Jasmin Ruch

Relatedness facilitates cooperation in the subsocial spider, Stegodyphus tentoriicola

BackgroundCooperative hunting and foraging in spiders is rare and prone to cheating such that the actions of selfish individuals negatively affect the whole group. The resulting social dilemma may be mitigated by kin selection since related individuals lose indirect fitness benefits by acting selfishly. Indeed, cooperation with genetic kin reduces the disadvantages of within-group competition in the subsocial spider Stegodyphus lineatus, supporting the hypothesis that high relatedness is an important pre-adaptation in the transition to sociality in spiders. In this study we examined the consequences of group size and relatedness on cooperative feeding in the subsocial spider S. tentoriicola, a species suggested to be at the transition to permanent sociality.ResultsWe formed groups of 3 and 6 spiders that were either siblings or non-siblings. We found that increasing group size negatively affected feeding efficiency but that these negative effects were reduced in sib-groups. Sib groups were more likely to feed cooperatively and all group members grew more homogenously than groups of unrelated spiders. The measured differences did not translate into differential growth or mortality during the experimental period of 8 weeks.ConclusionThe combination of our results with those from previous studies indicates that the conflict between individual interests and group interests may be reduced by nepotism and that the latter promote the maintenance of the social community.

Offspring dynamics affect food provisioning, growth and mortality in a brood-caring spider

In brood-caring species, family members are faced with a conflict over resource distribution. While parents are selected to adapt the amount of care according to their offsprings needs, offspring might be selected to demand more care than optimal for parents. Recent studies on birds have shown that the social network structure of offspring affects the amount of care and thus the fitness of families. Such a network structure of repeated interactions is probably influenced by within-brood relatedness. We experimentally manipulated the group composition in a brood-caring spider to test how the presence of unrelated spiderlings affects the dynamics between female and brood as well as within broods. Broods consisting of siblings grew better and had a lower mortality compared with mixed broods, no matter whether the caring female was a genetic or foster mother. Interestingly, we found that foster mothers lost weight when caring for sibling broods, whereas females caring for mixed broods gained weight. This indicates that females may be willing to share more prey when the brood contains exclusively siblings even if the entire brood is unrelated to the female. Resource distribution may thus be negotiated by offspring dynamics that could have a signalling function to females.

Families hunt more successfully : effect of group composition on hunting and communal feeding

Group activities that require an initial investment are liable to be exploited. This situation can, for example, be found in group-hunting lions, but also in subsocial and social spiders, in which several individuals capture single large prey items. Individuals could save investment by contributing less to the hunt but also during feeding by saving their external digestive enzymes. Such dynamics have been partly explored in subsocial and social web-building spiders, but are likely to differ when groups hunt in the absence of a web. Subsocial crab spiders hunt without webs and forage communally. Their nests usually comprise related individuals, although groups accept immigrating spiderlings from foreign nests, which may affect competition among group members. We aimed to test whether hunting and communal feeding differ depending on spiderling group composition and formed experimental treatments consisting of either (1) family members, (2) family members including two foreigners or (3) assorted spiderlings. Group hunting was more frequently found among family members and was positively correlated with spiderling mass increase. Family groups fed in consistently larger numbers and grew better compared with the assorted groups, but also compared with family groups including only two foreigners. The latter finding suggests that even a few immigrants may negatively affect communal activities.

Site Selection and Foraging in the Eresid Spider Stegodyphus tentoriicola

Habitat selection has profound consequences for survival and reproductive success. We investigated web relocation behaviour in relation to plant structure and body condition as well as the plasticity of foraging behaviour of the spider Stegodyphus tentoriicola. Spiders inhabiting thorny vegetation were larger, built larger webs than spiders in thornless plants and relocated their webs less frequently. Web relocation affected reproductive success through a delay of oviposition. Spiders supplemented with extra food improved body condition and built smaller webs than control spiders implying a crucial role of food in limiting fitness of S. tentoriicola. Reduced investment in webs suggests a trade-off between the benefit of more food against the cost of web-construction. We propose that S. tentoriicola exhibit a “silk and energy saving” strategy when saturated.

Multiple origins of subsociality in crab spiders (Thomisidae).

Determining factors that facilitate the transition from a solitary to a social lifestyle is a major challenge in evolutionary biology, especially in taxa that are usually aggressive towards conspecifics. Most spiders live solitarily and few species are known to be social. Nevertheless, sociality has evolved multiple times across several families and nearly all studied social lineages have originated from a periodically social (subsocial) ancestor. Group-living crab spiders (Thomisidae) are exclusively found in Australia and differ from most other social spiders because they lack a communal capture web. Three of the group-living species were placed in the genus Diaea and another in the genus Xysticus. Most Australian thomisids are, however, difficult to identify as most descriptions are old and of poor quality, and the genera Diaea and Xysticus may not correspond to monophyletic groups. Here, we clarify the phylogenetic relationships of the four group-living Australian thomisids and conclude that amongst these subsociality has evolved two to three times independently. The subsocial Xysticus bimaculatus is not closely related to any of the social Diaea and an independent origin of subsociality is likely in this case. The presented data indicates that within Diaea two origins of subsociality are possible. Our results help to understand the evolution of sociality in thomisids and support the hypothesis that permanent sociality in spiders has evolved multiple times relatively recently from subsocial ancestors.

Social network structure in group-feeding spiders

In group-living animals, an individual’s fitness is predicted by non-random interactions with other group members and social network analysis has become a powerful tool to study these interactions. We experimentally studied the social network structure in group-foraging subsocial spiders that naturally live in kin groups but accept immigrants. Spiderlings were individually marked and we observed interactions during six foraging trials in groups comprising (i) siblings, (ii) siblings with two non-siblings, and (iii) assorted spiderlings. In this foraging context, we found a higher social network structure in sibling groups compared with assorted groups or sibling groups containing two non-siblings. We asked whether non-siblings in the treatment containing mostly siblings and two immigrants are excluded or less connected, which would explain the overall reduced social network structure of the whole group. We found that non-siblings were not generally excluded but that their presence negatively affected the network structure of the whole group. The connectivity of foreign individuals in this treatment was moreover predicted by their size relative to the other group members with very small and very large spiderlings being well connected. Our findings support the idea that siblings have an advantage over unrelated individuals and that the social network structure may play a role in the evolution of social behaviour in spiders.

Cuticular Antifungals in Spiders: Density- and Condition Dependence

Animals living in groups face a high risk of disease contagion. In many arthropod species, cuticular antimicrobials constitute the first protective barrier that prevents infections. Here we report that group-living spiders produce cuticular chemicals which inhibit fungal growth. Given that cuticular antifungals may be costly to produce, we explored whether they can be modulated according to the risk of contagion (i.e. under high densities). For this purpose, we quantified cuticular antifungal activity in the subsocial crab spider Diaea ergandros in both natural nests and experimentally manipulated nests of varying density. We quantified the body-condition of spiders to test whether antifungal activity is condition dependent, as well as the effect of spider density on body-condition. We predicted cuticular antifungal activity to increase and body-condition to decrease with high spider densities, and that antifungal activity would be inversely related to body-condition. Contrary to our predictions, antifungal activity was neither density- nor condition-dependent. However, body-condition decreased with density in natural nests, but increased in experimental nests. We suggest that pathogen pressure is so important in nature that it maintains high levels of cuticular antifungal activity in spiders, impacting negatively on individual energetic condition. Future studies should identify the chemical structure of the isolated antifungal compounds in order to understand the physiological basis of a trade-off between disease prevention and energetic condition caused by group living, and its consequences in the evolution of sociality in spiders.

Re-description of Xysticus bimaculatus L. Koch, 1867 (Araneae, Thomisidae) and characterization of its subsocial lifestyle.

Abstract Spiders have become an important model to study the evolution of sociality, but a lack of their detailed natural history and taxonomy hinders broader comparative studies. Group-living crab spiders (Thomisidae) provide an excellent contrast to other social spiders since they lack a communal capture web, which was thought to be a critical factor in the evolution of sociality. Only three non-webbuilding crab-spider species are known to be subsocial or social, all of which belong to the genus Diaea Thorell, 1869. The aim of this study is to describe the social lifestyle of Xysticus bimaculatus L. Koch, 1867 for the first time. Furthermore, we present a detailed re-description of this species and discuss its taxonomic implications. Like other subsocial crab spiders, X. bimaculatus builds nests from tree leaves. Nests contain up to 38 spiders and sometimes several adult females, indicating the species may be at a transitory stage between subsociality and permanent sociality.