David B. McDonald’s research while affiliated with University of Wyoming and other places

We present novel metrics for analysis of weighted social networks that focus explicitly on the distribution of edge weights at hierarchical scales from node to egonet to community and to the network as a whole. The formulae are adapted from existing measures, originally developed in the context of population genetics to analyse variance in gene frequencies at different levels of organization. Our metrics, including 'effective degree' (by analogy to effective number of alleles), 'concentration' (by analogy to the inbreeding coefficient), 'observed' and 'expected edge weight diversity' (by analogy to observed and expected gene diversity) and F statistics allow one to partition the variance in edge weights among hierarchical levels of organization within networks. They provide a quantitative method for addressing issues as diverse as disease transmission, social complexity, the spread of learned behaviours and the evolution of cooperation. We illustrate the utility of these new metrics by applying them to three empirical social networks: long-tailed manakins, Chiroxiphia linearis, monk parakeets, Myiopsitta monachus, and mountain goats, Oreamnos americanus.

Fission–fusion dynamics, consisting of regular shifts in the size and composition of social groups, are prevalent in animal societies and have implications for foraging success, disease and information transfer, and the fitness of individuals. Thus, the individual and environmental factors that drive social dynamics have become a focus of recent investigations. River otters, Lontra canadensis, in coastal Alaska have a plastic social system, influenced by forage fish availability. These carnivores also regularly frequent terrestrial latrine sites, where they associate and communicate through deposition of odorous substances. To investigate fission–fusion dynamics in this system we (1) deployed camera traps to record social behaviours at latrine sites and (2) attached proximity tags to quantify encounter rates among individuals. Camera detections demonstrated that most latrine visits were of single otters and small groups (2–8 individuals). Fusion events into large groups (up to 18 individuals) were infrequent. Larger groups were recorded at crossover latrines, where trails connected bodies of water, whereas social behaviour was more frequent at spatially central latrines. Visiting otters performed signalling behaviours more frequently than social behaviours, especially at crossover sites. Proximity tag data revealed that the timing of fission and fusion events coincided with latrine visits and that spatial overlap was a good predictor of social interaction. Thus, the structural and spatial features of latrines influence their function as centres of information exchange, social activity hubs and meeting places among small social units, with implications for river otter group dynamics. We conclude that shifting social and environmental conditions may lead to high communication complexity. This unique social system provides novel evidence of the role of olfactory communication in mediating social decisions.

The widespread existence of dominance hierarchies has been a central puzzle in social evolution, yet we lack a framework for synthesizing the vast empirical data on hierarchy structure in animal groups. We applied network motif analysis to compare the structures of dominance networks from data published over the past 80 years. Overall patterns of dominance relations, including some aspects of non-interactions, were strikingly similar across disparate group types. For example, nearly all groups exhibited high frequencies of transitive triads, whereas cycles were very rare. Moreover, pass-along triads were rare, and double-dominant triads were common in most groups. These patterns did not vary in any systematic way across taxa, study settings (captive or wild) or group size. Two factors significantly affected network motif structure: the proportion of dyads that were observed to interact and the interaction rates of the top-ranked individuals. Thus, study design (i.e. how many interactions were observed) and the behaviour of key individuals in the group could explain much of the variations we see in social hierarchies across animals. Our findings confirm the ubiquity of dominance hierarchies across all animal systems, and demonstrate that network analysis provides new avenues for comparative analyses of social hierarchies. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

Hybridization between diverged taxa tests the strength of reproductive isolation, and can therefore reveal mechanisms of reproductive isolation. However, it remains unclear how consistent reproductive isolation is across species' ranges, and to what extent reproductive isolation might remain polymorphic as species diverge. To address these questions, we compared outcomes of hybridization across species pairs of Catostomus fishes in three rivers in the Upper Colorado River basin, where an introduced species, C. commersoni, hybridizes with at least two native species, C. discobolus and C. lattipinis. We observed substantial heterogeneity in outcomes of hybridization, both between species pairs and across geographically separate rivers within each species pair. We also observed hybridization of additional related species with our focal species, suggesting that reproductive isolation in this group involves interactions of multiple evolutionary and ecological factors. These findings suggest that a better understanding of the determinants of variation in reproductive isolation is needed, and that studies of reproductive isolation in hybrids should consider how the dynamics and mechanisms of reproductive isolation vary over ecological space and over evolutionary time. Our results also have implications for the conservation and management of native catostomids in the Colorado River basin. Heterogeneity in outcomes of hybridization suggests that the threat posed by hybridization and genetic introgression to the persistence of native species likely varies with extent of reproductive isolation, both across rivers and across species pairs. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.

Social networks arise from complex interactions among multiple individuals and affect the emergent properties of groups (e.g. cooperation, disease spread, information transfer, etc.). Cooperation among nonkin is generally predicted to be favoured in structured social networks where individuals primarily interact only with certain individuals. Long-tailed manakins, Chiroxiphia linearis, form lek groups of as many as 15 unrelated males, whose members can attend multiple leks. At each lek, several top-ranked males perform the majority of obligate cooperative courtship displays. We used exponential random graph (ERG) modelling to analyse manakin cooperation networks constructed from 2-year time intervals over a 14-year study period. ERG modelling evaluates how local processes contribute to formation of global social network structure. We found that four local processes of link formation largely explained the overall structure of male manakin cooperation networks: (1) the spatial proximity of birds: males were more likely to cooperate if they primarily displayed at the same or neighbouring leks; (2) social status of birds: males were more likely to cooperate as they moved up the social queue at leks; (3) triad closure: males were more likely to cooperate with a ‘friend of a friend’ than with males with which they did not share a mutual partner; and (4) link persistence: males were more likely to cooperate with males whom they had cooperated with in the past. Other plausible mechanisms, such as selective mixing (the tendency to interact with individuals of similar or dissimilar social status) and preferential attachment by degree (whereby individuals with many social links gain additional links) did not consistently explain the structure of male cooperation networks at leks. These local processes may facilitate cooperation among long-tailed manakins by creating structured social networks in which males interact with only a subset of the population.

Social network analysis provides a broad and complex perspective on animal sociality that is widely applicable to almost any species. Recent applications demonstrate the utility of network analysis for advancing our understanding of the dynamics, selection pressures, development, and evolution of complex social systems. However, most studies of animal social networks rely primarily on a descriptive approach. To propel the field of animal social networks beyond exploratory analyses and to facilitate the integration of quantitative methods that allow for the testing of ecologically and evolutionarily relevant hypotheses, we review methodological and conceptual advances in network science, which are underutilized in studies of animal sociality. First, we highlight how the use of statistical modeling and triadic motifs analysis can advance our understanding of the processes that structure networks. Second, we discuss how the consideration of temporal changes and spatial constraints can shed light on the dynamics of social networks. Third, we consider how the study of variation at multiple scales can potentially transform our understanding of the structure and function of animal networks. We direct readers to analytical tools that facilitate the adoption of these new concepts and methods. Our goal is to provide behavioral ecologists with a toolbox of current methods that can stimulate novel insights into the ecological influences and evolutionary pressures structuring networks and advance our understanding of the proximate and ultimate processes that drive animal sociality.

Dominance is a social relation between a subordinate animal and the dominant to which it submits. Animal groups seem regularly to form dominance hierarchies in which dominance relations are transitive and stable, but comparative studies are rare. Dominance hierarchies can be formalized as social networks, with arrows (directed edges) pointing from dominant animals (nodes) to subordinates. Using this network perspective, we explored the orderliness of 40 published datasets for taxa from ants to elephants. To quantify orderliness, we used the triad census, a technique from sociology, that enumerates the proportion of orderly (transitive) triads (e.g., A dominates B and C, B dominates C, yielding clear top, middle, and bottom rankings) versus disorderly (cyclic) triads (e.g., A dominates B, B dominates C, but C in turn dominates A). All 40 datasets showed a significant excess of orderly (transitive) triads and a deficit of disorderly (cyclic) triads compared with the null model of random networks. Most datasets showed relatively high rank stability (mean stability index of 0.81 on a scale from 0 to 1). Steep hierarchies arise when the scores used to rank contestants differ sharply, further promoting stability. All 40 dominance hierarchies were steeper than expected from randomized sequences of contests. The overwhelming conclusion was that animal groups are orderly, as assessed by a high proportion of transitive relations, a paucity of disorderly cycles, and high temporal stability in rankings. Thus, a certain degree of self-organization may characterize even agonistic interactions across many different kinds of animal societies.

Dominance is a social relation between a subordinate animal and the dominant to which it submits. Animal groups seem regularly to form dominance hierarchies in which dominance relations are transitive and stable, but comparative studies are rare. Dominance hierarchies can be formalized as social networks, with arrows (directed edges) pointing from dominant animals (nodes) to subordinates. Using this network perspective, we explored the orderliness of 40 published datasets for taxa from ants to elephants. To quantify orderliness, we used the triad census, a technique from sociology, that enumerates the proportion of orderly (transitive) triads (e.g., A dominates B and C, B dominates C, yielding clear top, middle, and bottom rankings) versus disorderly (cyclic) triads (e.g., A dominates B, B dominates C, but C in turn dominates A). All 40 datasets showed a significant excess of orderly (transitive) triads and a deficit of disorderly (cyclic) triads compared with the null model of random networks. Most datasets showed relatively high rank stability (mean stability index of 0.81 on a scale from 0 to 1). Steep hierarchies arise when the scores used to rank contestants differ sharply, further promoting stability. All 40 dominance hierarchies were steeper than expected from randomized sequences of contests. The overwhelming conclusion was that animal groups are orderly, as assessed by a high proportion of transitive relations, a paucity of disorderly cycles, and high temporal stability in rankings. Thus, a certain degree of self-organization may characterize even agonistic interactions across many different kinds of animal societies. [Includes supplemental materials; Appendix 1-3]