Female-Biased Sex Ratios Increase Colony Survival and Reproductive Output in the Spider Anelosimus studiosus.

Negative frequency-dependent selection acting on the sexes is hypothesized to drive populations toward a balanced sex ratio. However, numerous examples of female-biased sex ratios pepper the arthropods. Theoretical examinations have proposed that female-biased populations or groups can have higher chances of surviving and propagating that may be advantageous. We evaluated this hypothesis in the semisocial spider Anelosimus studiosus by creating artificial colonies of varying sex ratios and sizes and observing colony performance at sites with high versus low group extinction rates. We also tested whether colony extinction rates and sex ratios were correlated across 25 collection sites, spanning 10° latitude. We found that colonies with female-biased sex ratios produced more egg cases and were more likely to survive the duration of a field season, suggesting that female-biased sex ratios confer both survival and reproductive advantages on colonies. The effect of sex ratio on colony survival and reproductive output was strongest for small colonies in high extinction areas. Moreover, we found that female-biased sex ratios correlated with greater extinction rates across 25 sites, indicating that female-biased sex ratios may have evolved at some sites in response to high extinction rates. These findings suggest that selection favoring groups with female-biased sex ratios may operate in A. studiosus, shedding light on some of the factors that may drive the evolution of biased sex ratios.

Genomic tools for behavioural ecologists to understand repeatable individual differences in behaviour.

Behaviour is a key interface between an animal's genome and its environment. Repeatable individual differences in behaviour have been extensively documented in animals, but the molecular underpinnings of behavioural variation among individuals within natural populations remain largely unknown. Here, we offer a critical review of when molecular techniques may yield new insights, and we provide specific guidance on how and whether the latest tools available are appropriate given different resources, system and organismal constraints, and experimental designs. Integrating molecular genetic techniques with other strategies to study the proximal causes of behaviour provides opportunities to expand rapidly into new avenues of exploration. Such endeavours will enable us to better understand how repeatable individual differences in behaviour have evolved, how they are expressed and how they can be maintained within natural populations of animals.

Activity trends and movement distances in the Arizona bark scorpion (Scorpiones: Buthidae).

The bark scorpion, Centruroides sculpturatus Ewing, is a nocturnal, cryptic, nonburrowing, mobile species that is common in urban landscapes spanning the desert southwest. Bark scorpions are often found in dense localized populations in cities, but the question of whether this is because the species is metabolically movement limited or choose to aggregate has not been addressed. Field observations lead us to believe that the scorpions move very little. Their ability to move is tested here. A circular pacing ring was constructed to observe the distance individuals could move in 2 h under both dark and light conditions. Observations under light motivate the arthropods to move, and significantly greater distances were observed in light trials, the maximum travel distance being 104.37 m, while the maximum distance in dark trials was 14.63 m. To monitor movement in the field, telemetry tags were used to mark female and male scorpions over 21 d during which relocation distances were recorded daily. Additionally, 12-h and 6-h overnight observational periods took place during which, scorpion movements were recorded hourly. Overall, it was found that scorpions moved significantly more in the pacing ring than in the field, indicating that field individuals are not moving at their maximum potential. Movement limitation does not explain their distribution pattern. In both the pacing ring and field, gender and pregnancy status had significant influence on distances moved. We conclude that C. sculpturatus is capable of much greater movement than is typically observed in the field.

Exploration of refuge preference in the Arizona bark scorpion (Scorpiones: Buthidae).

The ongoing difficulty in understanding how Centruroides sculpturatus (Ewing) uses the built environment has prompted this study in exploring refuge choices in a school environment and in laboratory behavioral assays. Radio telemetry tags were used at an urban site heavily populated with C. sculpturatus to track scorpions for a period of 21 d from 1 to 21 August 2013. Complimentary laboratory work tested scorpions in refuge choice arenas targeting variables of four crevice widths--14.0, 9.3, 7.0, and 4.6 mm--or of a vertical versus horizontal orientation preference. These crevice sizes were picked as a response to crevices being naturally used in the field. Telemetry and observations tallied significant preference for artificial structural harborage. Ninety-five percent of the structure refuge use occurred in hollow block walls. Vegetative harborage, debris, and underground burrows were not selected with any significance compared with each other or structures. Generalized additive models (GAMs) indicated the strongest predictive power from individual preference. The behavioral choice assays yielded a significant preference for the largest of crevice widths offered, 14.0 mm and to a lesser extent 9.3 mm, both horizontally and vertically. GAMs for these assays indicated size as the strongest predictive factor in choices. The orientation tests and GAMs showed individual preference driving choice favoring vertical planes. Observations about negative geotaxis in assay and refuge use details from the field are also reported.

Behavioural syndromes and social insects: personality at multiple levels.

Animal personalities or behavioural syndromes are consistent and/or correlated behaviours across two or more situations within a population. Social insect biologists have measured consistent individual variation in behaviour within and across colonies for decades. The goal of this review is to illustrate the ways in which both the study of social insects and of behavioural syndromes has overlapped, and to highlight ways in which both fields can move forward through the synergy of knowledge from each. Here we, (i) review work to date on behavioural syndromes (though not always referred to as such) in social insects, and discuss mechanisms and fitness effects of maintaining individual behavioural variation within and between colonies; (ii) summarise approaches and principles from studies of behavioural syndromes, such as trade-offs, feedback, and statistical methods developed specifically to study behavioural consistencies and correlations, and discuss how they might be applied specifically to the study of social insects; (iii) discuss how the study of social insects can enhance our understanding of behavioural syndromes-research in behavioural syndromes is beginning to explore the role of sociality in maintaining or developing behavioural types, and work on social insects can provide new insights in this area; and (iv) suggest future directions for study, with an emphasis on examining behavioural types at multiple levels of organisation (genes, individuals, colonies, or groups of individuals).

Group size and its effects on collective organization.

Many insects and arthropods live in colonies or aggregations of varying size. Group size may affect collective organization either because the same individual behavior has different consequences when displayed in a larger group or because larger groups are subject to different constraints and selection pressures than smaller groups. In eusocial colonies, group size may have similar effects on colony traits as body size has on organismal traits. Social insects may, therefore, be useful to test theories about general principles of scaling, as they constitute a distinct level of organization. However, there is a surprising lack of data on group sizes in social insects and other group-living arthropods, and multiple confounding factors have to be controlled to detect effects of group size. If such rigorous studies are performed, group size may become as important to understanding collective organization as is body size in explaining behavior and life history of individual organisms.