Yael Lubin

The Evolution of Sociality in Spiders

Publisher Summary This chapter discusses the occurrence of group living in spiders. Group living has arisen in spiders in basically two different forms. Cooperative or nonterritorial permanent‐social species are the main focus of the chapter. The form of group living in spiders has been termed “colonial” or “communalterritorial.” Colonial species have been likened to foraging flocks of birds and are described as foraging societies. Social spiders can be viewed as an ideal evolutionary experiment with independent replicates, both within and across several families. The designation of species as nonterritorial permanent‐social and nonterritorial periodic‐social applies to groups of traits that generally occur together. The chapter illustrates that, as this terminology is also somewhat unwieldy, for convenience it revert to the commonly used shorthand designations of social and subsocial, for nonterritorial permanent‐social and territorial periodic‐social, respectively.

The functions of societies and the evolution of group living: spider societies as a test case

Many models have been advanced to suggest how different expressions of sociality have evolved and are maintained. 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 animals 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 particular social behaviour we can determine how that social behaviour will change 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 understand the social structure of spider groups and therefore the evolutionary potential for speciation in social spiders.

Intersexual conflict in spiders

Reproductive strategies of males and females usually differ and, as a consequence, may impose asymmetric costs of reproduction on the two sexes and result in conflict between the sexes. In spiders, males do not provide parental care and females can store sufficient sperm for several clutches. These characteristics define the stage for a conflict between males and females that occurs mainly over the frequency of mating. Factors such as sexual size dimorphism, operational sex ratio, mating system and life-history strategies are likely to influence the degree of conflict and its outcome for different species. Male spiders may suffer large costs of mating due to mate search, assessment of female condition, courtship and cannibalistic tendencies of their mates. Courtship may reduce cannibalism, although in some cases, males benefit from being cannibalised by having an increased fertilisation rate or greater offspring fitness. In some species, limited mating capacities will increase the value of the current mating relative to future reproduction. Apart from a possible benefit of genetic variability within a clutch, females may not benefit from multiple mating and multiple mating may even be costly. Exceptions occur if additional resources are provided by males or when offspring fitness increases with additional mating. Forced copulation, prey theft, loss of the web and reduction of foraging time can all result in reduced reproductive success for females. We discuss the interacting influences of life-history traits (especially patterns of growth and maturation and sexual size dimorphism) and the reproductive strategies of males and females, using a semelparous spider, Stegodyphus lineatus (Eresidae), as an example of a species in which males and females can have strongly conflicting interests.

Woody Species as Landscape Modulators and Their Effect on Biodiversity Patterns

ABSTRACT Ecological research on organism-environment interactions has developed asymmetrically. Modulation of organisms by the environment has received much attention, while theoretical studies on the environmental impact of organisms have until recently been limited. We propose a theoretical framework for studying the environmental impacts of woody plants in order to understand their effects on biodiversity. We adopt pattern formation theory to discuss how woody plants organize ecological systems on the patch and landscape levels through patch formation, and how organism patchiness creates resource patchiness that affects biodiversity. We suggest an integrative model that links organisms as landscape modulators through resource distribution and species filtering from larger to smaller spatial scales. Our “biodiversity cycling hypothesis” states that in organism-modulated landscapes, disturbance enables the coexistence of different developmental stages of vegetation patches, thereby increasing biodiversity. This hypothesis emphasizes that species and landscape diversity vary with the development, renewal, maturation, and decay of biotically induced patches.

Arthropods from the canopy of inundated and terra firme forests near Manaus, Brazil, with critical considerations on the pyrethrum‐fogging technique*

Samples of arthropods in canopies of selected trees were collected by pyrethrum‐fogging during the early part of the 1977 dry season from three types of neotropical forest: varzea and igapo forest which are inundated seasonally by white‐water and black‐water rivers respectively, and terra firme forest which is not subject to inundation. Sampling areas were within 30 km of Manaus, Brazil. Number and biomass of arthropods sampled was highest in terra firme, intermediate in igapo and lowest in varzea. Ants were dominant on all sites and a high percentage of their species were found only in one type of forest. It is considered that many taxa are incompletely sampled with the fogging technique. Further statements on the canopy fauna of Central Amazonian forest types should be based on fogging data gathered from all seasons and compared with material sampled with other methods at the same time and locality.

THE TRANSITION TO SOCIAL INBRED MATING SYSTEMS IN SPIDERS: ROLE OF INBREEDING TOLERANCE IN A SUBSOCIAL PREDECESSOR

Abstract The social spiders are unusual among cooperatively breeding animals in being highly inbred. In contrast, most other social organisms are outbred owing to inbreeding avoidance mechanisms. The social spiders appear to originate from solitary subsocial ancestors, implying a transition from outbreeding to inbreeding mating systems. Such a transition may be constrained by inbreeding avoidance tactics or fitness loss due to inbreeding depression. We examined whether the mating system of a subsocial spider, in a genus with three social congeners, is likely to facilitate or hinder the transition to inbreeding social systems. Populations of subsocial Stegodyphus lineatus are substructured and spiders occur in patches, which may consist of kin groups. We investigated whether male mating dispersal prevents matings within kin groups in natural populations. Approximately half of the marked males that were recovered made short moves (< 5m) and mated within their natal patch. This potential for inbreeding was counterbalanced by a relatively high proportion of immigrant males. In mating experiments, we tested whether inbreeding actually results in lower offspring fitness. Two levels of inbreeding were tested: full sibling versus non-sib matings and matings of individuals within and between naturally occurring patches of spiders. Neither full siblings nor patch mates were discriminated against as mates. Sibling matings had no effect on direct fitness traits such as fecundity, hatching success, time to hatching and survival of the offspring, but negatively affected offspring growth rates and adult body size of both males and females. Neither direct nor indirect fitness measures differed significantly between within patch and between-patch pairs. We tested the relatedness between patch mates and nonpatch mates using DNA fingerprinting (TE-AFLP). Kinship explained 30% of the genetic variation among patches, confirming that patches are often composed of kin. Overall, we found limited male dispersal, lack of kin discrimination, and tolerance to low levels of inbreeding. These results suggest a history of inbreeding which may reduce the frequency of deleterious recessive alleles in the population and promote the evolution of inbreeding tolerance. It is likely that the lack of inbreeding avoidance in subsocial predecessors has facilitated the transition to regular inbreeding social systems.

Foraging decisions and behavioural flexibility in trap-building predators: a review.

Foraging theory was first developed to predict the behaviour of widely‐foraging animals that actively search for prey. Although the behaviour of sit‐and‐wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap‐building predators (mainly pit‐building antlions and web‐building spiders), a specific group of sit‐and‐wait predators that construct traps as a foraging device, with those of widely‐foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap‐building and widely‐foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap‐building predators. In each chapter of this review, we summarize the response of trap‐building predators to a different factor, while contrasting it with the equivalent response characterizing widely‐foraging predators. We provide here evidence that the behaviour of trap‐building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individuals internal state and its interactions with external environmental factors.

Habitat Selection and the Life History of a Desert Spider, Stegodyphus lineatus (Eresidae)

We examined the habitat selection of a Negev desert web-building spider Stegodyphus lineatus in two adjacent habitats differing in vegetation structure and prey availability. We investigated the choice of web-sites by spiders in terms of both vegetation structure and expected availability of prey. Spiders built their webs preferentially in shrubs with dense branch architecture and chose the parts of shrubs with greatest potential availability of prey. We conducted food-supplementation experiments to determine the effects of food supply on time to reproduction and number of offspring produced. Spiders in the habitat with greater prey availability and more abundant annual vegetation grew larger and reproduced earlier

Dispersal by Swarming in a Social Spider

Groups of Achaearanea wau (Theridiidae) disperse and found new colonies by means of synchronized emigrations of adult and subadult females. Emigrations involve the construction of silk highways from parent colonies to new web sites, synchronized migrations along the highways, and the establishment of daughter colonies. Emigrations of Achaearanea wau are similar in timing, group composition, and in some behavior components to swarming of social bees and wasps.

Survival benefits select for group living in a social spider despite reproductive costs

The evolution of cooperation requires benefits of group living to exceed costs. Hence, some components of fitness are expected to increase with increasing group size, whereas others may decrease because of competition among group members. The social spiders provide an excellent system to investigate the costs and benefits of group living: they occur in groups of various sizes and individuals are relatively short‐lived, therefore life history traits and Lifetime Reproductive Success (LRS) can be estimated as a function of group size. Sociality in spiders has originated repeatedly in phylogenetically distant families and appears to be accompanied by a transition to a system of continuous intra‐colony mating and extreme inbreeding. The benefits of group living in such systems should therefore be substantial. We investigated the effect of group size on fitness components of reproduction and survival in the social spider Stegodyphus dumicola in two populations in Namibia. In both populations, the major benefit of group living was improved survival of colonies and late‐instar juveniles with increasing colony size. By contrast, female fecundity, female body size and early juvenile survival decreased with increasing group size. Mean individual fitness, estimated as LRS and calculated from five components of reproduction and survival, was maximized for intermediate‐ to large‐sized colonies. Group living in these spiders thus entails a net reproductive cost, presumably because of an increase in intra‐colony competition with group size. This cost is traded off against survival benefits at the colony level, which appear to be the major factor favouring group living. In the field, many colonies occur at smaller size than expected from the fitness curve, suggesting ecological or life history constraints on colony persistence which results in a transient population of relatively small colonies.