes spp. has only a foraging function.
The group does not need to be large to be effective at
capturing prey and exploiting a rich food source because
Argyrodes spp. have taken the concept of cheating, in a
cooperative hunting sense, to an extreme by letting another
species catch the prey for them. In addition, dispersal
costs seem low for Argyrodes spp. as all life stages regularly
move between host webs (Whitehouse & Jackson, 1993).
Consequently, constraints on the breeding system, group
structure and dispersal found in the social spiders have less
of an effect on kleptoparasitic Argyrodes spp. and ther efore
would not restr ict their potential for adaptive radiation.
VIII. CONCLUS IONS
(1) All-encomp assing models of social behaviour in
animals ignore the function or role of sociality for each
particular group.
(2) We argue that sociality may have primaril y a repro-
ductive, protective, or foraging function, depending on
whether it enhances the reproductive, protective or foraging
aspect of the animal’s life (sociality may serve a mixture of
these functions).
(3) By identifying which function infl uences a particular
social behaviour, we can determine how that behaviour
should change under different conditions and we can
identify which general model of the evolution of sociality is
most appropriate to understand that society.
(4) The foraging function of group living plays an im-
portant role in defining the characteristics of all group-living
spiders, whether co lonial, cooperative or kleptoparasitic.
(5) Kleptopa rasitic spider societies have taken the for-
aging function of group living to an extreme by using other
species to gain a foraging advantage.
(6) Cooperative spider societies differ from colonial spi-
der societies owing to the legacy of the reproductive function
of the subsocial pathway (maternal care, few young). This
yields a syndrome of traits that limit the potential for
adaptive radiation.
IX. ACKNOWLEDGEMENTS
We are very grateful to Theo Evans for extensive thoughtful
comments on the manuscript. We also thank Leticia Avile
´
s, Jutta
Schneider, Tamar Erez and Alexei Maklakov for constructive
criticisms of the manuscript. This is contribution no. 448 of the
Mitrani Department of Desert Ecology.
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Function of spider societies 15
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