Salomon, Maxence F.

A major goal of evolutionary research is to elucidate the processes involved in the evolution of group versus solitary living, by examining the selective forces driving or constraining a particular type of social system. Species with intermediate group living tendencies are particularly interesting because they offer insight into evolutionary transitions and, in particular, how different selective environments may modulate group living behaviours. This dissertation explores the different factors that shape the social strategies of the western black widow spider, Latrodectus hesperus, a facultatively group- living spider. Over a period of four years, I characterized the social structure of coastal British Columbia populations of L. hesperus, and show that individuals live either solitarily or in groups depending on the time of year, their reproductive status, and age. I then used an experimental approach to investigate the dynamics and adaptive value of facultative group-living behaviours, by testing different hypotheses about the decisions involved in social interactions, web building, microhabitat settlement, movement, and foraging. Several factors were manipulated and shown to influence the patterns of group living in L. hesperus, including individual nutritional state, prey availability, group size, population density, and neighbour proximity. Spiders adjusted their group living behaviours according to changes in these factors, and responded strategically to the presence and proximity of conspecifics. Based on the results of these experiments, I developed a model of group living in spiders that specifically considers the dynamic and strategic nature of social interactions in the context of frequency-dependent selection. The research presented in this thesis furthers our understanding of the evolution of social behaviour by providing new evidence on the mechanisms that promote and regulate facultative group-living behaviours.

Evolutionary and ecological transitions from carnivorous to omnivorous feeding may be constrained by the ability of the animal to cope with disparate types of foods, even if preadaptations for such behaviour exist. The omnivorous true bug, Dicyphus hesperus (Hemiptera: Miridae) requires both animals (small, soft-bodied insects) and plants in its diet and obtains the majority of its dietary and metabolic water from plant feeding. Serrations on the lateral margins of the mandibular stylets wear with age, and this wear is exacerbated when the insects feed on plants compared to those provided free water and no plants. D. hesperus that feed on plants attack fewer prey but consumed similar amounts of prey tissue compared to individuals that were provided free water. Although others have shown mandible wear for plant-chewing animals we show for the first time that plant feeding can impose similar wear on plant-piercing animals as well.

It is well recognized that feeding rate has a major influence on the amount of movement between microhabitats for many animals. However, the role of other extrinsic and intrinsic factors, and particularly how these factors may interact, is not well understood. This three-part study examines the movement decisions of a web-building spider, Latrodectus hesperus, by assessing microhabitat tenacity in established spiders and by testing how the presence of conspecific neighbours and the combined influence of individual feeding state (determined by prior feeding experience) and neighbour presence influence microhabitat residence time in unestablished spiders. The results show that naturally established spiders did not leave their microhabitats readily, emphasizing the importance of choosing a profitable location. Unestablished spiders stayed longer in microhabitats occupied by conspecifics than in unoccupied ones, and there was practically no cannibalism even though neighbours shared webs. Furthermore, feeding state and neighbour presence showed an interactive effect on microhabitat residence time. When spiders were housed alone, microhabitat residence time increased with feeding state. However, in the presence of conspecifics, spiders had a low propensity to move, regardless of feeding state. Together, these results demonstrate the combined importance of grouping dynamics and feeding state in shaping movement decisions. [ABSTRACT FROM AUTHOR]

Animal social systems come in a wide range of forms characterized by different types of group-living relationships. Species that express facultative group-living behaviours, where individuals only associate under certain conditions or at certain times, are especially interesting for studying social evolution. We investigated the social structure of the western black widow spider (Latrodectus hesperus Chamberlin and Ivie, 1935) over 5 years in a coastal British Columbia habitat, and present the first comprehensive evidence of facultative group living in the genus Latrodectus Walckenaer, 1805. Latrodectus hesperus have a flexible social structure that varies seasonally, wherein individuals (mostly females) either form groups or live solitarily. In the fall and early winter, females spontaneously form groups of 2-8 individuals, but live alone during the oviposition season in the spring and summer. When living in groups spiders share large webs and are tolerant of each other, although they appear to forage individually. We also report on the relationships between different ecological factors and the social structure of L. hesperus, including the interaction of these spiders with two co-occurring species of introduced spiders (Tegenaria agrestis (Walckenaer, 1802) and Tegenaria duellica Simon, 1875). Our findings suggest that L. hesperus is a good model system with which to explore the mechanisms involved in the evolution of sociality. [ABSTRACT FROM AUTHOR]

The life history of the funnel weaver Eratigena agrestis (Walckenaer) (Araneae: Agelenidae) is not well studied despite its widespread occurrence in Europe and its establishment and spread in the Pacific Northwest of North America since its introduction in the early 20th century. We report phenology and life-history data for E. agrestis and another co-occurring funnel weaver, Eratigena atrica (Koch), in two study sites in British Columbia, Canada. The most notable difference in phenology between the two Eratigena species was the timing of emergence: E. atrica spiderlings emerge in the fall whereas E. agrestis spiderlings emerge in the spring. Surprisingly, the contrasting densities of E. atrica in the two study sites and the presence of the western black widow spider, Latrodectus hesperus Chamberlin and Ivie (Araneae: Theridiidae), in one study site had little effect on the life history of E. agrestis. This unexpected finding may be explained by (i) low overall competition pressure in the study habitats, (ii) conspecifics and heterospecifics exerting equivalent competition or predation pressures; and/or (iii) aggregations of heterospecifics providing benefits that offset costs associated with any competition.

Web-building spiders are territorial animals whose webs serve several purposes including foraging, reproduction and protection. Two factors are predicted to be of primary importance in determining a spider's web-building strategy: (1) current nutritional state (a function of prior feeding rate), which determines the net value of a web, and (2) the presence of co-occurring spiders with whom it may have to interact and partition resources. I conducted a laboratory experiment to test whether western black widow spiders, Latrodectus hesperus, express state-dependent web-building strategies that vary with the presence of conspecific or heterospecific (Tegenaria agrestis and T. duellica) spiders in a microhabitat. I maintained L. hesperus females on two different diets (high- versus low-food) and tested their web-building behaviour in the presence or absence of neighbours. When sharing a microhabitat with con- or heterospecific spiders, L. hesperus increased their web-building investment by producing higher-density webs containing more silk and by initiating web-building sooner. Web building was further influenced by nutritional state, such that well-fed spiders produced more silk and built denser webs than their poorly fed counterparts. Furthermore, microhabitat occupancy level and nutritional state showed a combined effect on the different components of web-building behaviour in L. hesperus. I discuss how this behavioural plasticity might optimize microhabitat settlement investments in the context of constraints on web-building activities imposed by nutritional state and cohabitation.

Gammadelta T cells are implicated to play crucial roles during early immune responses to pathogens. A subset of human gammadelta T cells carrying the Vgamma9Vdelta2 TCR recognize small, phosphorylated nonpeptidic Ags. However, the precise role of these cells and the ligands recognized in human immune responses against pathogens remains unclear because of the lack of suitable animal models. We have analyzed the reactivity of spleen cells of the New World monkey Aotus nancymaae against isopentenyl pyrophosphate (IPP), a phosphorylated microbial metabolite selectively activating Vgamma9Vdelta2 T cells. Spleen cells were stimulated by IPP and the expanding cell population expressed the Vgamma9 TCR. TRGV-J and TRDV-D-J rearrangements expressed by IPP-stimulated cells of Aotus were analyzed by RT-PCR and DNA sequencing. The TRGV-J and TRDV-D-J rearrangements expressed by IPP-stimulated Aotus and human gammadelta T cells were similar with respect to 1) TCR gene segment usage, 2) a high degree of germline sequence homology of the TCR gene segments used, and 3) the diversity of the CDR3 regions. Phylogenetic analysis of human, Pan troglodytes, and A. nancymaae TRGV gene segments showed that the interspecies differences are smaller than the intraspecies differences with TRGV9 gene segments located on a distinct clade of the phylogenetic tree. The structural and functional conservation of Vgamma9Vdelta2 T cells in A. nancymaae and humans implicates a functionally important and evolutionary conserved mechanism of recognition of phosphorylated microbial metabolites.