Describing the factors that shape collective behaviour is central to our understanding of animal societies. Countless studies have demonstrated an effect of group size in the emergence of collective ...behaviours, but comparatively few have accounted for the composition/diversity of behavioural phenotypes, which is often conflated with group size. Here, we simultaneously examine the effect of personality composition and group size on nest architecture and collective foraging aggressiveness in the social spider Stegodyphus dumicola. We created colonies of two different sizes (10 or 30 individuals) and four compositions of boldness (all bold, all shy, mixed bold and shy, or average individuals) in the field and then measured their collective behaviour. Larger colonies produced bigger capture webs, while colonies containing a higher proportion of bold individuals responded to and attacked prey more rapidly. The number of attackers during collective foraging was determined jointly by composition and size, although composition had an effect size more than twice that of colony size: our results suggest that colonies of just 10 bold spiders would attack prey with as many attackers as colonies of 110 ‘average’ spiders. Thus, personality composition is a more potent (albeit more cryptic) determinant of collective foraging in these societies.
Behavioural ecologists often note that one or a few group members appear to shape the collective behaviour of social groups differentially. Our understanding of these keystone individuals is largely ...taken from meticulous field observations and semi-scientific anecdotes. In this study we experimentally test whether the behavioural tendencies of putative keystone individuals shift the collective behaviour of colonies using the social spider Stegodyphus dumicola. Prior studies on Stegodyphus demonstrated that the single best predictor of colonies' collective behaviour is the behaviour of colonies' boldest individual. Here, we probe the causal relationship between the traits of extremely bold individuals and colonies' collective behaviour by experimentally creating colonies of identical size and personality composition in the laboratory and then adding a single individual of varying boldness (the putative keystone individual). Experimentally adding just one extremely bold individual increased the foraging aggressiveness of entire colonies and altered the total mass gained by fellow group members, relative to the addition of a less bold individual. Additionally, our data suggest that bold individuals are capable of such influence because they catalyse variation in the behavioural tendencies of fellow group members.
•In the social spider Stegodyphus dumicola, individuals vary in their boldness.•Extremely bold individuals influence their groups more than shyer individuals.•Colonies with one extreme individual gain more mass and behave more aggressively.•The effect of extreme individuals scales positively to their degree of boldness.
The concept of keystone individuals offers a unifying framework to study the evolution and persistence of individuals that have a disproportionately large, irreplaceable effect on group dynamics. ...Although the literature is teeming with examples of these individuals, disparate terminologies have impeded a major synthesis of this topic across fields. To allow a strict classification of potential keystone individuals, we offer herein some general terminology, outline practical methodological approaches to distinguish between keystone individuals and generic individuals that only occupy a keystone role, and propose ways to measure the effect of keystones on group dynamics. In particular, we suggest that keystone individuals should be classified as ‘fixed’ or ‘episodic’ according to the duration of time over which they impact their group. We then venture into the existing literature to identify distinctive keystone roles that generic and/or keystone individuals can occupy in a group (e.g. dominant individual, leader or superspreader), and describe traits that can give rise to keystone individuals. To highlight the ecological implications, we briefly review some of the effects that keystone individuals can have on their group and how this could affect other levels of organization such as populations and communities. In looking at their diverse evolutionary origins, we discuss key mechanisms that could explain the presence of keystone individuals. These mechanisms include traditional Darwinian selection on keystone-conferring genotypes, experience and state- or context-dependent effects. We close our review by discussing various opportunities for empirical and theoretical advancement and outline concepts that will aid future studies on keystone individuals.
•We outline the concept of keystone individuals.•Keystones are defined as having a disproportionally large effect on groups.•Keystones can occupy a diverse array of functional roles in a group.•We propose methodological approaches to study keystone's impacts.•We explore keystone's evolutionary and ecological implications.
Antagonistic interactions impose pressures that can trigger shifts in defensive phenotypes. For instance, one natural enemy may activate defensive phenotypes that influence defenses that protect ...against other enemies. Socially parasitic ants (
Temnothorax americanus
) are both parasites and predators of other coevolved
Temnothorax
species, whose brood they either consume as prey or steal during raids to utilize as a work force in their own colonies. Since these social parasites impose a significant threat to host colonies, we explored whether exposing a
T. americanus
worker to
T. curvispinosus
host colonies could impact nest hygiene behavior, a component of collective disease defense. Specifically, we measured the latency to remove colony-mate corpses from the nest. We measured corpse removal twice before and twice after exposure to a
T. americanus
worker collected in sympatry to the focal host colonies. We found that simulating the initial stage of a scout raid had no effect on this measure of collective nest hygiene. These results indicate that some measures of social immunity may remain robust after a potentially stressful antagonistic interaction from a coevolved heterospecific.
Social animals are extraordinarily diverse and ecologically abundant. In understanding the success of complex animal societies, task differentiation has been identified as a central mechanism ...underlying the emergence and performance of adaptive collective behaviors. In this study, we explore how individual differences in behavior and body size determine task allocation in the social spider Stegodyphus dumicola. We found that individuals with high body condition indices were less likely to participate in prey capture, and individuals’ tendency to engage in prey capture was not associated with either their behavioral traits or body size. No traits were associated with individuals’ propensity to participation in web repair, but small individuals were more likely to engage in standard web-building. We also discovered consistent, differences among colonies in their collective behavior (i.e., colony-level personality). At the colony level, within-colony variation in behavior (aggressiveness) and body size were positively associated with aggressive foraging behavior. Together, our findings reveal a subtly complex relationship between individual variation and collective behavior in this species. We close by comparing the relationship between individual variation and social organization in nine species of social spider. We conclude that intraspecific variation is a major force behind the social organization of multiple independently derived lineages of social spider.
The success of a social group is often driven by its collective characteristics and the traits of its individuals. Thus, understanding how collective behavior is influenced by the behavioral ...composition of group members is an important first step to understand the ecology of collective personalities. Here, we investigated how the efficiency of several group behaviors is influenced by the aggressiveness of its members in two species of Temnothorax ants. In our manipulation of group composition, we created two experimentally reconstituted groups in a split-colony design, i.e., each colony was split into an aggressive and a docile group of equal sizes. We found strong species-specific differences in how collective behaviors were influenced by its group members. In Temnothorax longispinosus, having more aggressive individuals improved colony defense and nest relocation efficiency. In addition, source colony identity strongly influenced group behavior in T. longispinosus, highlighting that manipulations of group compositions must control for the origin of the chosen individuals. In contrast, group composition and source colony did not influence collective behaviors in Temnothorax curvispinosus. This suggests that the mechanisms regulating collective behaviors via individual differences in behavior might differ among even closely related species.
The vast majority of species interactions in nature go unnoticed because they occur under circumstances unamenable to observation. This is unfortunate, as our understanding of trophic ecology is ...predicated on our ability to quantify the nature and magnitude of species interactions. Here, we use specimens from museums and private collections to estimate prey breadth and morphological patterns between predator and prey pairs of the malacophagous Cychrini beetles collected alongside their snail prey. We identified each pair, measured a series of morphological traits on each, and tested for relationships between the morphological characteristics of beetles and the snails they were found eating. Of 38 specimen pairs, we identified 12 species of Cychrini beetles from two genera (Scaphinotus and Cychrus) eating 22 species of snail prey from 12 genera and ranging from 1 to 9 species of snail prey per beetle species. We found 29 unique predator–prey species pairs. Irrespective of species identity, we found that female beetles were discovered eating larger snails compared to male beetles. We detected two trends in which larger beetles were found eating snails with relatively larger aperture openings, and beetles with more slender body shapes (longer, thinner mandibles, heads, and pronota) were found eating snails whose shells had relatively smaller aperture openings. This suggests that, even within the carabid tribe Cychrini, variation in the cychrine body form may influence prey availability. This study provides the most comprehensive list to date of predator–prey pairs in this understudied group of beetles and also demonstrates the utility of museum collections for documenting cryptic species interactions.
Determining the drivers of complex systems is a major challenge felt by virtually all of biology. For complex societies, there is considerable interest in how individuals' traits determine their ...social roles and how the milieu of trait variants within groups shapes emergent group-level phenotypes. In this study, we tested in spiders whether individual differences in behaviour, morphology, or both shape individuals' participation in foraging. We then tested which aspects of group composition (e.g. morphological composition, personality composition, group size) predict colony-level foraging and escape behaviour. We performed these studies both under (1) standardized laboratory conditions, where colonies' ability to build webs was constrained, and (2) under more free-living conditions in the field. Regardless of the laboratory/field distinction, we found that individual task participation was regulated by individuals' body size and group size: larger individuals and individuals in smaller groups tended to participate more in foraging tasks. At the colony level, colonies composed of bolder spiders attacked prey with a larger number of attackers regardless of the laboratory/field distinction. However, our field assessments of colony foraging revealed that the speed with which colonies responded to prey was determined by their group size, morphological composition and personality composition; none of these effects were detected under laboratory conditions. Finally, we found that colonies took longer to retreat from aversive stimuli when more spiders were outside the nest. Taken together, our results suggest that colonies' social organization and collective behaviour are determined by spiders' morphology, behavioural tendencies and colony demographics.
Long‐term interactions among individuals are a hallmark of animal societies, but groups rarely remain entirely stable over time. Individuals die or emigrate, or groups become spatially fragmented. ...Group fragmentation can alter the phenotypic composition of subgroups by separating well‐connected individuals or altering sex ratios, which may alter the execution of collective behaviors. Over 10 days, we measured the aggregation behavior and collective prey capture of experimentally fragmented social spider (Stegodyphus dumicola) colonies collected from different populations in South Africa and Namibia. Colonies were fragmented for 4 weeks, after which subgroups were allowed to aggregate into a single group over time in a shared novel environment. Namibian colonies aggregated more rapidly than South African colonies. Across both populations, colonies containing individuals with higher average boldness values (faster recovery time after an antagonistic stimulus) attacked prey stimuli with more participants. However, bolder colonies from South Africa attacked prey stimuli faster, whereas attack latency in Namibian colonies was unaffected by colony boldness. These data suggest that fragmentation events, which are a common phenomenon in this species and other animal societies, can influence how individuals interact to accomplish collective tasks. Further, collective behavior and group fusion after fragmentation events can differ among groups from different populations.
Over 10 days, we measured the aggregation behavior and collective prey capture of experimentally fragmented social spider (Stegodyphus dumicola) colonies collected from South Africa and Namibia. Namibian colonies aggregated more rapidly than South African colonies. Across both populations, colonies containing individuals with higher average boldness values (faster recovery after an antagonistic stimulus) attacked prey stimuli with more participants. Bolder colonies from South Africa attacked prey stimuli faster; attack speed in Namibian colonies was unaffected by colony boldness.
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