Thomas C. Jones

Behavioural syndromes and their fitness consequences in a socially polymorphic spider, Anelosimus studiosus

The temperate comb-footed spider, Anelosimus studiosus, shows a social behaviour polymorphism: individuals may defend asocial nests against intrusion by conspecifics or cooperate with them in multifemale nests. A suite of behavioural traits, including response to predators and prey, degree of superfluous killing, exploratory behaviour and general level of activity, was examined in laboratory trials to: (1) establish the extent to which these traits are correlated with social phenotype and (2) explore the potential adaptive value of this trait suite to the respective asocial and social phenotypes. Populations from the two latitudes studied, 26° and 36°, showed similar suites of correlated traits or syndromes. The individuals tested generally fell into two groups in the behavioural trials, although the asocial individuals from the mixed-phenotype population at 36° latitude were intermediate with respect to some of the measures. Individuals scored as ‘social’ generally showed less aggressive behaviour towards predatory cues, were less responsive to prey encountering their webs, showed little evidence of superfluous killing of prey and showed lower levels of activity than did most individuals scored as ‘asocial’. These results suggest linkage or pleiotropy between social behaviour and these traits. The behaviour of individuals in staged, mixed-phenotype feeding pairs indicated that social individuals may suffer fitness consequences in polymorphic populations. These results are discussed with respect to the stability and dynamics of the respective phenotypes in polymorphic populations.

Fostering model explains variation in levels of sociality in a spider system

We developed an ‘assured fitness returns’ model for the evolution of sociality independent of high relatedness within colonies. We first developed this model based on parameters from the northern social spider Anelosimus studiosus, which shows a higher incidence of multiple-female colonies caring for a common brood at higher latitudes and colder conditions within latitudes. The mathematical model developed predicts that multiple females will cooperatively care for a brood in those environments in which a single female has a high probability of dying before her offspring are able to care for themselves. The frequency of multiple-female nests observed in variable temperature sites in eastern Tennessee, U.S.A., is consistent with that found in our latitudinal censuses: a higher frequency of multiple-female colonies was present at cold-water sites than at warm-water sites. A test of a critical model assumption found a positive correlation of temperature and juvenile development rate in both the field and the laboratory. Colony success in field studies was also consistent with model predictions.

Extra-pair paternity in waved albatrosses.

We estimated the rate of extra‐pair fertilizations (EPFs) in waved albatrosses (Phoebastria irrorata) on Isla Española, Galápagos, Ecuador, using multilocus minisatellite DNA fingerprinting. Waved albatrosses are socially monogamous, long‐lived seabirds whose main population is on Española. Aggressive extra‐pair copulation (EPC) attempts have been observed in the breeding colony during the days preceding egg‐laying. Our genetic analyses of 16 families (single chicks and their attending parents) revealed evidence of EPFs in four families. In all cases males were the excluded parent. These data suggest that waved albatrosses have an unusually high rate of EPF relative to taxa with similar life histories. Future behavioural observations will determine the extent to which forced vs. unforced EPCs contribute to this high EPF rate.

High gene flow levels lead to gamete wastage in a desert spider system

Field censuses, breeding experiments, and a quantitative model are used to obtain insight into the extent and consequences of genetic mixing between locally adapted populations of a desert spider. Typically, 9% of the matings of desert riparian spiders (non-aggressive phenotype) in native habitat involve an arid-land partner (aggressive phenotype). Mating was found to be random with respect to behavioral phenotype, but linearly related to both the rate of immigration and survival of immigrants from surrounding arid habitats in the riparian area. Genetic mixing between riparian and arid-land spiders produces offspring that exhibit lower rates of survival in riparian habitat. Two extreme behavioral phenotypes were also observed in the field studies: approximately 5% of the female spiders attacked all males they encountered while another 22% ran from all potential mates. Punnett square analyses of the potential genotypes produced by introgression between arid- and riparian-adapted spiders indicate that these extreme phenotypes appear in F2 generation hybrids and backcrosses. Because there is a costly wastage of gametes in the case of mixed phenotype mating, model results indicate that within three generations of the cessation of gene flow, the riparian population would be free of mixed genotypes and moving towards genetic differentiation.

Patterns of reproductive success associated with social structure and microclimate in a spider system

Species that vary in social structure within and among populations can provide unparalleled insight into the evolution of sociality. The theridiid spider Anelosimus studiosus varies widely in its social structure at the northern edge of its range. Colony sizes range from the solitary/territorial female with her offspring to cooperative colonies of tens to hundreds of adult females. In previous work, we developed an assured fitness returns ‘brood-fostering model’ that predicts that in cooler environments mothers in multiple-female colonies will have a selective advantage over solitary female nests. According to the model, at cool sites the rate of juvenile maturation is slowed, increasing the probability that the mother will die before the brood reaches independence. In her absence, other females would foster her brood. We tested this hypothesis by manipulating colony size and by monitoring the reproductive success of naturally occurring solitary and multifemale colonies in cold and warm temperature environments. Our results indicate that while multiple-female colonies have higher fitness at cool sites, the solitary female nests achieve higher fitness at warmer sites. The higher reproductive success of multifemale colonies at cold sites further reflects the total failure of solitary female nests at these sites. Solitary female nests that survived generally had higher reproductive success than multifemale colonies at all temperatures. In natural colonies, fitness was highest for smaller multifemale colonies in the colder environments and decreased in the larger colonies. We use these data to refine the brood-fostering model and discuss the results with regard to the observed polymorphism in social structure.

COSTS AND BENEFITS OF FORAGING ASSOCIATED WITH DELAYED DISPERSAL IN THE SPIDER ANELOSIMUS STUDIOSUS (ARANEAE, THERIDIIDAE)

Abstract In the theridiid spider, Anelosimus studiosus, most juveniles remain in their natal web, forming temporary colonies in which individuals cooperate in web maintenance and prey capture until they disperse at maturity. There is natural variation in age at dispersal, and subadult spiders removed from their natal webs build webs and continue to develop. To explore the costs and benefits of delayed dispersal, we compared the rate of prey capture and developmental rate for individuals in colonies and those isolated at the fourth instar. Rate of prey capture by colonies increased with colony size and age; this result was driven primarily by the enhanced capture of large prey by larger and older colonies. The presence of juveniles increased the overall productivity of webs, an effect which remained after the juveniles were removed from the web. Despite the overall increase in prey capture, per-individual prey capture decreased with colony size. The variance in prey capture success decreased significantly with colony size, but not with colony age. Spiders in colonies captured more prey per juvenile than singletons experimentally dispersed at the fourth instar; however, this did not result in increased development rate of colonial juveniles over isolated juveniles. These data suggest that juvenile A. studiosus benefit from delayed dispersal by acquiring more resources and acquiring them more steadily. The productivity of webs of females whose juveniles were removed at the fourth instar remained higher than those of similarly aged females who never produced juveniles. This suggests that delayed dispersal of juveniles enhances the resources which the female could allocate to her next egg mass.

INTERACTIONS BETWEEN THE SOCIAL SPIDER ANELOSIMUS STUDIOSUS (ARANEAE, THERIDIIDAE) AND FOREIGN SPIDERS THAT FREQUENT ITS NESTS

Abstract Because competition for resources generally leads to the spatial exclusion of species using similar resources, it is surprising that foreign spider species are frequently observed in or near the nests of the social spider, Anelosimus studiosus (Hentz 1850) (Theridiidae). In this study, we quantified the frequency of A. studiosus-foreign spider co-occurrence and completed experiments designed to explain the nature of the host-foreign spider species associations. Four families were numerically prominent nest associates of A. studiosus: Salticidae, Anyphaenidae, Araneidae and Tetragnathidae. These families and the Agelenidae (intermediate association rate) and (Philodromidae) (infrequent association rate) were subjected to further study. In choice trials, no foreign spider family discriminated nest silk or A. studiosus, itself, from the foliage the nest is built in. Predation events and maximum inter-individual spacing were significant outcomes of induced pair-wise interactions between host and foreign spiders for all families. Predation events were almost exclusively foreign on host. Field census results show that a close correspondence exists between the rate of loss of A. studiosus nests over time and the association rate of anyphaenids and agelenids at particular sites: these two families were the prominent predators on A. studiosus in laboratory trials. In reciprocal predation tests, juveniles suffered less predation in the presence of a mother; in the reciprocal trials, mothers also suffered less predation from foreign spiders when juveniles were present.

Diel and life-history characteristics of personality: consistency versus flexibility in relation to ecological change

Despite the potential benefits of modifying behaviour according to changing ecological conditions, many populations comprise individuals that differ consistently in behaviour across situations, contexts and points in time (i.e. individuals show personality). If personalities are adaptive, the balance between consistency and flexibility of behavioural traits should reflect the ability of individuals to detect and respond to changing conditions in an appropriate and timely manner and, thus, depend upon the pace and predictability of changing conditions. We investigated the balance between individual consistency and flexibility in the subsocial spider Anelosimus studiosus by assaying boldness across the diel cycle and correlating these data with patterns of prey and threat abundance in the natural habitat. We found significant diel flexibility in boldness correlating with drastic and predictable changes in prey availability. Moreover, the strength of within-individual flexibility in boldness was comparable to the strength of rank-order consistency among individuals. We also found evidence that mean boldness level and among-individual variation in boldness are correlated with reproductive status. These data emphasize the interplay between behavioural consistency and flexibility and suggest that temporal characteristics of ecological conditions may be vital in assessing the strength, stability and adaptive value of animal personalities.

Epaulet Color and Sexual Selection in the Red-Winged Blackbird: A Field Experiment

Abstract. The epaulets of male Red-winged Blackbirds (Agelaius phoeniceus) function in both intersexual and intrasexual contexts, but there is little evidence that they covary with reproductive success in this well-studied species. We used path analysis of male/territory traits, mating success, and reproductive success of unmanipulated males to estimate current directional selection. Territory size had a positive effect on number of within-pair mates. Number of extra-pair mates had a positive effect on number of extra-pair fledglings, and number of within-pair and extra-pair fledglings had positive effects on total number of fledglings. We also reddened epaulets of free-living territorial males to determine whether manipulated and control males differ in territorial behavior, mating success, or reproductive success. Compared with control males, males with reddened epaulets incurred elevated rates of trespassing and territorial challenges, lost their territories more often, and were unable to produce extra-pair offspring. Despite these differences, however, the realized reproductive success of experimental and control males did not differ significantly, perhaps because males with reddened epaulets devoted more time to anti-predator vigilance and were more aggressive toward a simulated predator than were control males. An apparent lack of current sexual selection on epaulet color may be a form of counter-balancing sexual selection in which male aggression against redder epaulets opposes female preference for redder epaulets, or it might be the result of males compensating for lost extra-pair fertilizations by increasing their parental care.

PopGen Fishbowl: A Free Online Simulation Model of Microevolutionary Processes.

ABSTRACT Natural selection and other components of evolutionary theory are known to be particularly challenging concepts for students to understand. To help illustrate these concepts, we developed a simulation model of microevolutionary processes. The model features all the components of Hardy-Weinberg theory, with population size, selection, gene flow, nonrandom mating, and mutation all being demonstrated in the simulations. By using this freely available computer model, students can develop and test hypotheses with replicated virtual experiments. Because the model is an agent-based simulation, there is biologically realistic variability in the results. Students using the model see results both numerically and graphically and these are reinforced by an animation of the virtual fish in the simulated experiment.