Papageorgiou, Danai
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A guide to sampling design for GPS-based studies of animal societies
1. GPS-based tracking is widely used for studying wild social animals. Much like traditional observational methods, using GPS devices requires making a number of decisions about sampling that can affect the robustness of a study's conclusions. For example, sampling fewer individuals per group across more distinct social groups may not be sufficient to infer group- or subgroup-level behaviours, while sampling more individuals per group across fewer groups limits the ability to draw conclusions about populations.
2. Here, we provide quantitative recommendations when designing GPS-based tracking studies of animal societies. We focus on the trade-offs between three fundamental axes of sampling effort: (1) sampling coverage—the number and allocation of GPS devices among individuals in one or more social groups; (2) sampling duration—the total amount of time over which devices collect data and (3) sampling frequency—the temporal resolution at which GPS devices record data.
3. We first test GPS tags under field conditions to quantify how these aspects of sampling design can affect both GPS accuracy (error in absolute positional estimates) and GPS precision (error in the estimate relative position of two individuals), demonstrating that GPS error can have profound effects when inferring distances between individuals. We then use data from whole-group tracked vulturine guineafowl Acryllium vulturinum to demonstrate how the trade-off between sampling frequency and sampling duration can impact inferences of social interactions and to quantify how sampling coverage can affect common measures of social behaviour in animal groups, identifying which types of measures are more or less robust to lower coverage of individuals. Finally, we use data-informed simulations to extend insights across groups of different sizes and cohesiveness.
4. Based on our results, we are able to offer a range of recommendations on GPS sampling strategies to address research questions across social organizational scales and social systems—from group movement to social network structure and collective decision-making.
5. Our study provides practical advice for empiricists to navigate their decision-making processes when designing GPS-based field studies of animal social behaviours, and highlights the importance of identifying the optimal deployment decisions for drawing informative and robust conclusions.
Collective movement and social decision-making in the Vulturine Guineafowl (Acryllium vulturinum)
Animals living in stable groups need to maintain cohesion while simultaneously acquiring resources critical for survival. Yet, little is known about how groups overcome the challenges of maintaining cohesion and making decisions when faced with heterogeneity in the environment. Heterogeneity can arise both spatially, e.g. based on how resources are distributed, and temporally, e.g. based on the seasonal variation in the spatial distribution and/or abundance of food and water. Groups themselves can also be heterogeneous, fluctuating in size and composition. For this thesis work, I studied multiple groups of vulturine guineafowl (Acryllium vulturinum) that range freely in the Kenyan savannah. I used a combination of data collection methods, including high-resolution GPS tracking, field observations of habituated and nonhabituated groups, video tracking and environmental data from meteorological stations. I first described the multilevel social structure of this species, in which stable groups comprising of multiple breeding units associate preferentially with specific other groups. Herewith, vulturine guineafowl is the first avian species discovered to live in a multilevel society, which was so far described for largebrained mammals. I then tested the effect of environmental and social drivers of collective movement. Groups used larger areas, travelled longer distances, and moved to new places more often during drier than wetter seasons. Further, I found an optimal group size for collective movement with groups of intermediate size expressing the largest home-range size, greater variation in space use and higher reproductive success, in comparison to large and small groups. Despite these emergent differences in group movement characteristics, vulturine guineafowl groups always followed a shared decision-making process, according to which all group members could contribute to decisions. Finally, I demonstrated the importance of shared decision-making for allowing groups to satisfy the needs of each group member, and thus enable groups to maintain cohesion even when faced with heterogeneous environments and individual differences in priority access to resources. By following a multi-scale approach 4 (within-groups, between-groups, population-level), I showed that the interaction between the social and the physical environment can shape both the outcomes and the micro-dynamics of collective behaviour in wild group-living animals.
The importance of individual-to-society feedbacks in animal ecology and evolution
1. The social decisions that individuals make—who to interact with and how frequently—give rise to social structure. The resulting social structure then determines how individuals interact with their surroundings—resources and risks, pathogens and predators, competitors and cooperators.
2. However, despite intensive research on (a) how individuals make social decisions and (b) how social structure shapes social processes (e.g. cooperation, competition and conflict), there are still few studies linking these two perspectives. These perspectives represent two halves of a feedback loop: individual behaviour scales up to define the social environment, and this environment, in turn, feeds back by shaping the selective agents that drive individual behaviour.
3. We first review well‐established research areas that have captured both elements of this feedback loop—host–pathogen dynamics and cultural transmission. We then highlight areas where social structure is well studied but the two perspectives remain largely disconnected. Finally, we synthesise existing research on 14 distinct research topics to identify new prospects where the interplay between social structure and social processes are likely to be important but remain largely unexplored.
4. Our review shows that the inherent links between individuals’ traits, their social decisions, social structure and social evolution, warrant more consideration. By mapping the existing and missing connections among many research areas, our review highlights where explicitly considering social structure and the individual‐to‐society feedbacks can reveal new dimensions to old questions in ecology and evolution.
The multilevel society of a small-brained bird
Animal societies can be organised in multiple hierarchical tiers [1]. Such multilevel societies, where stable groups move together through the landscape, overlapping and associating preferentially with specific other groups, are thought to represent one of the most complex forms of social structure in vertebrates. For example, hamadryas baboons (Papio hamadryas) live in units consisting of one male and one or several females, or of several solitary males, that group into clans. These clans then come together with solitary bachelor males to form larger bands [2]. This social structure means that individuals have to track many different types of relationships at the same time [1,3]. Here, we provide detailed quantitative evidence for the presence of a multilevel society in a small-brained bird, the vulturine guineafowl (Acryllium vulturinum). We demonstrate that this species lives in large, multi-male, multi-female groups that associate preferentially with specific other groups, both during the day and at night-time communal roosts.
Costs dictate strategic investment in dominance interactions
Dominance is important for access to resources. As dominance interactions are costly, individuals should be strategic in whom they interact with. One hypothesis is that individuals should direct costly interactions towards those closest in rank, as they have most to gain-in terms of attaining or maintaining dominance-from winning such interactions. Here, we show that male vulturine guineafowl (Acryllium vulturinum), a gregarious species with steep dominance hierarchies, strategically express higher-cost aggressive interactions towards males occupying ranks immediately below themselves in their group's hierarchy. By contrast, lower-cost aggressive interactions are expressed towards group members further down the hierarchy. By directly evaluating differences in the strategic use of higher- and lower-cost aggressive interactions towards competitors, we show that individuals disproportionately use highest-cost interactions-such as chases-towards males found one to three ranks below themselves. Our results support the hypothesis that the costs associated with different interaction types can determine their expression in social groups with steep dominance hierarchies. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.
Multilevel Societies in Birds
There is growing interest in the study of multilevel societies, where social units comprising several individual animals (human and non-human) come together to form higher-level groupings. Grueter et al. [ 1 ] provided a useful definition of multilevel societies, highlighting that key characteristics of multilevel societies should include stability across different social levels, and that stability should be driven by active social preferences rather than attraction to the same resource or spatial overlap. Despite their framework being predominately focused on mammals, and in particular primates, we argue it applies to a much wider range of taxa. Here, we highlight the important contribution that studies on birds can make to distinguish different evolutionary pathways that can lead to multilevel societies and mechanisms that might maintain these.
Seasonality impacts collective movements in a wild group-living bird
Background
A challenge faced by animals living in groups with stable long-term membership is to effectively coordinate their actions and maintain cohesion. However, as seasonal conditions alter the distribution of resources across a landscape, they can change the priority of group members and require groups to adapt and respond collectively across changing contexts. Little is known about how stable group-living animals collectively modify their movement behaviour in response to environment changes, such as those induced by seasonality. Further, it remains unclear how environment-induced changes in group-level movement behaviours might scale up to affect population-level properties, such as a population’s footprint.
Methods
Here we studied the collective movement of each distinct social group in a population of vulturine guineafowl (Acryllium vulturinum), a largely terrestrial and non-territorial bird. We used high-resolution GPS tracking of group members over 22 months, combined with continuous time movement models, to capture how and where groups moved under varying conditions, driven by seasonality and drought.
Results
Groups used larger areas, travelled longer distances, and moved to new places more often during drier seasons, causing a three-fold increase in the area used at the population level when conditions turned to drought. By contrast, groups used smaller areas with more regular movements during wetter seasons.
Conclusions
The consistent changes in collective outcomes we observed in response to different environments raise questions about the role of collective behaviour in facilitating, or impeding, the capacity for individuals to respond to novel environmental conditions. As droughts will be occurring more often under climate change, some group living animals may have to respond to them by expressing dramatic shifts in their regular movement patterns. These shifts can have consequences on their ranging behaviours that can scale up to alter the footprints of animal populations.
Social network architecture and the tempo of cumulative cultural evolution
The ability to build upon previous knowledge -- cumulative cultural evolution -- is a hallmark of human societies. While cumulative cultural evolution depends on the interaction between social systems, cognition and the environment, there is increasing evidence that cumulative cultural evolution is facilitated by larger and more structured societies. However, such effects may be interlinked with patterns of social wiring, thus the relative importance of social network architecture as an additional factor shaping cumulative cultural evolution remains unclear. By simulating innovation and diffusion of cultural traits in populations with stereotyped social structures, we disentangle the relative contributions of network architecture from those of population size and connectivity. We demonstrate that while more structured networks, such as those found in multilevel societies, can promote the recombination of cultural traits into high-value products, they also hinder spread and make products more likely to go extinct. We find that transmission mechanisms are therefore critical in determining the outcomes of cumulative cultural evolution. Our results highlight the complex interaction between population size, structure and transmission mechanisms, with important implications for future research.
Shared decision-making allows subordinates to lead when dominants monopolize resources
The concepts of leadership and dominance are often conflated, with individuals high in the social hierarchy assumed to be decision-makers. Dominants can exclusively benefit from monopolizing food resources and, therefore, induce an intragroup conflict when leading their group to these resources. We demonstrate that shared decision-making reduces such conflicts by studying movement initiations of wild vulturine guineafowl, a species that forms large, stable social groups with a steep dominance hierarchy. When dominant individuals displace subordinates from monopolizable food patches, the excluded subordinates subsequently initiate collective movement. The dominants then abandon the patch to follow the direction of subordinates, contrasting with nonmonopolizable resources where no individuals are excluded, and dominant individuals contribute extensively to group decisions. Our results demonstrate the role of shared decision-making in maintaining the balance of influence within animal societies.