Safety in numbers: the dilution effect and other drivers of group life in the face of danger

Shoal size as a key variable in fish behavioral ecotoxicology: an example using sertraline

A significant limitation of behavioral ecotoxicology is the challenge of obtaining reproducible results due to a wide range of testing conditions. In particular, shoal size affects almost all aspects of fish behavior, but is rarely considered as a factor in ecotoxicological studies. In the present study, we compared the swimming and antipredator responses of different sized shoals of Arabian killifish (Aphaniops stoliczkanus) after exposure to environmentally realistic concentrations of the antidepressant medication sertraline. Groups of fish (1, 3 or 5 individuals) were exposed to either 5 or 50 ng/L sertraline. After 37 days, swimming behavior and responses to a predator alarm were measured. We found that the effects of group size were much stronger than the effects of sertraline on swimming. Group size was also the major factor influencing responses to the predator alarm, with single fish showing the strongest responses. Sertraline directly affected acceleration, turning speed and average distance to the arena wall. For all three parameters, there were significant interactions with shoal size, demonstrating that responses differed depending on the size of the group. We also found that effects of sertraline could still be observed 14 days after cessation of exposure. The study highlights the importance of considering social context and specifically shoal size when designing behavioral studies on chemicals. Failure to consider this may result in over- or under-estimation of risks.

Spatial memory obviates following behaviour in an information centre of wild fruit bats

According to the information centre hypothesis (ICH), colonial species use social information in roosts to locate ephemeral resources. Validating the ICH necessitates showing that uninformed individuals follow informed ones to the new resource. However, following behaviour may not be essential when individuals have a good memory of the resources' locations. For instance, Egyptian fruit bats forage on spatially predictable trees, but some bear fruit at unpredictable times. These circumstances suggest an alternative ICH pathway in which bats learn when fruits emerge from social cues in the roost but then use spatial memory to locate them without following conspecifics. Here, using an unique field manipulation and high-frequency tracking data, we test for this alternative pathway: we introduced bats smeared with the fruit odour of the unpredictably fruiting Ficus sycomorus trees to the roost, when they bore no fruits, and then tracked the movement of conspecifics exposed to the manipulated social cue. As predicted, bats visited the F. sycomorus trees with significantly higher probabilities than during routine foraging trips (of >200 bats). Our results show how the integration of spatial memory and social cues leads to efficient resource tracking and highlight the value of using large movement datasets and field experiments in behavioural ecology. This article is part of the theme issue 'The spatial-social interface: a theoretical and empirical integration'.

The impact of predators and vegetation on shoaling in wild zebrafish

In their natural habitats, animals experience multiple ecological factors and regulate their social responses accordingly. To unravel the impact of two ecological factors on the immediate behavioural response of groups, we conducted experiments on wild zebrafish shoals in arenas with vegetation, predator cues, and both factors simultaneously or neither (control treatments). Analysis of 297 trials revealed that while shoals formed significantly larger subgroups in the presence of predator cues, their subgroup size was comparable to control treatments when they faced predator cues and vegetation. Shoals were highly polarized in open arenas, in the absence of either ecological factors and in the presence of predator cues (with/without vegetation). The presence of vegetation alone, however, significantly reduced shoal polarization. Furthermore, food intake was significantly reduced when predator cues and/or vegetation were present. Tracking individuals revealed that (i) individuals within shoals receiving predator cues had a significantly higher probability to continue being in a group compared with control treatments and (ii) individuals occupying the front positions deviated less from their median position within a shoal as compared with other individuals regardless of predator cues. The adaptability of animals depends on behavioural responses to changing environments, making this study significant in the context of environmental changes.

Behavioural changes of native freshwater prawn in the presence of a predator fish

Predation pressure influences animal behaviour in relation to feeding, exposure to predators and habitat use. In the presence of a predator, prey usually decreases feeding and activity, avoiding predation-risk areas. This study evaluated the visual effects of a predator fish (red-bellied piranha Pygocentrus nattereri) on the behaviour and habitat use of Macrobrachium jelskii prawn. It was hypothesized that prawns would modify their behaviour in the presence of a predator, decreasing their general activity (foraging and locomotion) and seeking shelter more frequently. Twenty behavioural tests were carried out without and with the presence of a predator using five different M. jelskii individuals per trial. Behavioural data were collected using scan sampling with instantaneous recording of behaviours every 15 s during 5 min in each trial. The presence of the predator caused prawns to decrease their locomotion and feeding and increase environment exploration and predator inspection. Prawns used the shelter more frequently when the predator was present. Red-bellied piranha elicited anti-predator behaviour in M. jelskii. The patterns observed suggested that the most common anti-predatory behaviours exhibited by M. jelskii are beneficial as they reduce vigilance and active anti-predation strategies, such as escape.

The sociality of sleep in animal groups

Group-living animals sleep together, yet most research treats sleep as an individual process. Here, we argue that social interactions during the sleep period contribute in important, but largely overlooked, ways to animal groups' social dynamics, while patterns of social interaction and the structure of social connections within animal groups play important, but poorly understood, roles in shaping sleep behavior. Leveraging field-appropriate methods, such as direct and video-based observation, and increasingly common on-animal motion sensors (e.g., accelerometers), behavioral indicators can be tracked to measure sleep in multiple individuals in a group of animals simultaneously. Sleep proximity networks and sleep timing networks can then be used to investigate the collective dynamics of sleep in wild group-living animals.

Inferring stochastic group interactions within structured populations via coupled autoregression

The internal behaviour of a population is an important feature to take account of when modelling its dynamics. In line with kin selection theory, many social species tend to cluster into distinct groups in order to enhance their overall population fitness. Temporal interactions between populations are often modelled using classical mathematical models, but these sometimes fail to delve deeper into the, often uncertain, relationships within populations. Here, we introduce a stochastic framework that aims to capture the interactions of animal groups and an auxiliary population over time. We demonstrate the model's capabilities, from a Bayesian perspective, through simulation studies and by fitting it to predator-prey count time series data. We then derive an approximation to the group correlation structure within such a population, while also taking account of the effect of the auxiliary population. We finally discuss how this approximation can lead to ecologically realistic interpretations in a predator-prey context. This approximation also serves as verification to whether the population in question satisfies our various assumptions. Our modelling approach will be useful for empiricists for monitoring groups within a conservation framework and also theoreticians wanting to quantify interactions, to study cooperation and other phenomena within social populations.

Chemical and Visual Cues as Modulators of the Stress Response to Social Isolation in the Marine Medaka, Oryzias melastigma

The marine medaka is emerging as a potential behavioral model organism for ocean studies, namely on marine ecotoxicology. However, not much is known on the behavior of the species and behavioral assays lack standardization. This study assesses the marine medaka as a potential model for chemical communication. We investigated how short exposure to visual and chemical cues mediated the stress response to social isolation with the light/dark preference test (LDPT) and the open field test (OFT). After a 5-day isolation period, and 1 h before testing, isolated fish were randomly assigned to one of four groups: (1) placed in visual contact with conspecifics; (2) exposed to a flow of holding water from a group of conspecifics; (3) exposed to both visual and chemical cues from conspecifics; or (4) not exposed to any stimuli (controls). During the LDPT, the distance traveled and transitions between zones were more pronounced in animals exposed to the conspecific's chemical stimuli. The time spent in each area did not differ between the groups, but a clear preference for the bright area in all animals indicates robust phototaxis. During the OFT, animals exposed only to chemical cues initially traveled more than those exposed to visual or both stimuli, and displayed lower thigmotaxis. Taken together, results show that chemical cues play a significant role in exploratory behavior in this species and confirm the LDPT and OFT as suitable tests for investigating chemical communication in this species.

The relative importance of social information use for population abundance in group-living and non-grouping prey

Predator-prey relationships are fundamental components of ecosystem functioning, within which the spatial consequences of prey social organization can alter predation rates. Group-living (GL) species are known to exploit inadvertent social information (ISI) that facilitates population persistence under predation risk. Still, the extent to which non-grouping (NG) prey can benefit from similar processes is unknown. Here we built an individual-based model to explore and compare the population-level consequences of ISI use in GL and NG prey. We differentiated between GL and NG prey only by the presence or absence of social attraction toward conspecifics that drives individual movement patterns. We found that the extent of the benefits of socially acquired predator information in NG highly depends on the prey's ability to detect nearby predators, prey density and the occurrence of false alarms. Conversely, even moderate probabilities of ISI use and predator detection can lead to maximal population-level benefits in GL prey. This theoretical work provides additional insights into the conditions under which ISI use can facilitate population persistence irrespective of prey social organisation.

Widow spiders alter web architecture and attractiveness in response to same-sex competition for prey and mates, and predation risk

Female-female competition in animals has rarely been studied. Responses of females that compete context-dependently for mates and prey, and seek safety from predators, are ideally studied with web-building spiders. Cobwebs possess unique sections for prey capture and safety, which can be quantified. We worked with Steaoda grossa females because their pheromone is known, and adjustments in response to mate competition could be measured. Females exposed to synthetic sex pheromone adjusted their webs, indicating a perception of intra-sexual competition via their sex pheromone. When females sequentially built their webs in settings of low and high intra-sexual competition, they adjusted their webs to increase prey capture and lower predation risk. In settings with strong mate competition, females deposited more contact pheromone components on their webs and accelerated their breakdown to mate-attractant pheromone components, essentially increasing their webs' attractiveness. We show that females respond to sexual, social and natural selection pressures originating from intra-sexual competition.

Combined effects of light pollution and vegetation height on behavior and body weight in a nocturnal rodent

At a global scale, organisms are under threat due to various kinds of environmental changes, such as artificial light at night (ALAN), noise, climatic change and vegetation destruction. Usually, these changes co-vary in time and space and may take effect simultaneously. Although impacts of ALAN on biological processes have been well documented, our knowledge on the combined effects of ALAN and other environmental changes on animals remains limited. In this study, we conducted field experiments in semi-natural enclosures to explore the combined effects of ALAN and vegetation height on foraging behavior, vigilance, activity patterns and body weight in dwarf striped hamsters (Cricetulus barabensis), a nocturnal rodent widely distributed in East Asia. We find that ALAN and vegetation height affected different aspects of behavior. ALAN negatively affected search speed and positively affected handling speed, while vegetation height negatively affected giving-up density and positively affected body weight. ALAN and vegetation height also additively shaped total time spent in a food patch. No significant interactive effect of ALAN and vegetation height was detected. C. barabensis exposed to ALAN and short vegetation suffered a significant loss in body weight, and possessed a much narrower temporal niche (i.e. initiated activity later but became inactive earlier) than those under other combinations of treatments. The observed behavioral responses to ALAN and changes in vegetation height may bring fitness consequences, as well as further changes in structure and functioning of local ecosystems.

Guppies in large groups cooperate more frequently in an experimental test of the group size paradox

The volunteer's dilemma, in which a single individual is required to produce a public good, predicts that individuals in larger groups will cooperate less frequently. Mechanistically, this could result from trade-offs between costs associated with volunteering and costs incurred if the public good is not produced (nobody volunteers). During predator inspection, one major contributor to the cost of volunteering is likely increased probability of predation; however, a predator also poses a risk to all individuals if nobody inspects. We tested the prediction that guppies in larger groups will inspect a predator less than those in smaller groups. We also predicted that individuals in larger groups would perceive less threat from the predator stimulus because of the protective benefits of larger groups (e.g. dilution). Contrary to prediction, we found that individuals in large groups inspected more frequently than those in smaller groups, but (as predicted) spent less time in refuges. There was evidence that individuals in intermediate-sized groups made fewest inspections and spent most time in refuges, suggesting that any link between group size, risk and cooperation is not driven by simple dilution. Extensions of theoretical models that capture these dynamics will likely be broadly applicable to risky cooperative behaviour.

Warning Coloration, Body Size, and the Evolution of Gregarious Behavior in Butterfly Larvae

AbstractMany species gain antipredator benefits by combining gregarious behavior with warning coloration, yet there is debate over which trait evolves first and which is the secondary adaptive enhancement. Body size can also influence how predators receive aposematic signals and potentially constrain the evolution of gregarious behavior. To our knowledge, the causative links between the evolution of gregariousness, aposematism, and larger body sizes have not been fully resolved. Here, using the most recently resolved butterfly phylogeny and an extensive new dataset of larval traits, we reveal the evolutionary interactions between important traits linked to larval gregariousness. We show that larval gregariousness has arisen many times across butterflies, and aposematism is a likely prerequisite for gregariousness to evolve. We also find that body size may be an important factor for determining the coloration of solitary, but not gregarious, larvae. Additionally, by exposing artificial larvae to wild avian predation, we show that undefended, cryptic larvae are heavily predated when aggregated but benefit from solitariness, whereas the reverse is true for aposematic prey. Our data reinforce the importance of aposematism for gregarious larval survival while identifying new questions about the roles of body size and toxicity in the evolution of grouping behavior.

Imperfect ant mimicry contributes to local adaptation in a jumping spider

Putative ant mimicry is a remarkable example of an evolutionary strategy that can be well integrated into the framework of natural selection and adaptation. However, challenges remain in understanding imperfect ant mimicry. Here, we combine trait quantification and behavioral assays to investigate imperfect ant mimicry in the jumping spider Siler collingwoodi. We performed trajectory analysis and gait analysis to show that the locomotor characters of S. collingwoodi generally resemble those of the putative ant models, supporting the multiple models hypothesis. We then performed background-matching analysis, revealing that body coloration may be involved in background camouflage. We further carried out antipredation assays and found that S. collingwoodi had a significantly lower risk of predation than nonmimetic salticids, suggesting an overall protective effect of Batesian mimicry. Our findings quantitatively demonstrate a combination of mimicry and camouflage in S. collingwoodi and thus highlight the significance of a complex phenomenon driven by natural selection.

Could non-native species boost their chances of invasion success by socializing with natives?

Most invasions start with the introduction of a few individuals and the majority fail to establish and become invasive populations. A possible explanation for this is that some species are subject to Allee effects-disadvantages of low densities-and fail to perform vital activities due to the low availability of conspecifics. We propose that 'facilitation' from native individuals to non-natives through heterospecific sociability could enhance chances of the latter establishing in novel environments by helping them avoid Allee effects and even reducing the minimum number of non-native individuals necessary to achieve the density for a viable population (the Allee effect threshold). There is evidence from experiments carried out with freshwater fish, snails, lizards, mussels and bird that supports the idea of heterospecific sociability between native and non-native species as a process to promote invasion success. We propose that to understand invasion success in social non-native species we need to investigate how they integrate into the recipient community. Furthermore, to manage them, it may be necessary to reduce population density not just below the Allee effect threshold but also to understand how natives could help them shift the conspecific Allee effect threshold to their benefit. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Mechanisms of collective learning: how can animal groups improve collective performance when repeating a task?

Learning is ubiquitous in animals: individuals can use their experience to fine-tune behaviour and thus to better adapt to the environment during their lifetime. Observations have accumulated that, at the collective level, groups can also use their experience to improve collective performance. Yet, despite apparent simplicity, the links between individual learning capacities and a collective's performance can be extremely complex. Here we propose a centralized and broadly applicable framework to begin classifying this complexity. Focusing principally on groups with stable composition, we first identify three distinct ways through which groups can improve their collective performance when repeating a task: each member learning to better solve the task on its own, members learning about each other to better respond to one another and members learning to improve their complementarity. We show through selected empirical examples, simulations and theoretical treatments that these three categories identify distinct mechanisms with distinct consequences and predictions. These mechanisms extend well beyond current social learning and collective decision-making theories in explaining collective learning. Finally, our approach, definitions and categories help generate new empirical and theoretical research avenues, including charting the expected distribution of collective learning capacities across taxa and its links to social stability and evolution. This article is part of a discussion meeting issue 'Collective behaviour through time'.

Density-dependent habitat selection alters drivers of population distribution in northern Yellowstone elk

Although it is well established that density dependence drives changes in organismal abundance over time, relatively little is known about how density dependence affects variation in abundance over space. We tested the hypothesis that spatial trade-offs between food and safety can change the drivers of population distribution, caused by opposing patterns of density-dependent habitat selection (DDHS) that are predicted by the multidimensional ideal free distribution. We addressed this using winter aerial survey data of northern Yellowstone elk (Cervus canadensis) spanning four decades. Supporting our hypothesis, we found positive DDHS for food (herbaceous biomass) and negative DDHS for safety (openness and roughness), such that the primary driver of habitat selection switched from food to safety as elk density decreased from 9.3 to 2.0 elk/km² . Our results demonstrate how population density can drive landscape-level shifts in population distribution, confounding habitat selection inference and prediction and potentially affecting community-level interactions.

Testing the environmental buffering hypothesis of cooperative breeding in the Seychelles warbler

Species are facing environmental challenges caused by rapidly changing environments. Globally, extreme weather events, like droughts or extreme rainfall, are increasing in frequency. Natural selection usually acts slowly, while adaptations through phenotypic plasticity are limited. Therefore, organisms may utilise other mechanisms to cope with such rapid change. Cooperative breeding is hypothesised to be one such mechanism, as helpers could increase survival probabilities of offspring, especially in harsh years. Rainfall is a cue for onset of breeding in many tropical species, to ensure young are born when food abundance is highest. Using 21 years of data, we investigate the effect of rainfall on social behaviour and life history in the insectivorous Seychelles warbler (Acrocephalus sechellensis), a facultative cooperative breeder. We found that low rainfall is associated with reduced reproductive output and possibly with decreased survival. However, there were no statistical differences in response between groups with helpers, groups with only non-helping subordinates, and breeding pairs without subordinates. With low rainfall, more sons (the sex less likely to help) were produced, and those subordinate males already present were less likely to help. Thus, in contrast to expectations, cooperative breeding does not seem to buffer against harsh environments in Seychelles warblers, indicating that group living may be costly and thus not a mechanism for coping with changing environments. Our study showed that the interaction between the environment and life histories, including social behaviour, is complex, but that this interaction is important to consider when studying the impact of changing environments on species survival.

Anticipatory Threat Mitigates the Breakdown of Group Cooperation

Humans are exposed to environmental and economic threats that can profoundly affect individual survival and group functioning. Although anecdotal evidence suggests that threat exposure can increase collective action, the effects of threat on decision-making have been mainly investigated at the individual level. Here we examine how threat exposure and concomitant physiological responses modulate cooperation in small groups. Individuals (N = 105, ages 18-34 years) in groups of three were exposed to threat of electric shocks while deciding how much to contribute to a public good. Threat of shock induced a state of physiological freezing and, compared with no-threat conditions, reduced free riding and enabled groups to maintain higher cooperation over time. Exploratory analyses revealed that more cooperative responses under threat were driven by stronger baseline prosociality, suggesting that habitual prosociality is reinforced under threat. The current results support the view that human groups respond to outside threat with increased cooperation.

Subordinate submissive responses are predicted by dominant behaviour in a cooperatively breeding fish

In complex social groups, animals rely on communication to facilitate priority access to resources and minimise the costs of conflict. Animals typically have more aggression signals than submission signals. However, some social species do show multiple submission signals, and the context in which these different signals are used is often not well understood. In the current study, we assessed agonistic interactions within groups of the cooperatively breeding daffodil cichlid fish (Neolamprologus pulcher) to investigate the relationship between the aggressive behaviours of the dominant breeding pair, and the submissive responses of the highest ranked subordinate within the group. Daffodil cichlids may respond to aggression by fleeing or by the production of either a tail quiver display or a head up display. Among the two submission signals, the tail quiver display was used more frequently in response to a threat display, while head up displays were produced approximately equally in response to both threat displays and overt aggression. An exaggerated version of the head up display was given more often in response to overt aggression, suggesting a graded submissive response both within and between the two submission signals. Within fish, the frequency of head up displays, but not tail quiver displays, correlated positively with the frequency of threat displays received. The current study helps us to better understand the use of submission signals in a highly social vertebrate and sheds light on submission as an understudied aspect of communication.

Female zebrafish (Danio rerio) demonstrate stronger preference for established shoals over newly-formed shoals in the three-tank open-swim preference test

Zebrafish (Danio rerio) share a considerable amount of biological similarity with mammals, including identical or homologous gene expression pathways, neurotransmitters, hormones, and cellular receptors. Zebrafish also display complex social behaviors like shoaling and schooling, making them an attractive model for investigating normal social behavior as well as exploring impaired social function conditions such as autism spectrum disorders. Newly-formed and established shoals exhibit distinct behavior patterns and inter-member interactions that can convey the group's social stability. We used a three-chamber open-swim preference test to determine whether individual zebrafish show a preference for an established shoal over a newly-formed shoal. Results indicated that both sexes maintained greater proximity to arena zones nearest to the established shoal stimulus. In addition, we report the novel application of Shannon entropy to discover sex differences in systematicity of responses not revealed by unit-based measurements; male subjects spent more time investigating between the two shoals than female subjects. This novel technique using established versus newly-formed shoals can be used in future studies testing transgenics and pharmacological treatments that mimic autism spectrum disorder and other disorders that affect social interaction.

Raptors avoid the confusion effect by targeting fixed points in dense aerial prey aggregations

Collective behaviours are widely assumed to confuse predators, but empirical support for a confusion effect is often lacking, and its importance must depend on the predator's targeting mechanism. Here we show that Swainson's Hawks Buteo swainsoni and other raptors attacking swarming Mexican Free-tailed Bats Tadarida brasiliensis steer by turning towards a fixed point in space within the swarm, rather than by using closed-loop pursuit of any one individual. Any prey with which the predator is on a collision course will appear to remain on a constant bearing, so target selection emerges naturally from the geometry of a collision. Our results show how predators can simplify the demands on their sensory system by decoupling steering from target acquisition when capturing prey from a dense swarm. We anticipate that the same tactic will be used against flocks and schools across a wide range of taxa, in which case a confusion effect is paradoxically more likely to occur in attacks on sparse groups, for which steering and target acquisition cannot be decoupled.

Timescale reverses the relationship between host density and infection risk

Host density shapes infection risk through two opposing phenomena. First, when infective stages are subdivided among multiple hosts, greater host densities decrease infection risk through 'safety in numbers'. Hosts, however, represent resources for parasites, and greater host availability also fuels parasite reproduction. Hence, host density increases infection risk through 'density-dependent transmission'. Theory proposes that these phenomena are not disparate outcomes but occur over different timescales. That is, higher host densities may reduce short-term infection risk, but because they support parasite reproduction, may increase long-term risk. We tested this theory in a zooplankton-disease system with laboratory experiments and field observations. Supporting theory, we found that negative density-risk relationships (safety in numbers) sometimes emerged over short timescales, but these relationships reversed to 'density-dependent transmission' within two generations. By allowing parasite numerical responses to play out, time can shift the consequences of host density, from reduced immediate risk to amplified future risk.

Influence of weather on the behaviour of reintroduced Przewalski’s horses in the Great Gobi B Strictly Protected Area (Mongolia): implications for conservation

Background

Reintroduction is a common technique for re-establishing threatened species. However, the adaptation to novel habitats with distinct conditions poses a risk of failure. Weather conditions affect the behaviour of animals, and thus, their adaptation to new conditions and survival. Reintroduced Przewalski’s horses living in Mongolia’s continental arid climate with extreme temperature and precipitation variability, serve as an ideal model species for studying the behavioural response of selected groups to these harsh conditions.

Methods

The research was conducted in The Great Gobi B Strictly Protected Area, Mongolia. In summer 2018, three groups were recorded (Azaa, Tsetsen and Mares18) involving 29 individuals. In Spring 2019, 4 groups were recorded (Azaa, Tsetsen, Hustai1 and Mares19) involving 34 individuals. In Autumn 2019, 4 groups were recorded (Azaa, Tsetsen, Hustai2 and Tanan) involving 35 individuals. Thirteen weather variables were recorded in 10-min intervals, together with the percentage representation of selected behavioural categories (feeding, locomotion, resting, and social). The effect of weather on behaviour was analysed through GLMM. Influence of the group-history factors (recently reintroduced, long-term reintroduced and wild-born) was also analysed.

Results

Feeding significantly increased with cloudy and windy conditions and was more frequent in autumn than spring and summer. Locomotion was positively explained by temperature and cloudiness and was higher in summer than spring and autumn. Resting behaviour decreased with altitude and cloudiness, and the dispersion of the group was lower when resting. Increased social interactions were observed with higher temperatures and were more frequent in summer compared to spring and autumn. Differences were found in the display of the behaviours among the selected harems, showing interesting patterns when grouping them according to their origin and experience.

Conclusions

Weather patterns seem to influence the behaviour of Przewalski’s horse. These results might assist in further management plans for the species, especially in the view of intensifying climate change and alteration of weather patterns. As previously suggested, after approximately 1 year, horses adapt to novel conditions and display the typical behavioural pattern of wild-born Przewalski’s horses.

Kidnapping intergroup young: an alternative strategy to maintain group size in the group-living pied babbler (Turdoides bicolor)

Both inter- and intragroup interactions can be important influences on behaviour, yet to date most research focuses on intragroup interactions. Here, we describe a hitherto relatively unknown behaviour that results from intergroup interaction in the cooperative breeding pied babbler: kidnapping. Kidnapping can result in the permanent removal of young from their natal group. Since raising young requires energetic investment and abductees are usually unrelated to their kidnappers, there appears no apparent evolutionary advantage to kidnapping. However, kidnapping may be beneficial in species where group size is a critically limiting factor (e.g. for reproductive success or territory defence). We found kidnapping was a highly predictable event in pied babblers: primarily groups that fail to raise their own young kidnap the young of others, and we show this to be the theoretical expectation in a model that predicts kidnapping to be facultative, only occurring in those cases where an additional group member has sufficient positive impact on group survival to compensate for the increase in reproductive competition. In babblers, groups that failed to raise young were also more likely to accept extragroup adults (hereafter rovers). Groups that fail to breed may either (i) kidnap intergroup young or (ii) accept rovers as an alternative strategy to maintain or increase group size. This article is part of the theme issue 'Intergroup conflict across taxa'.

Ecological and social pressures interfere with homeostatic sleep regulation in the wild

Sleep is fundamental to the health and fitness of all animals. The physiological importance of sleep is underscored by the central role of homeostasis in determining sleep investment – following periods of sleep deprivation, individuals experience longer and more intense sleep bouts. Yet, most sleep research has been conducted in highly controlled settings, removed from evolutionarily relevant contexts that may hinder the maintenance of sleep homeostasis. Using triaxial accelerometry and GPS to track the sleep patterns of a group of wild baboons (Papio anubis), we found that ecological and social pressures indeed interfere with homeostatic sleep regulation. Baboons sacrificed time spent sleeping when in less familiar locations and when sleeping in proximity to more group-mates, regardless of how long they had slept the prior night or how much they had physically exerted themselves the preceding day. Further, they did not appear to compensate for lost sleep via more intense sleep bouts. We found that the collective dynamics characteristic of social animal groups persist into the sleep period, as baboons exhibited synchronized patterns of waking throughout the night, particularly with nearby group-mates. Thus, for animals whose fitness depends critically on avoiding predation and developing social relationships, maintaining sleep homeostasis may be only secondary to remaining vigilant when sleeping in risky habitats and interacting with group-mates during the night. Our results highlight the importance of studying sleep in ecologically relevant contexts, where the adaptive function of sleep patterns directly reflects the complex trade-offs that have guided its evolution.

The acoustic behavior of the Brazilian caatinga big rodent is incongruent to its actual position in Hydrochoerinae

Our knowledge about the acoustic behavior of Caviinae species drove us to investigate Kerodon rupestris's (Caviidae: Hydrochoerinae) repertoire. This species is endemic to Caatinga and states as vulnerable in the Brazilian list of endangered species. We recorded sounds uttered by individuals from Santa Luzia, State of Paraíba, Brazil. We promoted interactions among 13 animals during intra and intersexual pairing sessions, under undisturbed interactions in captivity, and in free-living conditions. We found a repertoire of 13 call types, most of them (all except three) shared with Cavia and only five possibly shared also with Hydrochoerus: (1) Close contact and cohesion: tonal and noisy contact calls, tweet, slow-whistle; (2) Social regulation (incitement, subordination or auto-defense): whine, peepy-squeak, yelp; (3) Offensive aggression and status display: roar, snort, and teeth-chattering (4) Warning or intimidation: alarm-whistle, drrr, and drumming; (5) Courtship: purr and slow-whistle. The similarity of Kerodon signals to Hydrochoerinae species, despite the ecological differences, needs to be understood in comparative phylogenetic studies tracing back the origin of the courtship display in Caviidae. Thus, future research should focus on playback studies to test signals' biological function hypothesis.

Predation risk shapes the use of conflicting personal risk and social safety information in guppies

When faced with uncertainty, animals can benefit from using multiple sources of information in order to make an optimal decision. However, information sources (e.g., social and personal cues) may conflict, while also varying in acquisition cost and reliability. Here, we assessed behavioral decisions of Trinidadian guppies (Poecilia reticulata), in situ, when presented with conflicting social and personal information about predation risk. We positioned foraging arenas within high- and low-predation streams, where guppies were exposed to a personal cue in the form of conspecific alarm cues (a known indicator of risk), a novel cue, or a control. At the same time, a conspecific shoal (a social safety cue) was either present or absent. When social safety was absent, guppies in both populations showed typical avoidance responses towards alarm cues, and high-predation guppies showed their typical avoidance of novel cues (i.e., neophobia). However, the presence of social safety cues was persuasive, overriding the neophobia of high-predation guppies and emboldening low-predation guppies to ignore alarm cues. Our experiment is one of the first to empirically assess the use of safety and risk cues in prey and suggests a threshold level of ambient risk which dictates the use of conflicting social and personal information.

Motion: enhancing signals and concealing cues

Animal colour patterns remain a lively focus of evolutionary and behavioural ecology, despite the considerable conceptual and technical developments over the last four decades. Nevertheless, our current understanding of the function and efficacy of animal colour patterns remains largely shaped by a focus on stationary animals, typically in a static background. Yet, this rarely reflects the natural world: most animals are mobile in their search for food and mates, and their surrounding environment is usually dynamic. Thus, visual signalling involves not only animal colour patterns, but also the patterns of animal motion and behaviour, often in the context of a potentially dynamic background. While motion can reveal information about the signaller by attracting attention or revealing signaller attributes, motion can also be a means of concealing cues, by reducing the likelihood of detection (motion camouflage, motion masquerade and flicker-fusion effect) or the likelihood of capture following detection (motion dazzle and confusion effect). The interaction between the colour patterns of the animal and its local environment is further affected by the behaviour of the individual. Our review details how motion is intricately linked to signalling and suggests some avenues for future research.

This Review has an associated Future Leader to Watch interview with the first author.

Summary: While motion can reveal information about the signaller, motion can also be a means of concealing cues by reducing the likelihood of detection or the likelihood of capture following detection.

Is starvation a cause of overmortality of the Mediterranean sardine?

Animal mortality is difficult to observe in marine systems, preventing a mechanistic understanding of major drivers of fish population dynamics. In particular, starvation is known to be a major cause of mortality at larval stages, but adult mortality is often unknown. In this study, we used a laboratory food-deprivation experiment, on wild caught sardine Sardina pilchardus from the Gulf of Lions. This population is interesting because mean individual phenotype shifted around 2008, becoming dominated by small, young individuals in poor body condition, a phenomenon that may result from declines in energy availability. Continuous monitoring of body mass loss and metabolic rate in 78 captive food-deprived individuals revealed that sardines could survive for up to 57 days on body reserves. Sardines submitted to long-term caloric restriction prior to food-deprivation displayed adaptive phenotypic plasticity, reducing metabolic energy expenditure and enduring starvation for longer than sardines that had not been calorie-restricted. Overall, entry into critical fasting phase 3 occurred at a body condition of 0.72. Such a degree of leanness has rarely been observed over 34 years of wild population monitoring. Still, the proportion of sardines below this threshold has doubled since 2008 and is maximal in January and February (the peak of the reproductive season), now reaching almost 10 % of the population at that time. These results indicate that the demographic changes observed in the wild may result in part from starvation-related adult mortality at the end of the winter reproductive period, despite adaptive plastic responses.

Certainty and integration of options in animal movement

Physical energy defines the energy landscape and determines the species-specific cost of movement, thus influencing movement decisions. In unpredictable and dynamic environments, observing the locomotion of others increases individual certainty in the distribution of physical energy to increase movement efficiency. Beyond the physical energy landscape, social sampling increases certainty in all ecological landscapes that influence animal movement (including fear and resource landscapes), and individuals use energy to express each of these. We call for the development of an 'optimal movement theory' (OMT) that integrates the multidimensional reality of movement decisions by combining ecological landscapes according to a single expectation of energy cost-benefit, where social sampling provides up-to-date information under uncertain conditions. This mechanistic framework has implications for predicting individual movement patterns and for investigating the emergence of aggregations.

Parental Investment Under Predation Threat in Incubating Common Eiders (Somateria mollissima): A Hormonal Perspective

Predation risk affects the costs and benefits of prey life-history decisions. Predation threat is often higher during reproduction, especially in conspicuous colonial breeders. Therefore, predation risk may increase the survival cost of breeding, and reduce parental investment. The impact of predation risk on avian parental investment decisions may be hormonally mediated by prolactin and corticosterone, making them ideal tools for studying the trade-offs involved. Prolactin is thought to promote parental care and commitment in birds. Corticosterone is involved in allostasis and may either mediate reduced parental investment (corticosterone-fitness hypothesis), or promote parental investment through a reallocation of resources (corticosterone-adaptation hypothesis). Here, we used these hormonal proxies of incubation commitment to examine the impact of predation risk on reproduction in common eiders (Somateria mollissima) breeding in the Baltic Sea. This eider population is subject to high but spatially and temporally variable predation pressure on adults (mainly by the white-tailed eagle Haliaeetus albicilla and introduced mammalian predators) and nests (by the adult predators and exclusive egg predators such as hooded crows Corvus cornix). We investigated baseline hormonal levels and hatching success as a function of individual quality attributes (breeding experience, female and duckling body condition), reproductive investment (clutch weight), and predation risk. We expected individuals nesting in riskier environments (i.e., on islands where predation on adults or nests is higher, or in less concealed nests) to reduce their parental investment in incubation, reflected in lower baseline prolactin levels and either higher (corticosterone-fitness hypothesis) or lower (corticosterone-adaptation hypothesis) baseline corticosterone levels. Contrary to our predictions, prolactin levels showed a positive correlation with nest predation risk. The unexpected positive relationship could result from the selective disappearance of low-quality females (presumably having low prolactin levels) from risky sites. Supporting this notion, female body condition and hatching success were positively correlated with predation risk on females, and baseline prolactin concentrations were positively correlated with duckling body condition, a proxy of maternal quality. In line with the corticosterone-adaptation hypothesis, baseline corticosterone levels increased with reproductive investment, and were negatively associated with nest predation risk. Hatching success was lower on islands where nest predation risk was higher, consistent with the idea of reduced reproductive investment under increased threat. Long-term individual-based studies are now needed to distinguish selection processes occurring at the population scale from individually plastic parental investment in relation to individual quality and variable predation risk.

Is mimicry a diversification-driver in ants? Biogeography, ecology, ethology, genetics and morphology define a second West-Palaearctic Colobopsis species (Hymenoptera: Formicidae)

The West-Palaearctic Colobopsis ant populations have long been considered a single species (Colobopsis truncata). We studied the diversity of this species by employing a multidisciplinary approach and combining data from our surveys, museum and private collections, and citizen science platforms. As a result, we have revealed the existence of a second species, which we describe as Colobopsis imitans sp. nov., distributed allopatrically from Co. truncata and living in the Maghreb, Sicily and southern Iberia. While the pigmentation of Co. truncata is reminiscent of Dolichoderus quadripunctatus, that of Co. imitans is similar to Crematogaster scutellaris, with which Co. imitans lives in close spatial association, and whose foraging trails it habitually follows, similar to Camponotus lateralis and other ant-mimicking ants. The isolation between Co. imitans and Co. truncata seems to have occurred relatively recently because of significant, yet not extreme, morphometric differentiation, and to mtDNA polyphyly. Both Co. imitans and Co. truncata appear to employ mimicry of an unpalatable or aggressive ant species as an important defensive strategy; this ‘choice’ of a different model species is motivated by biogeographic reasons and appears to act as a critical evolutionary driver of their diversification.

High Survivorship of First-Generation Monarch Butterfly Eggs to Third Instar Associated with a Diverse Arthropod Community

Simple Summary

The eastern migratory population of the monarch butterfly has been the focus of extensive conservation efforts in recent years. However, there are gaps in our knowledge about the survival of first, or spring generation, monarchs in their core areas of Texas, Oklahoma, and Louisiana. This is important because the spring generation represents the first stage of annual recovery from overwinter mortality. It is, therefore, an important stage for monarch conservation efforts. This study showed that, in the context of a complex arthropod community in north Texas, first generation monarch survival was high. The study found that survival was not directly related to predators on the host plant, but was higher on host plants that harbored a greater number and variety of other, non-predatory arthropods. This is possibly because the presence of alternate, preferable prey enabled monarch eggs and larvae to be overlooked by predators. The implication is that, at least in the southern U.S., monarch conservation should consider strategies that promote diverse functional arthropod communities.

Abstract

Based on surveys of winter roost sites, the eastern migratory population of the monarch butterfly (Danaus plexippus) in North America appears to have declined in the last 20 years and this has prompted the implementation of numerous conservation strategies. However, there is little information on the survivorship of first-generation monarchs in the core area of occupancy in Texas, Oklahoma, and Louisiana where overwinter population recovery begins. The purpose of this study was to determine the survivorship of first-generation eggs to third instars at a site in north Texas and to evaluate host plant arthropods for their effect on survivorship. Survivorship to third instar averaged 13.4% and varied from 11.7% to 15.6% over three years. The host plants harbored 77 arthropod taxa, including 27 predatory taxa. Despite their abundance, neither predator abundance nor predator richness predicted monarch survival. However, host plants upon which monarchs survived often harbored higher numbers of non-predatory arthropod taxa and more individuals of non-predatory taxa. These results suggest that ecological processes may have buffered the effects of predators and improved monarch survival in our study. The creation of diverse functional arthropod communities should be considered for effective monarch conservation, particularly in southern latitudes.

Aerial attack strategies of hawks hunting bats, and the adaptive benefits of swarming

Aggregation can reduce an individual’s predation risk, by decreasing predator hunting efficiency or displacing predation onto others. Here, we explore how the behaviors of predator and prey influence catch success and predation risk in Swainson’s hawks Buteo swainsoni attacking swarming Brazilian free-tailed bats Tadarida brasiliensis on emergence. Lone bats including stragglers have a high relative risk of predation, representing ~5% of the catch but ~0.2% of the population. Attacks on the column were no less successful than attacks on lone bats, so hunting efficiency is not decreased by group vigilance or confusion. Instead, lone bats were attacked disproportionately often, representing ~10% of all attacks. Swarming therefore displaces the burden of predation onto bats outside the column—whether as isolated wanderers not benefitting from dilution through attack abatement, or as peripheral stragglers suffering marginal predation and possible selfish herd effects. In contrast, the hawks’ catch success depended only on the attack maneuvers that they employed, with the odds of success being more than trebled in attacks involving a high-speed stoop or rolling grab. Most attacks involved one of these two maneuvers, which therefore represent alternative rather than complementary tactics. Hence, whereas a bat’s survival depends on maintaining column formation, a hawk’s success does not depend on attacking lone bats—even though their tendency to do so is sufficient to explain the adaptive benefits of their prey’s aggregation behavior. A hawk’s success instead depends on the flight maneuvers it deploys, including the high-speed stoop that is characteristic of many raptors.

Swarming bats emerging from a massive desert roost reduce their predation risk by maintaining tight column formation, because the hawks that predate them attack peripheral stragglers and isolated wanderers disproportionately. Whereas a bat’s predation risk depends on maintaining its position within the column, the catch success of a hawk depends on how it maneuvers itself to attack, and is maximized by executing a high-speed dive or rolling grab maneuver.

Humans influence shrimp movement: a conservation behavior case study with “Shrimp Watching” ecotourism

An increase in ecotourism adversely impacts many animals and contributes to biodiversity loss. To mitigate these impacts, we illustrate the application of a conservation behavior framework toward the development of a sustainable ecotourism management plan. In Ubon Ratchathani, Thailand, thousands of tourists annually come to see a unique mass migration of shrimps on land (referred to as “shrimp parading”). Preliminary work suggests that this tourism has negatively impacted the shrimps. To reduce tourism-related impacts we studied: 1) the decisions shrimps make when parading and 2) how shrimps respond to different light intensities and colors. We created an artificial stream and tested the conditions that influence parading by experimentally varying the presence of light and systematically manipulating water velocity (10, 60, and 100 cm/s). Additionally, we conducted an in situ experiment to study how shrimps respond to tourists’ lights under three intensities (50,400, and 9,000 lux) and five colors (white, blue, green, orange, and red). We found most shrimps prefer to leave the river when it is dark and there is low water flow. Shrimps responded the least to red (λmax = 630 nm) and orange (λmax = 625 nm) light at 50 lux. These findings were used to develop a management plan by creating three different tourist zones, which maximize tourist needs and minimize the anthropogenic impacts on the shrimps. This work could be used as an example of the application of conservation behavior framework in developing management plan for sustainable ecotourism for other invertebrate taxa.

Alloparental care in glassfrogs: males care for unrelated clutches only when associated with their own

Parental care is costly, thus theory predicts that parents should avoid caring for unrelated offspring. However, alloparenting has been reported in many taxa because it may increase the caregiver mating success or offspring survival. We experimentally investigated the existence of allopaternal care in two glassfrog species, Hyalinobatrachium chirripoi and Centrolene peristicta, and discussed possible costs and benefits. Males mated with multiple females and cared for clutches, while continued to call. In the field, we randomly placed unrelated clutches in the territory of males already caring for their clutches and in the territory of non-attending males. Attending males adopted unrelated clutches, whereas non-attending males abandoned their territories. Once males adopted unrelated offspring, they cared for all clutches in a similar frequency and gained new clutches. Alloparenting was context-dependent, as only males already caring for their clutches adopted unrelated ones. We suggest that steroid hormonal levels might mediate the adoption of unrelated offspring by attending males. Additionally, our results suggest that males do not directly discriminate between related and unrelated offspring. Alloparenting has been widely investigated in different vertebrates, except for amphibians. Thus, our study sheds light on the roles of alloparenting for offspring survival and mating success in this group.

Numbers, neighbors, and hungry predators: What makes chemically defended aposematic prey susceptible to predation?

Many chemically defended aposematic species are characterized by relatively low toxin levels, which enables predators to include them in their diets under certain circumstances. Knowledge of the conditions governing the survival of such prey animals-especially in the context of the co-occurrence of similar but undefended prey, which may result in mimicry-like interactions-is crucial for understanding the initial evolution of aposematism. In a one-month outdoor experiment using fish (the common carp Cyprinus carpio) as predators, we examined the survival of moderately defended aposematic tadpole prey (the European common toad Bufo bufo) with varying absolute densities in single-species prey systems or varying relative densities in two-species prey systems containing morphologically similar but undefended prey (the European common frog Rana temporaria). The density effects were investigated in conjunction with the hunger levels of the predator, which were manipulated by means of the addition of alternative (nontadpole) food. The survival of the B. bufo tadpoles was promoted by increasing their absolute density in the single-species prey systems, increasing their relative density in the two-species prey systems, and providing ample alternative food for the predator. Hungry predators eliminated all R. temporaria individuals regardless of their proportion in the prey community; in treatments with ample alternative food, high relative B. bufo density supported R. temporaria survival. The results demonstrated that moderately defended prey did benefit from high population densities (both absolute and relative), even under long-term predation pressure. However, the physiological state of the predator was a crucial factor in the survival of moderately defended prey. While the availability of alternative prey in general should promote the spread and maintenance of aposematism, the results indicated that the resemblance between the co-occurring defended and undefended prey may impose mortality costs on the defended model species, even in the absence of actual mimicry.

Cellular interactions between social experience, alcohol sensitivity, and GABAergic inhibition in a crayfish neural circuit

We report here that prior social experience modified the behavioral responses of adult crayfish to acute alcohol exposure. Animals housed individually for 1 wk before alcohol exposure were less sensitive to the intoxicating effects of alcohol than animals housed in groups, and these differences are based on changes in the nervous system rather than differences in alcohol uptake. To elucidate the underlying neural mechanisms, we investigated the neurophysiological responses of the lateral giant (LG) interneurons after alcohol exposure. Specifically, we measured the interactions between alcohol and different GABA_A-receptor antagonists and agonists in reduced crayfish preparations devoid of brain-derived tonic GABAergic inhibition. We found that alcohol significantly increased the postsynaptic potential of the LG neurons, but contrary to our behavioral observations, the results were similar for isolated and communal animals. The GABA_A-receptor antagonist picrotoxin, however, facilitated LG postsynaptic potentials more strongly in communal crayfish, which altered the neurocellular interactions with alcohol, whereas TPMPA [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid], an antagonist directed against GABA_A-receptors with ρ subunits, did not produce any effects. Muscimol, an agonist for GABA_A-receptors, blocked the stimulating effects of alcohol, but this was independent of prior social history. THIP [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol], an agonist directed against GABA_A-receptors with δ subunits, which were not previously known to exist in the LG circuit, replicated the suppressing effects of muscimol. Together, our findings provide strong evidence that alcohol interacts with the crayfish GABAergic system, and the interplay between prior social experience and acute alcohol intoxication might be linked to changes in the expression and function of specific GABA_A-receptor subtypes.NEW & NOTEWORTHY The complex interactions between alcohol and prior social experience are still poorly understood. Our work demonstrates that socially isolated crayfish exhibit lower neurobehavioral sensitivity to acute ethanol compared with communally housed animals, and this socially mediated effect is based on changes in the nervous systems rather than on differences in uptake or metabolism. By combining intracellular neurophysiology and neuropharmacology, we investigated the role of the main inhibitory neurotransmitter GABA, and its receptor subtypes, in shaping this process.