Pairs of Fish Resolve Conflicts over Coordinated Movement by Taking Turns

Fitness benefits of alternated chick provisioning in cooperatively breeding carrion crows

In most bird species, parents raise offspring cooperatively. In some cases, this cooperation extends to helpers-at-the-nest who assist the breeders with a range of tasks. While cooperative food provisioning might merely arise incidentally, as a result of the efforts of carers that act independently from each other, recent studies suggest that birds may coordinate by taking turns in visiting the nest. However, evidence that such coordination emerges because individuals actively respond to each other's behaviour is controversial, and the potential benefits of carers' alternation remain unknown. We addressed this knowledge gap by analysing a multiyear dataset for cooperatively breeding carrion crows Corvus corone, comprising 8693 nest visits across 50 groups. Our results reveal that turn-taking does occur in this species and that all group members, regardless of their sex and social role (breeder/helper), tend to alternate at the nest with other carers rather than to make repeat visits. Importantly, we found that the body mass of nestlings increased significantly with the degree of carers' alternation, possibly because well-coordinated groups provided food at more regular intervals. Using earlier monitoring data, the observed increase in body mass is predicted to substantially boost postfledging survival rates. Our analyses demonstrate that alternation in nestling provisioning has measurable fitness benefits in this study system. This raises the possibility that cooperatively breeding carrion crows, as well as other bird species with similarly coordinated brood provisioning, exhibit specialized behavioural strategies that enable effective alternation.

Familiarity and sex modulate size-dependent following behaviour in the Mediterranean killifish

Living in a group offers the chance to follow the choices and the behaviours of other individuals. Following a group mate might confer fitness advantages if the group mate knows about resources such as food or shelters. Shoaling fish often follow larger group mates which, in most species, are generally older and therefore more experienced. Yet, the effect of individuals' characteristics other than size on following behaviour remains to be understood. For example, familiar fish and female shoals have been reported as more cohesive, which might be due to a differential tendency to follow in relation to familiarity and sex. Here, we investigated whether size, familiarity, sex, and the interaction between these factors affect fish following behaviour. We observed pairs of differently sized Mediterranean killifish, Aphanius fasciatus, exploring a new environment, and we recorded whether the rear fish followed the front fish when the latter changed swimming direction. In female and male pairs, and in unfamiliar pairs, smaller fish were more likely to follow the directional change of the larger fish than vice versa. In mixed-sex pairs and in familiar pairs, however, size did not affect following behaviour and larger fish followed as much as smaller fish did. Our results revealed that killifish's following decisions are determined by the size of the individuals, their level of familiarity, and their sex. These characteristics may have a notable impact on the behaviour of fish groups in nature.

Turn-taking in cooperative offspring care: by-product of individual provisioning behavior or active response rule?

Abstract

For individuals collaborating to rear offspring, effective organization of resource delivery is difficult because each carer benefits when the others provide a greater share of the total investment required. When investment is provided in discrete events, one possible solution is to adopt a turn-taking strategy whereby each individual reduces its contribution rate after investing, only increasing its rate again once another carer contributes. To test whether turn-taking occurs in a natural cooperative care system, here we use a continuous time Markov model to deduce the provisioning behavior of the chestnut-crowned babbler (Pomatostomus ruficeps), a cooperatively breeding Australian bird with variable number of carers. Our analysis suggests that turn-taking occurs across a range of group sizes (2–6), with individual birds being more likely to visit following other individuals than to make repeat visits. We show using a randomization test that some of this apparent turn-taking arises as a by-product of the distribution of individual inter-visit intervals (“passive” turn-taking) but that individuals also respond actively to the investment of others over and above this effect (“active” turn-taking). We conclude that turn-taking in babblers is a consequence of both their individual provisioning behavior and deliberate response rules, with the former effect arising through a minimum interval required to forage and travel to and from the nest. Our results reinforce the importance of considering fine-scale investment dynamics when studying parental care and suggest that behavioral rules such as turn-taking may be more common than previously thought.

Significance statement

Caring for offspring is a crucial stage in the life histories of many animals and often involves conflict as each carer typically benefits when others contribute a greater share of the work required. One way to resolve this conflict is to monitor when other carers contribute and adopt a simple “turn-taking” rule to ensure fairness, but natural parental care has rarely been studied in sufficient detail to identify such rules. Our study investigates whether cooperatively breeding chestnut-crowned babblers “take turns” delivering food to offspring, and (if so) whether this a deliberate strategy or simply a by-product of independent care behavior. We find that babblers indeed take turns and conclude that part of the observed turn-taking is due to deliberate responsiveness, with the rest arising from the species’ breeding ecology.

Electronic supplementary material

The online version of this article (10.1007/s00265-017-2391-4) contains supplementary material, which is available to authorized users.

Emergence of leadership in a robotic fish group under diverging individual personality traits

Variations of individual's personality traits have been identified before as one of the possible mechanisms for the emergence of leadership in an interactive collective, which may lead to benefits for the group as a whole. Complementing the large number of existing literatures on using simulation models to study leadership, we use biomimetic robotic fish to gain insight into how the fish's behaviours evolve under the influence of the physical hydrodynamics. In particular, we focus in this paper on understanding how robotic fish's personality traits affect the emergence of an effective leading fish in repeated robotic foraging tasks when the robotic fish's strategies, to push or not to push the obstacle in its foraging path, are updated over time following an evolutionary game set-up. We further show that the robotic fish's personality traits diverge when the group carries out difficult foraging tasks in our experiments, and self-organization takes place to help the group to adapt to the level of difficulties of the tasks without inter-individual communication.

Determinants of leadership in groups of female mallards

When moving in groups, social animals tend to follow a leader which successfully attracted them. Many variables are known to affect an individual’s propensity to act as a leader. Depending on their nature, these variables underlie two theoretical paradigms (i) ‘leadership according to social indifference’, characterised by differences in personality or sociability, or (ii) ‘leadership according to need’, characterised by differences in energetic requirements or information content. Currently, it is not clear under which circumstances each of the two paradigms plays a larger role. Here, we tried to understand these paradigms by observing collective movements in female mallards. Each of these mallards previously learned individually to associate one of four locations in a maze with food rewards. We then formed groups of various compositions (group size range: 2–5 individuals) with respect to personality, sociability, energetic requirements, motivation and information content. We found that groups remained cohesive, and that certain individuals were consistent leaders within and between trials. The order of entering the maze was mainly determined by energetic requirements. However, soon after entering the maze, the progression order changed. Then, more socially indifferent individuals took the lead and this new order remained constant until all individuals reached the final location, which was usually the one the leader had learned. In addition, we investigated the role of naïve individuals in group decision-making. In our setup, adding naïve individuals broke the leadership consistency between trials and increased fission events. Overall, our results show that the onset of collective movements may be driven by different mechanisms compared to the movement progression itself.

Behavioural plasticity across social contexts is regulated by the directionality of inter-individual differences

An individual's behavioural phenotype is a combination of its unique behavioural propensities and its responsiveness to environmental variation, also known as behavioural plasticity. In social species, we must not only explore how individuals respond to variations in the physical environment but also how they react to changes in their social environment. A growing body of work has demonstrated that the behavioural heterogeneity of a group can alter its responsiveness, decision making, and fitness. Whether an individual is more or less extreme than a partner - what we term its 'relative personality' - may also alter individual behavioural responses. We determined exploratory tendencies of individual zebrafish (Danio rerio) and then constructed pairs with varying differences in 'relative personality' to determine the effect of differences between partners on behavioural plasticity. We find that relative personality, but not the magnitude of the difference between partners, is the most important determinant of behavioural plasticity across social treatments. Despite this overall effect, pairs of fish exhibited no predictable leader-follower interactions, suggesting that details of the experimental paradigm may be important in shaping social dynamics.

Experience overrides personality differences in the tendency to follow but not in the tendency to lead

In many animal groups, coordinated activity is facilitated by the emergence of leaders and followers. Although the identity of leaders is to some extent predictable, most groups experience frequent changes of leadership. How do group members cope with such changes in their social role? Here, we compared the foraging behaviour of pairs of stickleback fish after a period of either (i) role reinforcement, which involved rewarding the shyer follower for following, and the bolder leader for leading, or (ii) role reversal, which involved rewarding the shyer follower for leading, and the bolder leader for following. We found that, irrespective of an individual's temperament, its tendency to follow is malleable, whereas the tendency to initiate collective movement is much more resistant to change. As a consequence of this lack of flexibility in initiative, greater temperamental differences within a pair led to improved performance when typical roles were reinforced, but to impaired performance when typical roles were reversed.

Schooling fish under attack are not all equal: some lead, others follow

Animal groups such as fish schools, bird flocks and insect swarms appear to move so synchronously that they have long been considered egalitarian, leaderless units. In schooling fish, video observations of their spatial-temporal organization have, however, shown that anti-predator manoeuvres are not perfectly synchronous and that individuals have spatial preferences within the school. Nonetheless, when facing life-or-death situations, it is not known whether schooling fish react to a threat following a random or a hierarchically-based order. Using high-speed video analysis, here we show that schooling fish (Golden grey mullet, Liza aurata) evade a threat in a non-random order, therefore individuals that are first or last to react tend to do so repeatedly over sequential stimulations. Furthermore, startle order is strongly correlated with individual positional preferences. Because school members are known to follow individuals that initiate a manoeuvre, early responders are likely to exert the strongest influence on the escape strategy of the whole school. Our results present new evidence of the intrinsic heterogeneity among school members and provide new rules governing the collective motion of gregarious animals under predator attack.

Determinants and outcomes of decision-making, group coordination and social interactions during a foraging experiment in a wild primate

Social animals have to coordinate joint movements to maintain group cohesion, but the latter is often compromised by diverging individual interests. A widespread behavioral mechanism to achieve coordination relies on shared or unshared consensus decision-making. If consensus costs are high, group fission represents an alternative tactic. Exploring determinants and outcomes of spontaneous group decisions and coordination of free-ranging animals is methodologically challenging. We therefore conducted a foraging experiment with a group of wild redfronted lemurs (Eulemur rufifrons) to study decision outcomes, coordination of movements, individual foraging benefits and social interactions in response to the presentation of drinking platforms with varying baiting patterns. Behavioral observations were complemented with data from recordings of motion detector cameras installed at the platforms. The animal's behavior in the experimental conditions was compared to natural group movements. We could not determine the type of consensus decision-making because the group visited platforms randomly. The group fissioned during 23.3% of platform visits, and fissioning resulted in more individuals drinking simultaneously. As under natural conditions, adult females initiated most group movements, but overtaking by individuals of different age and sex classes occurred in 67% of movements to platforms, compared to only 18% during other movements. As a result, individual resource intake at the platforms did not depend on departure position, age or sex, but on arrival order. Aggression at the platforms did not affect resource intake, presumably due to low supplanting rates. Our findings highlight the diversity of coordination processes and related consequences for individual foraging benefits in a primate group living under natural conditions.

The role of individuality in collective group movement

How different levels of biological organization interact to shape each other's function is a central question in biology. One particularly important topic in this context is how individuals' variation in behaviour shapes group-level characteristics. We investigated how fish that express different locomotory behaviour in an asocial context move collectively when in groups. First, we established that individual fish have characteristic, repeatable locomotion behaviours (i.e. median speeds, variance in speeds and median turning speeds) when tested on their own. When tested in groups of two, four or eight fish, we found individuals partly maintained their asocial median speed and median turning speed preferences, while their variance in speed preference was lost. The strength of this individuality decreased as group size increased, with individuals conforming to the speed of the group, while also decreasing the variability in their own speed. Further, individuals adopted movement characteristics that were dependent on what group size they were in. This study therefore shows the influence of social context on individual behaviour. If the results found here can be generalized across species and contexts, then although individuality is not entirely lost in groups, social conformity and group-size-dependent effects drive how individuals will adjust their behaviour in groups.

Decision-making in pigeon flocks: a democratic view of leadership

When travelling in groups, animals frequently have to make decisions on the direction of travel. These decisions can be based on consensus, when all individuals take part in the decision (i.e. democratic decision; social information), or leadership, when one member or a minority of members make the decision (i.e. despotic decision; personal information). Here we investigated whether decision-making on the navigation of small flocks is based on democratic or despotic decisions. Using individual and flock releases as the experimental approach, we compared the homing performances of homing pigeons that fly singly and in groups of three. Our findings show that although small groups were either governed (i.e. when individuals in the flock had age differences) or not (i.e. when individuals in the flock had the same age) by leaders, with concern to decision-making they were all ruled by democratic decisions. Moreover, the individual homing performances were not associated with leadership. Because true leaders did not assume right away the front position in the flock, we suggest that as in human groups, starting from a central position is more effective as it allows leaders to not only transmit their own information but also to average the tendencies of the other group members. Together, the results highlight the importance of democratic decisions in group decision-making.

Temperament and hunger interact to determine the emergence of leaders in pairs of foraging fish

Studies on leadership have focused either on physiological state as the key predictor (i.e. “leading according to need”), or else on temperamental asymmetries among group members (i.e. intrinsic leadership). In this paper, we explore how both factors interact in determining the emergence of leaders. We observed pairs of sticklebacks with varying degrees of temperamental difference, and recorded their movements back and forth between a safe covered area and a risky foraging area, both before and after satiating one of the two pair members (but not the other). Before satiation, when the fish had similar hunger levels, temperament was a good predictor of social roles, with the bolder member of a pair leading and the shyer member following. The effect of satiation depended on which fish received the additional food. When the shyer member of a pair was fed, and consequently became less active, the bolder fish did not change its behaviour but continued to lead. By contrast, when the bolder member of a pair was fed, and consequently initiated fewer trips out of cover, the shyer partner compensated by initiating trips more frequently itself. In pairs that differed only a little in temperament, feeding the bolder fish actually led to a role reversal, with the shyer fish emerging as a leader in the majority of joint trips out of cover. Our results show that leadership emerges as the consequence of multiple factors, and that their interaction can be complex.

Identification of learning mechanisms in a wild meerkat population

Vigorous debates as to the evolutionary origins of culture remain unresolved due to an absence of methods for identifying learning mechanisms in natural populations. While laboratory experiments on captive animals have revealed evidence for a number of mechanisms, these may not necessarily reflect the processes typically operating in nature. We developed a novel method that allows social and asocial learning mechanisms to be determined in animal groups from the patterns of interaction with, and solving of, a task. We deployed it to analyse learning in groups of wild meerkats (Suricata suricatta) presented with a novel foraging apparatus. We identify nine separate learning processes underlying the meerkats’ foraging behaviour, in each case precisely quantifying their strength and duration, including local enhancement, emulation, and a hitherto unrecognized form of social learning, which we term ‘observational perseverance’. Our analysis suggests a key factor underlying the stability of behavioural traditions is a high ratio of specific to generalized social learning effects. The approach has widespread potential as an ecologically valid tool to investigate learning mechanisms in natural groups of animals, including humans.

Initiative, personality and leadership in pairs of foraging fish

Studies of coordinated movement have found that, in many animal species, bolder individuals are more likely to initiate movement and shyer individuals to follow. Here, we show that in pairs of foraging stickleback fish, leadership is not merely a passive consequence of temperamental differences. Instead, the act of initiating a joint foraging trip out of cover itself brings about a change in the role that an individual plays throughout the subsequent trip, and success in recruiting a partner affects an individual's tendency to initiate the next trip. On each joint trip, whichever fish took the initiative in leading out of cover gains greater influence over its partner's behaviour, which persists even after several changes in position (i.e. termination attempts and re-joining). During any given trip, the initiator is less responsive to its partner's movements than during trips initiated by the partner. An individual's personality had an important effect on its response to failure to recruit a partner: while bold fish were unaffected by failures to initiate a joint trip, shy individuals were less likely to attempt another initiation after a failure. This difference provides a positive feedback mechanism that can partially stabilise social roles within the pair, but it is not strong enough to prevent occasional swaps, with individuals dynamically adjusting their responses to one another as they exchange roles.

The evolution of cooperative turn-taking in animal conflict

BACKGROUND

A fundamental assumption in animal socio-ecology is that animals compete over limited resources. This view has been challenged by the finding that individuals might cooperatively partition resources by "taking turns". Turn-taking occurs when two individuals coordinate their agonistic behaviour in a way that leads to an alternating pattern in who obtains a resource without engaging in costly fights. Cooperative turn-taking has been largely ignored in models of animal conflict and socio-ecological models that explain the evolution of social behaviours based only on contest and scramble competition. Currently it is unclear whether turn-taking should be included in socio-ecological models because the evolution of turn-taking is not well understood. In particular, it is unknown whether turn-taking can evolve when fighting costs and assessment of fighting abilities are not fixed but emerge from evolved within-fight behaviour. We address this problem with an evolutionary agent-based model.

RESULTS

We found that turn-taking evolves for small resource values, alongside a contest strategy that leads to stable dominance relationships. Turn-taking leads to egalitarian societies with unclear dominance relationships and non-linear dominance hierarchies. Evolutionary stability of turn-taking emerged despite strength differences among individuals and the possibility to evolve within-fight behaviour that allows good assessment of fighting abilities. Evolutionary stability emerged from frequency-dependent effects on fitness, which are modulated by feedbacks between the evolution of within-fight behaviour and the evolution of higher-level conflict strategies.

CONCLUSIONS

Our results reveal the impact of feedbacks between the evolution of within-fight behaviour and the evolution of higher-level conflict strategies, such as turn-taking. Similar feedbacks might be important for the evolution of other conflict strategies such as winner-loser effects or coalitions. However, we are not aware of any study that investigated such feedbacks. Furthermore, our model suggests that turn-taking could be used by animals to partition low value resources, but to our knowledge this has never been tested. The existence of turn-taking might have been overlooked because it leads to societies with similar characteristics that have been expected to emerge from scramble competition. Analyses of temporal interaction patterns could be used to test whether turn-taking occurs in animals.

Evolution of personality differences in leadership

When members of a group differ in their preferred course of action, coordination poses a challenge. Leadership offers one way to resolve this difficulty, but the evolution of leaders and followers is itself poorly understood. Existing discussions have frequently attributed leadership to differences in information or need among individuals. Here, however, we show that in an n-player, repeated coordination game, selection leads to evolutionary branching and diversification in intrinsic leadership among the members of a population even in the absence of any variation in state. When individuals interact in pairs, repeated branching is possible; when individuals interact in larger groups, the typical outcome is a single branching event leading to a dimorphism featuring extreme intrinsic leaders and followers. These personality types emerge and are maintained by frequency-dependent selection, because leaders gain by imposing their preferences on followers, but fail to coordinate effectively when interacting with other leaders. The fraction of intrinsic leaders in the population increases with the degree of conflict among group members, with both types common only at intermediate levels of conflict; when conflict is weak, most individuals are intrinsic followers, and groups achieve high levels of coordination by randomly converging on one individual's preferred option, whereas when conflict is strong, most individuals are intrinsic leaders, and coordination breaks down because members of a group are rarely willing to follow another.

Process-based approach reveals directional effects of environmental factors on movement between habitats

1. Understanding the effects of environmental factors on animal distributions is a central issue in ecology. However, movement rules inferred from distribution patterns do not reveal the processes through which animal distribution is realized. 2. We investigated individual movement rules using a process-based approach. In experiments, coastal fish larvae (red drum, Sciaenops ocellatus) were matched with an intraspecific competitor of different sizes, and time series of habitat transition of individuals were fitted with a continuous-time Markov chain model to evaluate the effects of the presence of a competitor, behavioural interactions and habitat quality on the likelihoods of habitat transition. 3. The process-based approach revealed that these factors did not simply act as a 'slope' between habitats that makes it easier to go in one direction and more difficult to return. Rather, these factors modify the movement rules differently depending on the directions of the movement. 4. Individuals were less likely to enter a better habitat in the presence of a larger conspecific, more likely to shift to a poorer habitat when they received aggressive behaviour and more likely to stay in a better habitat in the presence of food. However, no effect was found on the transition intensity for moving in the opposite direction. 5. The process-based approach to evaluating movement rules of animals allowed us to see the contrasting directional effects of different factors on the underlying movement rules used by animals, as opposed to pattern-based fitting of observed distributions. Consideration of these rules would improve the existing habitat-choice models.

Group decisions and individual differences: route fidelity predicts flight leadership in homing pigeons (Columba livia)

How social-living animals make collective decisions is currently the subject of intense scientific interest, with increasing focus on the role of individual variation within the group. Previously, we demonstrated that during paired flight in homing pigeons, a fully transitive leadership hierarchy emerges as birds are forced to choose between their own and their partner's habitual routes. This stable hierarchy suggests a role for individual differences mediating leadership decisions within homing pigeon pairs. What these differences are, however, has remained elusive. Using novel quantitative techniques to analyse habitual route structure, we show here that leadership can be predicted from prior route-following fidelity. Birds that are more faithful to their own route when homing alone are more likely to emerge as leaders when homing socially. We discuss how this fidelity may relate to the leadership phenomenon, and propose that leadership may emerge from the interplay between individual route confidence and the dynamics of paired flight.