OUP accepted manuscript

Naturwissenschaften, 2012

Cooperation and group living often evolves through kin selection. However, associations between unrelated organisms, such as different species, can evolve if both parties benefit from the interaction. Group living is rare in spiders, but occurs in cooperative, permanently social spiders, as well as in territorial, colonial spiders. Mixed species spider colonies, involving closely related species, have rarely been documented. We examined social interactions in newly discovered mixed-species colonies of theridiid spiders on Bali, Indonesia. Our aim was to test the degree of intra-and interspecific tolerance, aggression and cooperation through behavioural experiments and examine the potential for adoption of foreign brood. Morphological and genetic analyses confirmed that colonies consisted of two related species Chikunia nigra (O.P. Cambridge, 1880) new combination (previously Chrysso nigra) and a yet undescribed Chikunia sp. Females defended territories and did not engage in cooperative prey capture, but interestingly, both species seemed to provide extended maternal care of young and indiscriminate care for foreign brood. Future studies may reveal whether these species adopt only intraspecific young, or also inter-specifically. We classify both Chikunia species subsocial and intra-and interspecifically colonial, and discuss the evolutionary significance of a system where one or both species may potentially benefit from mutual tolerance and brood adoption.

Biotropica, 2006

Permanent cooperative sociality is rare in spiders. Here we describe sociality in a cobweb (Theridiidae) genus with no other known social species. In five areas of eastern Ecuador, we found nests of Theridion nigroannulatum containing from a single to several thousand adults living together in a communal web. Spiders cooperated in prey capture and shared their food. Subadult to young adult sex ratios were highly female biased, suggesting a strongly subdivided population structure as in other permanent-social spiders. Unusual aspects of T. nigroannulatum's biology include the existence of adult females of two discrete sizes, an extreme boom and bust pattern of colony growth, and the presence, within larger colonies, of populations of a predatory spider (Faiditus spp., Argyrodinae) that may play the role of a colony-level parasite. The existence of females of two sizes may be the result of alternative female reproductive strategies, intracolony competition for resources, or an incipient caste system in this species. A redescription and phylogenetic analysis corroborate the placement of this species in Theridion, indicating that sociality has evolved independently in at least three theridiid genera.

Advances in the Study of Behavior, Vol 37, 2007

Proceedings of the Royal Society B: Biological Sciences, 2007

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 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 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.

Evolution, 2006

Evolutionary ''dead ends'' result from traits that are selectively advantageous in the short term but ultimately result in lowered diversification rates of lineages. In spiders, 23 species scattered across eight families share a social system in which individuals live in colonies and cooperate in nest maintenance, prey capture, and brood care. Most of these species are inbred and have highly female-biased sex ratios. Here we show that in Theridiidae this social system originated eight to nine times independently among 11 to 12 species for a remarkable 18 to 19 origins across spiders. In Theridiidae, the origins cluster significantly in one clade marked by a possible preadaptation: extended maternal care. In most derivations, sociality is limited to isolated species: social species are sister to social species only thrice. To examine whether sociality in spiders represents an evolutionary dead end, we develop a test that compares the observed phylogenetic isolation of social species to the simulated evolution of social and non-social clades under equal diversification rates, and find that sociality in Theridiidae is significantly isolated. Because social clades are not in general smaller than their nonsocial sister clades, the ''spindly'' phylogenetic pattern-many tiny replicate social clades-may be explained by extinction rapid enough that a nonsocial sister group does not have time to diversify while the social lineage remains extant. In this case, this repeated origin and extinction of sociality suggests a conflict between the short-term benefits and long-term costs of inbred sociality. Although benefits of group living may initially outweigh costs of inbreeding (hence the replicate origins), in the long run the subdivision of the populations in relatively small and highly inbred colony lineages may result in higher extinction, thus an evolutionary dead end.

Journal of Ethology, 2008

A first case of subsociality is reported for the genus Latrodectus. Individuals were found sharing the same web and feeding together. In captivity they showed mutual tolerance and communal feeding. This finding is remarkable for two reasons. First, widow spiders, even compared with other spiders, are famously aggressive and cannibalistic so that social behavior in the genus was unexpected. Second, the genus nests outside the ''Anelosimus + lost colulus'' clade where all the other social theridiids are found.

Behavioral Ecology, 2012

Animals living in cooperative groups experience fundamentally different environments than their nonsocial relatives, potentially changing the strength of natural selection on some aspects of their behavior. Using a comparative approach, we examined a potential example of this phenomenon: an association between reduced levels of maternal care behavior and sociality in cobweb spiders. We compared 6 different measures of maternal care behavior between species from 2 independently derived social clades and subsocial species from sister clades. In natural nests, we measured the mean distance between egg sacs and the nearest female and the proportion of egg sacs being attended. In the lab, we measured a female's willingness to accept an egg sac, abandon her egg sac when disturbed, repair a damaged egg sac, and the speed at which a female reclaimed her egg sac when separated from it. Social species from both social clades scored significantly lower than subsocial species from sister clades on 6 and 4 of 6 of these assays of maternal care, respectively. We discuss alternative explanations of this pattern, including the potential role of relaxed natural selection in a social environment in permitting the evolution of a novel ''low-parenting'' phenotype.

Biological Journal of the Linnean Society, 2000

The concept of colony-level life history evolution is introduced for the cooperative spiders by describing the life cycle and demography of Aebutina binotata (Araneae: Dictynidae), a species living in groups containing up to several dozen adult females plus their offspring. In a life cycle remarkably similar to that of army ants, the colonies of A. binotata were found to reproduce by fission and to alternate nomadic and sedentary phases in tight association with their internal demography. Colonies of other cooperative spiders, on the other hand, remain stationary as they grow for a number of generations before producing propagules that are relatively small subsets of the maternal colony. It is suggested that A. binotata's peculiar life cycle may have unfolded as a consequence of the two-dimensional architecture of its nests. Expanding two-dimensional nests may fragment more easily than the three-dimensional nests characteristic of other species. A long distance group migration or nomadic phase, described here for the first time for a spider, may have followed as a mechanism to cope with potential disadvantages of fission while selecting for strict synchronization of individual life cycle stages within the nests. It is shown, however, that, as in other cooperative spiders, A. binotata's sex ratio is also highly female biased. The theoretical implications of biased sex ratios in a species with fissioning colonies are briefly discussed.

Behavioral Ecology and Sociobiology, 2013

Alloparental care-the care of other's offspringis a key aspect of sociality in many groups of animals. Understanding how this complex behavior arises requires identifying both the selective forces that may favor it, as well as characteristics of particular lineages that facilitate or hinder its evolution. One potential hindrance is the existence of discrimination against foreign offspring, an obstacle that would need to be overcome in order for alloparental care to evolve. In this study, we explored whether offspring discrimination may have constrained the evolution of alloparental care in social spiders in the genus Anelosimus. Social spiders are known for their cooperative behaviors, which include alloparental care. After quantitatively assessing the extent of alloparenting in the care of egg sacs in natural nests of these spiders, we investigated whether discrimination against foreign egg sacs existed in ancestral pre-social species in the genus. We did so by testing for discrimination between a female's own and foreign egg sacs in three subsocial sister taxa of each social species investigated. We found no detectable evidence of discrimination in the care of egg sacs by female Anelosimus, regardless of level of sociality. We used these data, along with those from previous studies, to infer that a lack of discrimination is likely the ancestral state in the genus Anelosimus. This supports the idea that offspring discrimination was not a constraint on the evolution of alloparental care in social Anelosimus species. We discuss the evolutionary implications of this finding, and suggest that lack of offspring discrimination may have eased the transition from solitary to cooperative breeding.

We report for the first time the existence of eggsac recognition and maternal care in Loxosceles gaucho. Spiders confronted simultaneously with their own and foreign eggsacs stay closer to their own eggsacs. This is unexpected since eggsac recognition should evolve among species with clumped distributions, high maternal investments and few breeding opportunities, features not present in this species. Despite this recognition, spiders with a single eggsac make no distinction between their own and foreign eggsacs: they adopt eggsacs from sympatric, conspecific females, and take care of them as their own. It seems that there is a readiness to perform maternal care that overrules the recognition system. We describe oviposition behavior and compare it with other descriptions in the literature. Seven behavioral characters related to eggsac building and/or guarding are mapped onto available phylogenies. Maternal care behaviors are quite conservative among spiders, useful for the grouping not only of families, but also of higher order ranks.