Humans use social information to adjust their quorum thresholds adaptively in a simulated predator detection experiment

Quorum sensing is used in many biological systems to increase decision accuracy. In quorum sensing, the probability that an individual adopts a behavior is a nonlinear function of the number of other individuals adopting this behavior. From an optimal decision-making perspective, individuals should adjust their quorum threshold to the particulars of the decision problem. Recent work predicts that a key factor here is the quality of social information. In particular, it is predicted that individuals should adjust their quorum thresholds such that it lies in between the average true and false positive rate of the other group members. We here test this prediction with a predator detection experiment. First, human groups observed a group of animals (projected on a white screen) in which a predator was present or absent, and each individual made an independent decision to escape or not. Second, individuals received social information on the decisions of their group members, after which individuals decided again. This social information, however, did not represent their own decisions but consisted of responses that either came from a high-performing group (i.e., many individuals detecting the predator) or from a low-performing group (i.e., few individuals detecting the predator). We found that individuals adaptively adjust their quorum threshold to the quality of the social information: when receiving social information from high-performing groups, individuals employed higher quorum thresholds than when receiving information from low-performing groups. Our study demonstrates that humans can quickly evaluate the quality of publicly available information and adaptively adjust their decision rules.     

Secondary sexual characters signal fighting ability and determine social rank in Alpine ibex (Capra ibex)

Social dominance is a fundamental aspect of male evolutionary ecology in polygynous mammals because it determines access to estrous females. As it is rarely possible to monitor marked individuals of known morphology, little is known about the determinants of male dominance. We studied the social structure of Alpine ibex males in Gran Paradiso National Park, Italy in 2003, 2006, and 2007. Dominance interactions produced a linear social hierarchy. In ibex males, body mass and horn length are key traits in male-male combat, and both increase with age. We explored the links between age, body mass, horn length, and social rank. Ibex males showed much age-independent phenotypic heterogeneity and rapidly growing males reached high rank at a younger age than slow-growing males. Because there is no trade-off between horn growth and longevity, fast-growing males may face weak potential costs of rapid growth and high fitness benefit of achieving high rank. Violent interactions were more likely to occur between males similar in both mass and horn length, independently of age. We suggest that only high-quality individuals can afford a strategy of rapid growth for both secondary sexual characters, and likely reap substantial fitness benefits.     

Ecological drivers of group living in two populations of the communally rearing rodent, Octodon degus

Intraspecific variation in sociality is thought to reflect a trade-off between current fitness benefits and costs that emerge from individuals' decision to join or leave groups. Since those benefits and costs may be influenced by ecological conditions, ecological variation remains a major, ultimate cause of intraspecific variation in sociality. Intraspecific comparisons of mammalian sociality across populations facing different environmental conditions have not provided a consistent relationship between ecological variation and group-living. Thus, we studied two populations of the communally rearing rodent Octodon degus to determine how co-variation between sociality and ecology supports alternative ecological causes of group living. In particular, we examined how variables linked to predation risk, thermal conditions, burrowing costs, and food availability predicted temporal and population variation in sociality. Our study revealed population and temporal variation in total group size and group composition that covaried with population and yearly differences in ecology. In particular, predation risk and burrowing costs are supported as drivers of this social variation in degus. Thermal differences, food quantity and quality were not significant predictors of social group size. In contrast to between populations, social variation within populations was largely uncoupled from ecological differences.     

A power-efficiency trade-off in resource use alters epidemiological relationships

Trade-offs play pivotal roles in the ecology and evolution of natural populations. However, trade-offs are probably not static, invariant relationships. Instead, ecological factors can shift, alter, or reverse the relationships underlying trade-offs and create critical genotype x environment (G x E) interactions. But which ecological factors alter trade-offs or create G x E interactions, and why (mechanistically) do they do this? We tackle these questions using resource quality as the central ecological factor and a case study of disease in the plankton. We show that clonal genotypes of a zooplankton host (Daphnia dentifera) exhibit a "power-efficiency" trade-off in resource use, where powerful (fast-feeding) host clones perform well on richer algal resources, but more efficient (slow-feeding) clones perform relatively well on poorer resources. This resource-based trade-off then influences epidemiological relationships due to fundamental connections between resources and fecundity, transmission rate (an index of resistance), and replication of a virulent fungal parasite (Metschnikowia bicuspidata) within hosts. For instance, using experiments and dynamic energy budget models, we show that the power-efficiency trade-off overturned a previously detected trade-off between fecundity and transmission risk of hosts to this parasite. When poorer resources were eaten, transmission risk and fecundity were negatively, not positively, correlated. Additionally, poor resource quality changed positive relationships between yield of infectious stages (spores) and host fecundity: those fecundity-spore relationships with poor food became negative or nonsignificant. Finally, the power-efficiency trade-off set up an interaction between host clone and resource quality for yield of fungal spores: powerful clones yielded relatively more spores on the better resource, while efficient clones yielded relatively more on the poorer resource. Thus, the physiological ecology of resource use can offer potent, mechanistic insight linking environmental factors to epidemiological relationships.     

Foraging in a variable environment: weather patterns and the behavioral ecology of baboons

We investigated the long-standing premise in behavioral ecology that the environment affects behavior and demography. We did this by evaluating the extent to which year-to-year variability in the behavioral ecology of a nonhuman primate population could be modeled from meteorological patterns. Data on activity profiles and home range use for baboons (Papio cynocephalus) in Amboseli, Kenya, were obtained over a 10-year period for three social groups: two completely wild-foraging ones, and a third that supplemented its diet with refuse from a nearby tourist lodge. The relationships across years among activity budgeting, travel distance, group size, and measures of temperature and rainfall patterns differed among the social groups. Although meteorological variation generally correlated with behavioral variation in the completely wild-foraging groups, different weather variables and direction of relationships resulted for each group. In addition, different relationships among variables were found before and after home-range shifts. The food-enhanced group spent half as much time foraging as did the other groups and therefore could be used to evaluate the relative extent to which foraging time was a limiting factor for resting and social time. Under their relaxed ecological conditions, the food-enhanced animals increased resting time much more than social time. These findings, combined with supplementary information on the population, lead us to suggest that baboons use a suite of interrelated responses to ecological variability that includes not only changes in activity budgets, but also home-range shifts, changes in the length of the active period, and changes in group size through fissions. Moreover, our results imply that group differences as well as interpopulational and interspecific differences in behavioral ecology provide significant sources of variability. Therefore, social groups rather than populations may be the appropriate unit of analysis for understanding the behavioral ecology of baboons and other highly social primates. The different patterns we observed among groups may have fitness consequences for the individuals in those groups and thereby affect population structure over time. Received: 18 February 1995/Accepted after revision: 6 January 1996     

Social organisation of thornbill-dominated mixed-species flocks using social network analysis

Mixed-species associations are a widespread phenomenon, comprising interacting heterospecific individuals which gain predator, foraging or social benefits. Avian flocks have traditionally been classified as monolithic species units, with species-wide functional roles, such as nuclear, active, passive, or follower. It has also been suggested that flocks are mutualistic interactions, where niches of participating species converge. However the species-level perspective has limited previous studies, because both interactions and benefits occur at the level of the individual. Social network analysis provides a set of tools for quantitative assessment of individual participation. We used mark-resighting methods to develop networks of nodes (colour-marked individuals) and edges (their interactions within flocks). We found that variation in flock participation across individuals within species, especially in the buff-rumped thornbill, encompassed virtually the entire range of variation across all individuals in the entire set of species. For example, female, but not male, buff-rumped thornbills had high network betweenness, indicating that they interact with multiple flocks, likely as part of a female-specific dispersal strategy. Finally, we provide new evidence that mixed-species flocking is mutualistic, by quantifying an active shift in individual foraging niches towards those of their individual associates, with implications for trade-off between costs and benefits to individuals derived from participating in mixed-species flocks. This study is, to our knowledge, the first instance of a heterospecific social network built on pairwise interactions.     

Social stimulation,nuptial colouration,androgens and immunocompetence in a sexual dimorphic cichlid fish

The nature of the costs maintaining honesty of sexual signalling in inter- and intrasexual interactions remains a contentious issue. For carotenoid-based colour ornaments, it has been hypothesized that the honesty of the signal is enforced when carotenoid allocation to colour expression is traded off against carotenoid availability for immunocompetence. In addition, honesty is enforced if androgens required for colour expression are immunosuppressive. We tested whether there is a trade-off between colour expression and immunocompetence in a lek-breeding haplochromine cichlid fish, Pundamilia nyererei, from Lake Victoria with a carotenoid-based nuptial dress. First, we showed that expression of red nuptial colouration and social rank in a group were positively correlated. We then successfully manipulated the level of colour advertisement by socially stimulating individually housed males with a rival male: Stimulated males developed larger areas of red nuptial colouration and had higher levels of circulating 11-ketotestosterone (11-KT) than non-stimulated males. We subsequently tested the humoral immune response to a novel antigen (sheep red blood cells). There was no overall significant effect of social stimulation on antibody production, but in the stimulated males, the degree of red colouration and the antibody response were negatively correlated. This significant interaction between colour and treatment thus shows that the negative correlation between colouration and antibody production is dependent on the (manipulated) social environment. This study provides correlational and experimental evidence for a trade-off between expression of a sexual trait and one component of immune function in fish. We found no evidence for a direct effect of 11-KT on antibody production, and we discuss alternative mechanisms that could mediate the trade-off.     

Flight initiation distance decreases during social activity in lizards (<Emphasis Type="Italic">Sceloporus virgatus</Emphasis>)

Flight initiation distance, the predator–prey distance when escape begins, is predicted by escape theory to decrease if fleeing entails loss of benefits. Shortening of flight initiation distance during social interactions is known only in males and only in a few species. In a previous study, male, but not female, Sceloporus virgatus lizards had shorter flight initiation distance when interacting with tethered conspecifics. Females in that study were not gravid or close to ovulating. I predicted that flight initiation distance would be shorter in gravid females that perform sidle-hopping displays to reject courtship than in lone females. I tested this prediction and examined effects of social interactions by males with free-ranging conspecifics to ensure that previous findings were not artifacts of tethering and experimental introduction of conspecifics. Flight initiation distance was shorter in females when interacting with males than when alone; it was also shorter in males interacting with either sex. Thus, when beneficial for reproductive reasons, social interaction affects flight initiation distance in females, but at other times, it does not. Lesser shortening of flight initiation distance in females than males may be a consequence of greater social benefit to males and protection of reproductive investment by females.     

Sexual voyeurs and copiers: social copying and the audience effect on male mate choice in the guppy

In most mating systems, males and females are commonly within signalling and receiving distance of conspecifics during courtship and mating activities. Although it is well known that females who observe sexual interactions between conspecifics will use public information obtained from these interactions when making their own mating decisions, much less is known about whether males use this type of information in making mating decisions. We used the Trinidadian guppy (Poecilia reticulata) to test whether males use public information to (i) copy the apparent mate choice of another male and (ii) modify their mating preference for a given female in the presence of one or two sexual rivals (potential copiers). We show that males use public information to copy the mate choice of other males and that males alter their mating preferences in response to the presence of an audience of sexual rivals, but find no evidence of a stronger audience effect when the number of sexual rivals increases. Collectively, these results indicate that males pay attention to their immediate social environment in making mating decisions and suggest that they avoid having another male copy their mate choice by weakening or even reversing their initial mating preference in the presence of eavesdropping male sexual competitors. Our findings highlight the importance of social context and public information in male mate-choice decisions and have implications for the evolution of male mating preferences and of social information use in populations.     

Network position: a key component in the characterization of social personality types

In recent years, animal social interactions have received much attention in terms of personality research (e.g. aggressive or cooperative interactions). However, other components of social behaviour such as those describing the intensity, frequency, directedness and individual repeatability of interactions in animal groups have largely been neglected. Network analysis offers a valuable opportunity to characterize individual consistency of traits in labile social groups and therein provide novel insights to personality research in ways previously not possible using traditional techniques. Should individual network positions be consistently different between individuals under changing conditions, they might reflect expressions of an individual's personality. Here, we discuss a conceptual framework for using network analyses to infer the presence of individual differences and present a statistical test based on randomization techniques for testing the consistency of network positions in individuals. The statistical tools presented are useful because if particular individuals consistently occupy key positions in social networks, then this is also likely to have consequences for their fitness as well as for that of others in the population. These consequences may be particularly significant since individual network position has been shown to be important for the transmission of diseases, socially learnt information and genetic material between individuals and populations.     

Social network theory: new insights and issues for behavioral ecologists

Until recently, few studies have used social network theory (SNT) and metrics to examine how social network structure (SNS) might influence social behavior and social dynamics in non-human animals. Here, we present an overview of why and how the social network approach might be useful for behavioral ecology. We first note four important aspects of SNS that are commonly observed, but relatively rarely quantified: (1) that within a social group, differences among individuals in their social experiences and connections affect individual and group outcomes; (2) that indirect connections can be important (e.g., partners of your partners matter); (3) that individuals differ in their importance in the social network (some can be considered keystone individuals); and (4) that social network traits often carry over across contexts (e.g., SN position in male–male competition can influence later male mating success). We then discuss how these four points, and the social network approach in general, can yield new insights and questions for a broad range of issues in behavioral ecology including: mate choice, alternative mating tactics, male–male competition, cooperation, reciprocal altruism, eavesdropping, kin selection, dominance hierarchies, social learning, information flow, social foraging, and cooperative antipredator behavior. Finally, we suggest future directions including: (1) integrating behavioral syndromes and SNT; (2) comparing space use and SNS; (3) adaptive partner choice and SNS; (4) the dynamics and stability (or instability) of social networks, and (5) group selection shaping SNS. This contribution is part of the special issue “Social networks: new perspectives” (Guest Editors: J. Krause, D. Lusseau and R. James).     

Social attributes and associated performance measures in marmots: bigger male bullies and weakly affiliating females have higher annual reproductive success

Studying the structure of social interactions is fundamental in behavioral ecology as social behavior often influences fitness and thus natural selection. However, social structure is often complex, and determining the most appropriate measures of variation in social behavior among individuals can be difficult. Social network analysis generates numerous, but often correlated, measures of individual connectedness derived from a network of interactions. We used measures of individual connectedness in networks of affiliative and agonistic interactions in yellow-bellied marmots, Marmota flaviventris, to first determine how variance was structured among network measures. Principal component analysis reduced our set of network measures to four “social attributes” (unweighted connectedness, affiliation strength, victimization, and bullying), which revealed differences between patterns of affiliative and agonistic interactions. We then used these extracted social attributes to examine the relationship between an individual’s social attributes and several performance measures: annual reproductive success, parasite infection, and basal stress. In male marmots, bullying was positively associated with annual reproductive success, while in females, affiliation strength was negatively associated with annual reproductive success. No other social attributes were significantly associated with any performance measures. Our study highlights the utility of considering multiple dimensions when measuring the structure and functional consequences of social behavior.     

Maintenance of familiarity and social bonding via communal latrine use in a solitary primate (Lepilemur leucopus)

Latrine use (i.e., the repeated use of specific defecation/urination sites) has been described for several mammals, including carnivores, ungulates, and primates. However, the functional significance of latrine use in primates has not been studied systematically yet. We, therefore, followed 14 radio-collared individuals of the pair-living white-footed sportive lemur (Lepilemur leucopus) for 1097 hours of continuous focal observations to investigate latrine distribution, seasonality of latrine use, as well as age and sex of users to test various hypotheses related to possible functions of latrine use, including territory demarcation, resource defense, signaling of reproductive state, social bonding, and mate defense. All individuals of a social unit exhibited communal use of latrines located in the core area of their territory, supporting the social boding hypothesis. Latrine use seems to facilitate familiarity and social bonding within social units via olfactory communication in this primate that lives in family units but exhibits low levels of spatial cohesion and direct social interactions. In addition, frequency of latrine visitation was higher during nights of perceived intruder pressure, supporting the mate defense hypothesis. However, animals did not react to experimentally introduced feces from neighboring or strange social units, indicating that urine may be the more important component of latrines than feces in this arboreal species. Based on a survey of latrine use and function in other mammals, we conclude that latrines facilitate communication particularly in nocturnal species with limited habitat visibility and in species where individuals are not permanently cohesive because they constitute predictable areas for information exchange.     

Individual and collective decision-making during nest site selection by the ant Leptothorax albipennis

Social insect colonies possess remarkable abilities to select the best among several courses of action. In populous societies with highly efficient recruitment behaviour, decision-making is distributed across many individuals, each acting on limited local information with appropriate decision rules. To investigate the degree to which small societies with less efficient recruitment can also employ distributed decision-making, we studied nest site selection in Leptothorax albipennis. Colonies were found to make sophisticated choices, taking into account not only the intrinsic qualities of each site, but also its value relative to the available options. In choices between two sites, individual ants were able to visit both sites, compare them and choose the better one. However, most ants encountered only one site in the course of an emigration. These poorly informed ants also contributed to the colony's decision, because their probability of initiating recruitment to a site depended on its quality. This led to shorter latencies between discovery and recruitment to a superior site, and so created greater amplification via positive feedback of the population at the better site. In short, these small colonies make use of a distributed mechanism of information processing, but also take advantage of direct decision-making by well-informed individuals. The latter feature may in part stem from the limitations of their social structure, but may also reflect the stringent demand for unanimous decisions by house-hunting colonies of any size.     

Social network theory in the behavioural sciences: potential applications

Social network theory has made major contributions to our understanding of human social organisation but has found relatively little application in the field of animal behaviour. In this review, we identify several broad research areas where the networks approach could greatly enhance our understanding of social patterns and processes in animals. The network theory provides a quantitative framework that can be used to characterise social structure both at the level of the individual and the population. These novel quantitative variables may provide a new tool in addressing key questions in behavioural ecology particularly in relation to the evolution of social organisation and the impact of social structure on evolutionary processes. For example, network measures could be used to compare social networks of different species or populations making full use of the comparative approach. However, the networks approach can in principle go beyond identifying structural patterns and also can help with the understanding of processes within animal populations such as disease transmission and information transfer. Finally, understanding the pattern of interactions in the network (i.e. who is connected to whom) can also shed some light on the evolution of behavioural strategies.     

Ambient noise, motor fatigue, and serial redundancy in chaffinch song

Many animal signals are performed in a highly redundant manner as in some bird species where males sing several renditions of one song type before switching to another. However, differences in signal redundancy between contexts and between individuals are only poorly understood. We found that chaffinches (Fringilla coelebs) in noisier areas (i.e., close to waterfalls and torrents) sang longer bouts of the same song type before switching to a new type, suggesting that they use increased serial redundancy to get the message across in noisy conditions. This is the first evidence of a noise-dependent adjustment of signal redundancy in a songbird. In addition, we found that song types with faster trills were sung in shorter bouts suggesting that the performance of highly redundant song series is probably limited by motor constraints. Thus, in noisy environments, serial redundancy in bird song may reflect a trade-off between successful signal transmission and preventing motor fatigue.     

Seasonal changes in the structure of rhesus macaque social networks

Social structure emerges from the patterning of interactions between individuals and plays a critical role in shaping some of the main characteristics of animal populations. The topological features of social structure, such as the extent to which individuals interact in clusters, can influence many biologically important factors, including the persistence of cooperation, and the rate of spread of disease. Yet, the extent to which social structure topology fluctuates over relatively short periods of time in relation to social, demographic, or environmental events remains unclear. Here, we use social network analysis to examine seasonal changes in the topology of social structures that emerge from socio-positive associations in adult female rhesus macaques (Macaca mulatta). Behavioral data for two different association types (grooming and spatial proximity) were collected for females in two free-ranging groups during two seasons: the mating and birth seasons. Stronger dyadic bonds resulted in social structures that were more tightly connected (i.e., of greater density) in the mating season compared to the birth season. Social structures were also more centralized around a subset of individuals and more clustered in the mating season than those in the birth season, although the latter differences were mostly driven by differences in density alone. Our results suggest a degree of temporal variation in the topological features of social structure in this population. Such variation may feed back on interactions, hence affecting the behaviors of individuals, and may therefore be important to take into account in studies of animal behavior.     

Nest-site selection of passerines: effects of geographic scale and public and personal information

Nest-site selection is an important determinant of individual fitness in birds. Understanding what information individuals use to choose nest sites is therefore important for understanding the evolution of nest-site selection, the dynamics of populations, and the conservation of species. We used five years of mark-recapture data for Mountain Bluebirds (Sialia currucoides) to examine how dispersal probability and nest-site selection vary with potential cues of nest-site quality. Dispersal distance between breeding seasons and nest-site selection were modeled as a function of personal reproductive success, conspecific density, conspecific reproductive success, and habitat type. Between years, the dispersal probability was related to personal reproductive success, not conspecific information, and individuals fledging fewer young dispersed longer distances. For dispersing individuals, the probability that a nest site was selected in year i was negatively related to distance from the nest site selected in year i - 1 for all age and sex classes, and positively related to conspecific density and reproductive success in year i - 1 for both second-year (SY) and after-hatch-year (AHY) females. However, nest-site selection in year i was more strongly related to conspecific density in year i- 1 for hatch-year (HY) females and was much more strongly related to the reproductive success of conspecifics in year i - 1 for AHY females. Nest-site selection of HY and AHY males was not consistently related to the metrics of conspecific information, but we suspect that relationships were obscured by competitive interactions. We found no evidence indicating that individuals respond differently to conspecific information at longer distances, suggesting that individuals limit dispersal to areas where they have prior knowledge. We predict that these patterns of nest-site selection will allow birds to loosely track nest-site quality and maintain an ideal free distribution, where average fitness is equal in all habitat types.     

Decision Making in a Human Population Living Sustainably

Abstract: The Tiwi people of northern Australia have managed natural resources continuously for 6000–8000 years. Tiwi management objectives and outcomes may reflect how they gather information about the environment. We qualitatively analyzed Tiwi documents and management techniques to examine the relation between the social and physical environment of decision makers and their decision‐making strategies. We hypothesized that principles of bounded rationality, namely, the use of efficient rules to navigate complex decision problems, explain how Tiwi managers use simple decision strategies (i.e., heuristics) to make robust decisions. Tiwi natural resource managers reduced complexity in decision making through a process that gathers incomplete and uncertain information to quickly guide decisions toward effective outcomes. They used management feedback to validate decisions through an information loop that resulted in long‐term sustainability of environmental use. We examined the Tiwi decision‐making processes relative to management of barramundi (Lates calcarifer) fisheries and contrasted their management with the state government's management of barramundi. Decisions that enhanced the status of individual people and their attainment of aspiration levels resulted in reliable resource availability for Tiwi consumers. Different decision processes adopted by the state for management of barramundi may not secure similarly sustainable outcomes.     

Supplanting by olive baboons: dominance rank difference and resource value

Summary Interactions in which one animal takes over a food resource from another (feeding supplants) are usually interpreted as evidence of feeding competition, and the level of competition is predicted to be proportionate to the value of the resource. However, in this study of juvenile olive baboons (Papio anubis) living in a troop near Gilgil, Kenya, individuals were less selective about the quality of the supplant food under certain social circumstances. Two major food classes, Grass and Corm, were compared. Corms were less common in the diet, more difficult to locate as a replacement food after a supplant, and then at greater distances than Grass: in general, corms were the favoured supplant food, as predicted. But in interactions between animals who were close in rank, supplants over Grass occurred more often than expected. It is suggested here that, in these particular interactions, the winner benefitted socially, even at some cost of time or energy. Further, such interactions which may serve to reinforce the winner's status will be most productively directed to those close in rank, where relationships are potentially less stable. These data provide evidence that baboons can discriminate the relative ranks of others, rather than simply distinguishing between individuals who are higher- or lower-ranking than themselves.