Cognition in Contests: Mechanisms, Ecology, and Evolution

How it ends: A review of behavioral and psychological phenomena, physiological processes and neural circuits in the termination of aggression in other animals and anger in people

More is known about aggression initiation and persistence in other animals, and anger in people, than about their cessation. This review summarizes knowledge of relevant factors in aggression, mostly in vertebrates, and anger termination in people. The latency, probability and intensity of offensive aggression in mice is controlled by activity in a neuronal subpopulation in ventromedial hypothalamus [VMH]. This activity instantiates an aggressive state termed angriffsbereitschaft ["attack-readiness"]. Fighting in many species is broken into bouts with interbout breaks due to fatigue and/or signals from dorsal raphe to VMH. Eventually, losers decide durations and outcomes of fighting by transitioning to submission or flight. Factors reducing angriffsbereitschaft and triggering these defeat behaviors could include metabolic costs, e.g., lactate accumulation and glucose depletion detected by the hypothalamus, central fatigue perhaps sensed by the Salience Network [insula and anterior cingulate gyrus] and pain of injuries, the latter insufficiently blunted by opioid and non-opioid stress analgesia and transduced by anterior VMH neurons. Winners' angriffsbereitschaft continue for awhile, as indicated by post-victory attacks and, perhaps, triumph displays of some species, including humans. In longer term situations, sensory and/or response habituation of aggression may explain the "Dear enemy" tolerance of competitive neighbors. Prolonged satiation of predatory behavior could involve habenula-regulated reduction of dopaminergic reward in nucleus accumbens. Termination of human anger involves at least three processes, metaphorically termed decay, quenching and catharsis. Hypothesized neural mechanisms include anger diminution by negative feedback from accumbens to anterior cingulate and/or activity in the Salience Network that controls anger's "accumulation/offset" phase.

High temperatures are associated with reduced cognitive performance in wild southern pied babblers

Global temperatures are increasing rapidly. While considerable research is accumulating regarding the lethal and sublethal effects of heat on wildlife, its potential impact on animal cognition has received limited attention. Here, we tested wild southern pied babblers (Turdoides bicolor) on three cognitive tasks (associative learning, reversal learning and inhibitory control) under naturally occurring heat stress and non-heat stress conditions. We determined whether cognitive performance was explained by temperature, heat dissipation behaviours, individual and social attributes, or proxies of motivation. We found that temperature, but not heat dissipation behaviours, predicted variation in associative learning performance. Individuals required on average twice as many trials to learn an association when the maximum temperature during testing exceeded 38°C compared with moderate temperatures. Higher temperatures during testing were also associated with reduced inhibitory control performance, but only in females. By contrast, we found no temperature-related decline in performance in the reversal learning task, albeit individuals reached learning criterion in only 14 reversal learning tests. Our findings provide novel evidence of temperature-mediated cognitive impairment in a wild animal and indicate that its occurrence depends on the cognitive trait examined and individual sex.

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.

Bullying as an advertisement of social dominance in common waxbills

Bullying consists of preferentially attacking individuals lowest in the dominance hierarchy, and its functions are unclear because the most subordinate individuals do not pose social challenges to the aggressor. Instead, conflict is expected mostly between individuals of similar dominance rank or socially distant (i.e. weakly associated), among whom dominance relationships may not be well established. A possible function of bullying is that it may be used as a low-risk strategy of showing-off dominance to relevant third parties. To study this hypothesis, we monitored aggressions during feeding, the composition of audiences, dominance hierarchy and social network of common waxbills (Estrilda astrild) in an open-air mesocosm, and tested (i) whether their aggressions show a pattern of bullying, and (ii) whether audience effects influence aggressiveness. Waxbills showed bullying, most often attacking the lowest ranking individuals rather than socially distant individuals or those of similar dominance rank, and aggressions increased when the audience included socially distant individuals, indicating a signalling function of bullying. Showing-off dominance in the presence of socially distant individuals may be a strategy to manage dominance hierarchies, avoiding direct fights with potentially dangerous opponents in the audience. We suggest that bullying is a safe manner of managing dominance hierarchies, by signalling dominance status to potential opponents.

Heuristics Facilitates the Evolution of Transitive Inference and Social Hierarchy in a Large Group

Transitive inference (TI) refers to social cognition that facilitates the discernment of unknown relationships between individuals using known relationships. It is extensively reported that TI evolves in animals living in a large group because TI could assess relative rank without deducing all dyadic relationships, which averts costly fights. The relationships in a large group become so complex that social cognition may not be developed adequately to handle such complexity. If members apply TI to all possible members in the group, TI requires extremely highly developed cognitive abilities especially in a large group. Instead of developing cognitive abilities significantly, animals may apply simplified TI we call reference TI in this study as heuristic approaches. The reference TI allows members to recognize and remember social interactions only among a set of reference members rather than all potential members. Our study assumes that information processes in the reference TI comprises (1) the number of reference members based on which individuals infer transitively, (2) the number of reference members shared by the same strategists, and (3) memory capacity. We examined how information processes evolve in a large group using evolutionary simulations in the hawk-dove game. Information processes with almost any numbers of reference members could evolve in a large group as long as the numbers of shared reference member are high because information from the others' experiences is shared. TI dominates immediate inference, which assesses relative rank on direct interactions, because TI could establish social hierarchy more rapidly applying information from others' experiences.

Unraveling the phenotypic and genomic background of behavioral plasticity and temperament in North American Angus cattle

BACKGROUND

Longitudinal records of temperament can be used for assessing behavioral plasticity, such as aptness to learn, memorize, or change behavioral responses based on affective state. In this study, we evaluated the phenotypic and genomic background of North American Angus cow temperament measured throughout their lifetime around the weaning season, including the development of a new indicator trait termed docility-based learning and behavioral plasticity. The analyses included 273,695 and 153,898 records for yearling (YT) and cow at weaning (CT) temperament, respectively, 723,248 animals in the pedigree, and 8784 genotyped animals. Both YT and CT were measured when the animal was loading into/exiting the chute. Moreover, CT was measured around the time in which the cow was separated from her calf. A random regression model fitting a first-order Legendre orthogonal polynomial was used to model the covariance structure of temperament and to assess the learning and behavioral plasticity (i.e., slope of the regression) of individual cows. This study provides, for the first time, a longitudinal perspective of the genetic and genomic mechanisms underlying temperament, learning, and behavioral plasticity in beef cattle.

RESULTS

CT measured across years is heritable (0.38-0.53). Positive and strong genetic correlations (0.91-1.00) were observed among all CT age-group pairs and between CT and YT (0.84). Over 90% of the candidate genes identified overlapped among CT age-groups and the estimated effect of genomic markers located within important candidate genes changed over time. A small but significant genetic component was observed for learning and behavioral plasticity (heritability = 0.02 ± 0.002). Various candidate genes were identified, revealing the polygenic nature of the traits evaluated. The pathways and candidate genes identified are associated with steroid and glucocorticoid hormones, development delay, cognitive development, and behavioral changes in cattle and other species.

CONCLUSIONS

Cow temperament is highly heritable and repeatable. The changes in temperament can be genetically improved by selecting animals with favorable learning and behavioral plasticity (i.e., habituation). Furthermore, the environment explains a large part of the variation in learning and behavioral plasticity, leading to opportunities to also improve the overall temperament by refining management practices. Moreover, behavioral plasticity offers opportunities to improve the long-term animal and handler welfare through habituation.

The importance of distinguishing individual differences in 'social impact' versus 'social responsiveness' when quantifying indirect genetic effects on the evolution of social plasticity

Social evolution and the dynamics of social interactions have previously been studied under the frameworks of quantitative genetics and behavioural ecology. In quantitative genetics, indirect genetic effects of social partners on the socially plastic phenotypes of focal individuals typically lack crucial detail already included in treatments of social plasticity in behavioural ecology. Specifically, whilst focal individuals (e.g. receivers) may show variation in their 'responsiveness' to the social environment, individual social partners (e.g. signallers) may have a differential 'impact' on focal phenotypes. Here we propose an integrative framework, that highlights the distinction between responsiveness versus impact in indirect genetic effects for a range of behavioural traits. We describe impact and responsiveness using a reaction norm approach and provide statistical models for the assessment of these effects of focal and social partner identity in different types of social interactions. By providing such a framework, we hope to stimulate future quantitative research investigating the causes and consequences of social interactions on phenotypic evolution.

Complex strategies: an integrative analysis of contests in Siamese fighting fish

Background

Animals use contests to attain resources and employ strategic decisions to minimise contest costs. These decisions are defined by behavioural response to resource value and competitive ability, but remain poorly understood. This is because the two factors are typically studied separately. Also, their study relies on overgeneralised assumptions that (i) strategies are fixed, (ii) modulated by the motivation or drive to fight and (iii) used to manage costs proportional to the timing of the loser’s retreat. To address these problems, we adopt an integrative sequential analysis that incorporates competitive ability and resource value factors, to characterise territorial contest decisions in male Siamese fighting fish (Betta splendens).

Results

Individuals exhibited a chronological organisation of behaviour, engaging opponents first with frontal display, then switching to lateral display before deciding to attack, and reserved retreats for later stages. Using asymmetries in retreats as a proxy for outcome, the likelihood of winning was found to be mostly dependent on display. However, resource and contest conditions affected initiation latency, display, attack and retreat, suggesting that strategic decisions influence all behaviour. Overall, sequential behaviour varied consistently with individual aggressiveness and resource-value factors, and increasingly with information on competitive ability collected during the contest. This enabled shifts in tactics, such as disadvantaged individuals responding first with aggression and later with submission. Motivation to continue fighting, after interruption by startle, was also adjusted to information gathered during the contest and progressively with energetic state. Two clusters of correlated behaviours were identified, cost-mitigation (display and retreat) and escalation (initiation and attack), but changes in motivation were associated only with cost mitigation.

Conclusions

Our findings contrast dominant assumptions that strategic decisions are fixed, controlled by motivational state and sufficiently described by outcome-dependent measures. We instead demonstrate that strategic decisions are complex, comprising functional changes in assessment, information use and motivational effects, which are not always inter-dependent.

Stag beetle Cyclommatus mniszechi employs both mutual- and self-assessment strategies in male-male combat

Animals may base contest decisions on their fighting ability alone (self-assessment) or also their opponents' (mutual assessment). Many male stag beetles develop disproportionately enlarged mandibles and use them as weapons. Information on their assessment strategy is limited. To investigate their assessment strategy and whether they adopt the same strategy at different stages of contests, we used food to encourage male Cyclommatus mniszechi of different (random pairings) or similar (ML-matched pairings) mandible length (ML) to interact. For the random pairings, losers had shorter mandibles than winners and were faster to feed. Overall contest duration and the tendency to escalate to tussles associated positively with winners' ML and average ML in the random and the ML-matched pairings, respectively, consistent with self-assessment. Non-tussle phase duration associated positively with average ML in the ML-matched pairings, consistent with self-assessment. Tussle phase duration, however, positively associated with losers' ML in the random pairings and had no association with average ML in the ML-matched pairings, consistent with mutual assessment. These results show that (1) the males employ both assessment strategies, (2) winners have more control over contest intensity than losers, and (3) males with shorter mandibles are quicker to feed and also more likely to lose fights.

Social behavior in farm animals: Applying fundamental theory to improve animal welfare

A fundamental understanding of behavior is essential to improving the welfare of billions of farm animals around the world. Despite living in an environment managed by humans, farm animals are still capable of making important behavioral decisions that influence welfare. In this review, we focus on social interactions as perhaps the most dynamic and challenging aspects of the lives of farm animals. Social stress is a leading welfare concern in livestock, and substantial variation in social behavior is seen at the individual and group level. Here, we consider how a fundamental understanding of social behavior can be used to: (i) understand agonistic and affiliative interactions in farm animals; (ii) identify how artificial environments influence social behavior and impact welfare; and (iii) provide insights into the mechanisms and development of social behavior. We conclude by highlighting opportunities to build on previous work and suggest potential fundamental hypotheses of applied relevance. Key areas for further research could include identifying the welfare benefits of socio–positive interactions, the potential impacts of disrupting important social bonds, and the role of skill in allowing farm animals to navigate competitive and positive social interactions. Such studies should provide insights to improve the welfare of farm animals, while also being applicable to other contexts, such as zoos and laboratories.

Social dominance has limited effects on spatial cognition in a wild food-caching bird

Social dominance has long been used as a model to investigate social stress. However, many studies using such comparisons have been performed in captive environments. These environments may produce unnaturally high antagonistic interactions, exaggerating the stress of social subordination and any associated adverse consequences. One such adverse effect concerns impaired cognitive ability, often thought to be associated with social subordination. Here, we tested whether social dominance rank is associated with differences in spatial learning and memory, and in reversal spatial learning (flexibility) abilities in wild food-caching mountain chickadees at different montane elevations. Higher dominance rank was associated with higher spatial cognitive flexibility in harsh environments at higher elevations, but not at lower, milder elevations. By contrast, there were no consistent differences in spatial learning and memory ability associated with dominance rank. Our results suggest that spatial learning and memory ability in specialized food-caching species is a stable trait resilient to social influences. Spatial cognitive flexibility, on the other hand, appears to be more sensitive to environmental influences, including social dominance. These findings contradict those from laboratory studies and suggest that it is critical to investigate the biological consequences of social dominance under natural conditions.

Reinstating reptiles: from clueless creatures to esteemed models of cognitive biology

Non-avian reptiles have long been neglect in cognitive science due to their reputation as slow and inflexible learners, but fortunately, this archaic view on reptile cognition is changing rapidly. The last two decades have witnessed a renewed interest in the cognitive capacities of reptiles, and more ecologically relevant protocols have been designed to measure such abilities. Now, we appreciate that reptiles possess an impressive set of cognitive skills, including problem-solving abilities, fast and flexible learning, quantity discrimination, and even social learning. This special issue highlights current research on reptiles in cognitive biology and showcases the diversity of research questions that can be answered by using reptiles as study model. Here, we briefly address (the key results of) the contributing articles and their role in the endeavour for total inclusion of reptiles in cognitive biological research, which is instrumental for our understanding of the evolution of animal cognition. We also discuss and illustrate the promising potential of reptiles as model organisms in various areas of cognitive research.

Cognition and covariance in the producer-scrounger game

The producer-scrounger game is a key element of foraging ecology in many systems. Producing and scrounging typically covary negatively, but partitioning this covariance into contributions of individual plasticity and consistent between individual differences is key to understanding population-level consequences of foraging strategies. Furthermore, little is known about the role cognition plays in the producer-scrounger game. We investigated the role of cognition in these alternative foraging tactics in wild mixed-species flocks of great tits and blue tits, using a production learning task in which we measured individuals' speed of learning to visit the single feeder in an array that would provide them with a food reward. We also quantified the proportion of individuals' feeds that were scrounges ('proportion scrounged'); scrounging was possible if individuals visited immediately after a previous rewarded visitor. Three learning experiments-initial and two reversal learning-enabled us to estimate the repeatability and covariance of each foraging behaviour. First, we examined whether individuals learned to improve their scrounging success (i.e. whether they obtained food by scrounging when there was an opportunity to do so). Second, we quantified the repeatability of proportion scrounged, and asked whether proportion scrounged affected production learning speed among individuals. Third, we used multivariate analyses to partition within- and among-individual components of covariance between proportion scrounged and production learning speed. Individuals improved their scrounging success over time. Birds with a greater proportion scrounged took longer to learn their own rewarding feeder. Although multivariate analyses showed that covariance between proportion scrounged and learning speed was driven primarily by within-individual variation, that is, by behavioural plasticity, among-individual differences also played a role for blue tits. This is the first demonstration of a cognitive trait influencing producing and scrounging in the same wild system, highlighting the importance of cognition in the use of alternative resource acquisition tactics. The results of our covariance analyses suggest the potential for genetic differences in allocation to alternative foraging tactics, which are likely species- and system-dependent. They also point to the need to control for different foraging tactics when studying individual cognition in the wild.

Cooperation, involuntary defection, and Polymorphic Evolutionary Games

The subject of involuntary defection in the context of cycles of interaction approach to direct reciprocal cooperation was introduced some time ago (J. Theor. Biol., 242: 873-879). Current work is motivated by the subsequent accumulation of empirical evidence and the advances in the methodology of evolutionary games. In recent decades it become clear that individuals in many animal species vary consistently in their behavioral responses to specific challenges-animal personality. Moreover, these differences have a hereditary component. Finally, investigations into the effects of neuropeptides on behavior suggest that the variations in animal personalities involve polymorphisms based on non-Mendelian heritability within the neuropeptide signaling systems. The last observation suggests that animal personalities can be productively analyzed via Polymorphic Evolutionary Games, which allow us to add genetic model(s) to standard (phenotypic) evolutionary games. Such an analysis of reciprocal cooperation is the subject of the current paper. The results indicate that there is a marked difference between models that assume Mendelian vs. non-Mendelian inheritance of the pertinent traits. Monomorphic and polymorphic-Mendelian models predict pure-strategy (single phenotype) ESS, whereas the non-Mendelian genetic model predicts a mixed strategy ESS exhibiting all three phenotypes.

Extrinsic stressors modulate resource evaluations: insights from territoriality under artificial noise

Background

Competition is considered to rely on the value attributed to resources by animals, but the influence of extrinsic stressors on this value remains unexplored. Although natural or anthropogenic environmental stress often drives decreased competition, assumptions that this relies on resource devaluation are without formal evidence. According to theory, physiological or perceptual effects may influence contest behaviour directly, but motivational changes due to resource value are expected to manifest as behavioural adjustments only in interaction with attainment costs and resource benefits. Thus, we hypothesise that stressor-induced resource devaluations will impose greater effects when attainment costs are high, but not when resource benefits are higher. Noise may elicit such effects because it impacts the acoustic environment and imposes physiological and behavioural costs to animals. Therefore, we manipulated the acoustic environment using playbacks of artificial noise to test our hypotheses in the territorial male Siamese fighting fish, Betta splendens.

Results

Compared to a no-playback control, noise reduced defense motivation only when territory owners faced comparatively bigger opponents that impose greater injury costs, but not when territories also contained bubble nests that offer reproductive benefits. In turn, nest-size decreases were noted only after contests under noise treatment, but temporal nest-size changes relied on cross-contest variation in noise and comparative opponent size. Thus, the combined effects of noise are conditional on added attainment costs and offset by exceeding resource benefits.

Conclusion

Our findings provide support for the hypothesised modulation of resource value under extrinsic stress and suggest implications for competition under increasing anthropogenic activity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00397-x.

Equal performance but distinct behaviors: sex differences in a novel object recognition task and spatial maze in a highly social cichlid fish

Sex differences in behavior and cognition can be driven by differential selection pressures from the environment and in the underlying neuromolecular mechanisms of decision-making. The highly social cichlid fish Astatotilapia burtoni exhibits dynamic and complex social hierarchies, yet explicit cognitive testing (outside of social contexts) and investigations of sex differences in cognition have yet to be fully explored. Here we assessed male and female A. burtoni in two cognitive tasks: a novel object recognition task and a spatial task. We hypothesized that males outperform females in a spatial learning task and exhibit more neophilic/exploratory behavior across both tasks. In the present study we find that both sexes prefer the familiar object in a novel object recognition task, but the time at which they exhibit this preference differs between the sexes. Females more frequently learned the spatial task, exhibiting longer decision latencies and quicker error correction, suggesting a potential speed-accuracy tradeoff. Furthermore, the sexes differ in space use in both tasks and in a principal component analysis of the spatial task. A model selection analysis finds that preference, approach, and interaction duration in the novel object recognition task reach a threshold of importance averaged across all models. This work highlights the need to explicitly test for sex differences in cognition to better understand how individuals navigate dynamic social environments.

Behavioral variation in natural contests: integrating plasticity and personality

Animals often interact aggressively when competing over limited resources. Aggressive decisions can be complex and may result from multiple sources of behavioral variation. The outcome of contests may be explained through contest theory and personality by considering conjointly plasticity and individual consistency. This integrative approach also allows investigating individual differences in responsiveness to environmental changes. Here, we observed multiple agonistic interactions occurring among eastern chipmunks (Tamias striatus) competing for food resources supplied at different distances from their burrows. Using an individual reaction norm approach, we found that the probability of winning a contest depended on an individual’s intrinsic characteristics (mass and age but not sex) but was also adjusted to characteristics of its opponents. Winning a contest also depended on extrinsic environmental characteristics, such as distance to the contestants’ burrows, but not the order of arrival at the feeding patch. We found consistent individual differences in the probability of winning, potentially related to differences in aggressiveness and territoriality. We also found that individuals differed in their plasticity level in response to changes in different characteristics of their social and physical environments. Plasticity, personality, and individual differences in responsiveness may thus all play a role in predicting contest outcome and in the evolution of animal contests.

Long-Term Dietary Restriction Leads to Development of Alternative Fighting Strategies

In competition for food, mates and territory, most animal species display aggressive behavior through visual threats and/or physical attacks. Such naturally-complex social behaviors have been shaped by evolution. Environmental pressure, such as the one imposed by dietary regimes, forces animals to adapt to specific conditions and ultimately to develop alternative behavioral strategies. The quality of the food resource during contests influence animals' aggression levels. However, little is known regarding the effects of a long-term dietary restriction-based environmental pressure on the development of alternative fighting strategies. To address this, we employed two lines of the wild-type Drosophila melanogaster Canton-S (CS) which originated from the same population but raised under two distinct diets for years. One diet contained both proteins and sugar, while the second one was sugar-free. We set up male-male aggression assays using both CS lines and found differences in aggression levels and the fighting strategies employed to establish dominance relationships. CS males raised on a sugar-containing diet started fights with a physical attack and employed a high number of lunges for establishing dominance but displayed few wing threats throughout the fight. In contrast, the sugar-free-raised males favored wing threats as an initial aggressive demonstration and used fewer lunges to establish dominance, but displayed a higher number of wing threats. This study demonstrates that fruit flies that have been raised under different dietary conditions have adapted their patterns of aggressive behavior and developed distinct fighting strategies: one favoring physical attacks, while the other one favoring visual threats.

Asymmetric interspecific competition drives shifts in signalling traits in fan-throated lizards

Interspecific competition can occur when species are unable to distinguish between conspecific and heterospecific mates or competitors when they occur in sympatry. Selection in response to interspecific competition can lead to shifts in signalling traits-a process called agonistic character displacement. In two fan-throated lizard species-Sitana laticeps and Sarada darwini-females are morphologically indistinguishable and male agonistic signalling behaviour is similar. Consequently, in areas where these species overlap, males engage in interspecific aggressive interactions. To test whether interspecific male aggression between Si. laticeps and Sa. darwini results in agonistic character displacement, we quantified species recognition and signalling behaviour using staged encounter assays with both conspecifics and heterospecifics across sympatric and allopatric populations of both species. We found an asymmetric pattern, wherein males of Si. laticeps but not Sa. darwini showed differences in competitor recognition and agonistic signalling traits (morphology and behaviour) in sympatry compared with allopatry. This asymmetric shift in traits is probably due to differences in competitive abilities between species and can minimize competitive interactions in zones of sympatry. Overall, our results support agonistic character displacement, and highlight the role of asymmetric interspecific competition in driving shifts in social signals.

Emotion in animal contests

Emotions encompass cognitive and behavioural responses to reward and punishment. Using contests as a case-study, we propose that short-term emotions underpin animals' assessments, decision-making and behaviour. Equating contest assessments to emotional 'appraisals', we describe how contestants appraise more than resource value and outcome probability. These appraisals elicit the cognition, drive and neurophysiology that governs aggressive behaviour. We discuss how recent contest outcomes induce long-term moods, which impact subsequent contest behaviour. Finally, we distinguish between integral (objectively relevant) and incidental (objectively irrelevant) emotions and moods (affective states). Unlike existing ecological models, our approach predicts that incidental events influence contest dynamics, and that contests become incidental influences themselves, potentially causing maladaptive decision-making. As affective states cross contexts, a more holistic ethology (incorporating emotions and moods) would illuminate animal cognition and behaviour.

Interactions with conspecific outsiders as drivers of cognitive evolution

The social intelligence hypothesis (SIH) posits that within-group interactions drive cognitive evolution, but it has received equivocal support. We argue the SIH overlooks a major component of social life: interactions with conspecific outsiders. Competition for vital resources means conspecific outsiders present myriad threats and opportunities in all animal taxa across the social spectrum (from individuals to groups). We detail cognitive challenges generated by conspecific outsiders, arguing these select for ‘Napoleonic’ intelligence; explain potential influences on the SIH; and highlight important considerations when empirically testing these ideas. Including interactions with conspecific outsiders may substantially improve our understanding of cognitive evolution.

The social intelligence hypothesis predicts that social organisms tend to be more intelligent because within-group interactions drive cognitive evolution. Here, authors propose that conspecific outsiders can be just as important in selecting for sophisticated cognitive adaptations.

Pheromonal Regulation of the Reproductive Division of Labor in Social Insects

The reproductive altruism in social insects is an evolutionary enigma that has been puzzling scientists starting from Darwin. Unraveling how reproductive skew emerges and maintains is crucial to understand the reproductive altruism involved in the consequent division of labor. The regulation of adult worker reproduction involves conspecific inhibitory signals, which are thought to be chemical signals by numerous studies. Despite the primary identification of few chemical ligands, the action modes of primer pheromones that regulate reproduction and their molecular causes and effects remain challenging. Here, these questions were elucidated by comprehensively reviewing recent advances. The coordination with other modalities of queen pheromones (QPs) and its context-dependent manner to suppress worker reproduction were discussed under the vast variation and plasticity of reproduction during colony development and across taxa. In addition to the effect of QPs, special attention was paid to recent studies revealing the regulatory effect of brood pheromones. Considering the correlation between pheromone and hormone, this study focused on the production and perception of pheromones under the endocrine control and highlighted the pivotal roles of nutrition-related pathways. The novel chemicals and gene pathways discovered by recent works provide new insights into the understanding of social regulation of reproductive division of labor in insects.

Nest construction and presence do not alter territorial aggression in male threespine stickleback

Constructing a nest within a mating territory provides a clear benefit to the resident, particularly by improving the opportunity to mate. It is unclear whether animals who use nests exclusively for reproductive purposes account for either the effort invested or the resulting increase in mating potential when valuing their territories. We sought to explicitly reveal a nest’s added subjective resource value through within-group comparison of aggressive behaviour before and after nest construction. An increase in aggression following construction would indicate that the resident perceives greater subjective value in his territory, and thus values the nest. Threespine stickleback, Gasterosteus aculeatus, fish demonstrate stereotypical aggressive behaviours during an easily induced territorial defence. The male’s nest is used exclusively for reproductive purposes, avoiding any confound of shelter. Contrary to our hypothesis, neither nest presence, timing of construction, nor nesting outcome was associated with differences in behavioural measures of territorial aggression. Assessed behaviours were robust, repeatable and inter-correlated. We conclude that territorial aggression is neither predictive of nor altered by nesting in threespine stickleback fish. Our results suggest that nests used transiently for a portion of the mating season add negligible subjective resource value to a territory. We additionally demonstrate that examinations of territorial aggression in sticklebacks do not need to control for nest building, improving statistical power by decreasing dropout rates. These results dovetail with recent work in other fish species to suggest that assessment of territorial aggression absent a nest may be practicable for fish in general.

Multiple factors affect discrimination learning performance, but not between-individual variation, in wild mixed-species flocks of birds

Cognition arguably drives most behaviours in animals, but whether and why individuals in the wild vary consistently in their cognitive performance is scarcely known, especially under mixed-species scenarios. One reason for this is that quantifying the relative importance of individual, contextual, ecological and social factors remains a major challenge. We examined how many of these factors, and sources of bias, affected participation and performance, in an initial discrimination learning experiment and two reversal learning experiments during self-administered trials in a population of great tits and blue tits. Individuals were randomly allocated to different rewarding feeders within an array. Participation was high and only weakly affected by age and species. In the initial learning experiment, great tits learned faster than blue tits. Great tits also showed greater consistency in performance across two reversal learning experiments. Individuals assigned to the feeders on the edge of the array learned faster. More errors were made on feeders neighbouring the rewarded feeder and on feeders that had been rewarded in the previous experiment. Our estimates of learning consistency were unaffected by multiple factors, suggesting that, even though there was some influence of these factors on performance, we obtained a robust measure of discrimination learning in the wild.

The role of personality, cognition, and affective state in same-sex contests in the red junglefowl

Abstract

Intra-species contests are common in the animal kingdom and can have fitness consequences. Most research on what predicts contest outcome focuses on morphology, although differences in personality and cognition may also be involved. Supporting this, more proactive individuals often have dominant status, although the causality of this relationship is rarely investigated. Contest initiators often win; thus, individuals that are more proactive in their personality (e.g., more aggressive, risk-taking) or cognition (e.g., more optimistic, impulsive) may initiate contests more often. To investigate this, we assayed the behavior and cognition of sexually mature male and female red junglefowl (Gallus gallus), a species in which both sexes contest over social status, before staging intra-sexual contests. We confirm that contest initiators were more likely to win. In males, individuals that behaved more boldly in a novel arena test were more likely to initiate and win contests. Female initiators tended to be less active in novel object test, more aggressive in a restrained opponent test, and respond less optimistically in a cognitive judgement bias test, whereas the main predictor of whether a female would win a contest was whether she initiated it. These results suggest that behaviors attributed to proactive and reactive personalities, and—at least for female red junglefowl—optimism, can affect contest initiation and outcome. Therefore, within species, and depending on sex, different aspects of behavior and cognition may independently affect contest initiation and outcome. The generality of these findings, and their fitness consequences, requires further investigation.

Significance statement

In red junglefowl, we explored how behavior previously shown to describe personality, cognition, and affective state affected initiation and outcome of intra-sexual contests, by staging contests between sexually mature individuals previously assayed in behavioral and cognitive tests. In both sexes, contest initiators usually won. Bolder males were more likely to initiate and win contests. Female contests initiators were less active, more aggressive, and less optimistic. Our results suggest that personality and cognition could affect the initiation and outcome of contests and that how this occurs may differ between sexes.

Self-deception in nonhuman animals: weak crayfish escalated aggression as if they were strong

Humans routinely deceive themselves when communicating to others, but no one knows whether other animals do the same. We ask whether dishonest signaling between crayfish meets a condition required for self-deception: dishonest individuals and honest individuals escalate aggression according to their signals of strength rather than actual strength. Using game theory, we predicted how an animal’s knowledge of its strength should affect its decision to escalate aggression. At the evolutionary equilibrium, an animal that knows its strength should escalate aggression according to its strength, relative to the expected strength of its opponent. By contrast, an animal that knows only its size should escalate aggression according to its size, relative to the size of its opponent. We tested these predictions by staging encounters between male crayfish (Cherax dispar) of known sizes and strengths. Consistent with a model of self-deception, crayfish escalated aggression based on the sizes of their claws relative to those of their opponents, despite the fact that size poorly predicts strength. Males who were weak for their size escalated disputes less often, but their aggression far exceeded the level predicted by a model of self-awareness, suggesting these crayfish were largely ignorant of their deception. Animals that fail to recognize their own dishonest signals may win disputes with stronger opponents without engaging in costly combat. Our game-theoretical approach can be used to identify potential cases of self-deception in nonhuman animals, enabling comparative studies of this behavior.

Further mismeasures of animal contests: a new framework for assessment strategies

Competition for resources is a ubiquitous feature of life, and a central topic in behavioral ecology. Organisms use assessment strategies to resolve contests, which can be delineated into two broad categories by the information individuals use to make decisions: mutual assessment (MA) or self-assessment (SA). Most research hitherto has worked to bin a species into one of these categories. In this review, we discuss the limitations of this approach and provide solutions. We posit that assessment strategies do not need to be fixed within a species, individuals, or interactions, and that many organisms should adjust their assessment strategy as the environment, opponent, and opportunities for information gathering change. We show that assessment strategies are an individual-level characteristic, can vary within and between contests, and are not mutually exclusive. We argue that MA is the midpoint along a spectrum of self only and opponent only assessment. We discuss the effects of resource distribution, demographics, experience, information transfer, and ontogeny on assessment strategy evolution and behavior. We conclude by providing empirical guidelines and an example with a simulated dataset.

How does cognition shape social relationships?

The requirements of living in social groups, and forming and maintaining social relationships are hypothesized to be one of the major drivers behind the evolution of cognitive abilities. Most empirical studies investigating the relationships between sociality and cognition compare cognitive performance between species living in systems that differ in social complexity. In this review, we ask whether and how individuals benefit from cognitive skills in their social interactions. Cognitive abilities, such as perception, attention, learning, memory, and inhibitory control, aid in forming and maintaining social relationships. We investigate whether there is evidence that individual variation in these abilities influences individual variation in social relationships. We then consider the evolutionary consequences of the interaction between sociality and cognitive ability to address whether bi-directional relationships exist between the two, such that cognition can both shape and be shaped by social interactions and the social environment. In doing so, we suggest that social network analysis is emerging as a powerful tool that can be used to test for directional causal relationships between sociality and cognition. Overall, our review highlights the importance of investigating individual variation in cognition to understand how it shapes the patterns of social relationships.This article is part of the theme issue 'Causes and consequences of individual differences in cognitive abilities'.

Advantages of social skills for contest resolution

Animal contests are natural interactions that occur to obtain or defend resources such as food and territory. Selection should favour individuals that can win contests with minimal costs in terms of energy expenditure or injuries. We hypothesized that social skills contribute to animals' assessment abilities in a contest situation and thereby will shorten contest duration. Animals were either raised in early life conditions stimulating the development of social skills, termed socialization or not (control). Contests between 342 pigs at eight weeks old (171 dyads) were studied for opponent assessment ability (using a game theoretical approach), examining duration and escalation, social behaviours performed, injuries and outcome. Contesting dyads were from the same treatment group and varied in body weight, a validated measure of resource holding potential (RHP). Socialized animals had shorter contests that were resolved with fewer injuries and they showed more ritualized display behaviour, consistent with mutual assessment. Furthermore, there was evidence of a novel form of opponent assessment in the socialized group revealed by a positive relationship between winner RHP and fight duration. In conclusion, social skills enabled more rapid establishment of dominance relationships at lower cost. Besides its evolutionary relevance, these findings may also contribute towards improving animal welfare.

Social experiences shape behavioral individuality and within-individual stability

Individual repeatability characterizes many behaviors. Repeatable behavior may result from repeated social interactions among familiar group members, owing to adaptive social niche specialization. In the context of aggression, in species like field crickets, social niche specialization should also occur when individuals repeatedly interact with unfamiliar individuals. This would require the outcome of social interactions to have carry-over effects on fighting ability and aggressiveness in subsequent interactions, leading to long-term among-individual differentiation. To test this hypothesis, we randomly assigned freshly emerged adult males of the southern field cricket Gryllus bimaculatus to either a solitary or social treatment. In social treatment, males interacted with a same-sex partner but experienced a new partner every 3 days. After 3 weeks of treatment, we repeatedly subjected treated males to dyadic interactions to measure aggression. During this time, we also continuously measured the 3-daily rate of carbohydrate and protein consumption. Individual differentiation was considerably higher among males reared in the social versus solitary environment for aggressiveness but not for nutrient intake. Simultaneously, social experience led to lower within-individual stability (i.e., increased within-individual variance) in carbohydrate intake. Past social experiences, thus, shaped both behavioral individuality and stability. While previous research has emphasized behavioral individuality resulting from repeated interactions among familiar individuals, our study implies that behavioral individuality, in the context of aggression, may generally result from social interactions, whether with familiar or unfamiliar individuals. Our findings thus imply that social interactions may have a stronger effect on individual differentiation than previously appreciated.

The coevolution of transitive inference and memory capacity in the hawk-dove game

Transitive inference (TI) that uses known relationships to deduce unknown ones (using A > B and B > C to infer A > C given no direct interactions between A and C) to assess the opponent's strength, or resource-holding potential (RHP), is widely reported in animals living in a group. This sounds counter-intuitive because TI seems to require social cognition and larger memory capacity than other inference that does not require social cognition as much as TI; individuals need abilities to identify others, observe contests among others and keep the results in memory. We examine the coevolution of memory and transitive inference by the evolutionary simulations, using the asymmetric hawk-dove game. When a cost for losers is higher than a reward for winners, we found that the immediate inference strategy (II), which estimates the opponent's strength based on the past history of the direct fights, evolves with the large memory capacity, while the TI strategy, which estimates the unknown opponent's strength by transitive inference, evolves with the limited memory capacity. When a cost for losers is slightly higher than a reward for winners, the II strategy with the large memory capacity has an evolutionary advantage over the TI strategy with the limited memory capacity. It is because the direct fights are not so costly that more information about the fights leads to more accurate estimation of the opponent's strength and results in the accurate rank of the RHPs. When a cost for losers is much higher than a reward for winners, the TI strategy with the limited memory capacity has an evolutionary advantage. It is because a good way to avoid the costly fights is the prompt formation of the dominance hierarchy which does not necessarily reflect the actual rank of the RHPs; the TI strategy builds the dominance hierarchy much faster than the II strategy regardless of memory capacity, and the large amounts of information are not required for the TI strategy to form the dominance hierarchy promptly. Our study suggests that even smaller memory capacity is evolutionarily favored in TI. The TI strategy tends to reinforce the hierarchy once it is built, regardless of whether it is consistent with RHP or not, because results of direct fights are always counted. Smaller memory capacity allows players to adjust the hierarchy well if it does not represent RHP. These results prove that TI can evolve without a requirement for large memory.

Lateralization influences contest behaviour in domestic pigs

Cerebral lateralization, i.e. hemispheric asymmetries in structure and function, relates in many species to a preference to attack from their left. Lateralization increases cognitive capacity, enabling the simultaneous processing of multiple sources of information. Therefore, lateralization may constitute a component of fighting ability (Resource Holding Potential), and/or influence the efficiency of information-gathering during a contest. We hypothesized that lateralization will affect contest outcome and duration, with an advantage for more strongly lateralized individuals. In 52 dyadic contests between weight-matched pigs (Sus scrofa; n = 104; 10 wk age), the direction of orientation towards the opponent was scan sampled every 10 s. Laterality indexes (LI) were calculated for the direction and strength of lateralization. Up to 12.5% of the individuals showed significant lateralization towards either the right or left but lateralization was absent at the population level. In line with our hypothesis, animals showing strong lateralization (irrespective of direction) had a shorter contest duration than animals showing weak lateralization. Winners did not differ from losers in their strength or direction of lateralization. Overall the results suggest that cerebral lateralization may aid in conflict resolution, but does not directly contribute to fighting ability, and will be of value in the study of animal contests.

Zebrafish aggression on the sub-second time scale: evidence for mutual motor coordination and multi-functional attack manoeuvres

Most animals fight by repeating complex stereotypic behaviours, yet the internal structure of these behaviours has rarely been dissected in detail. We characterized the internal structure of fighting behaviours by developing a machine learning pipeline that measures and classifies the behaviour of individual unmarked animals on a sub-second time scale. This allowed us to quantify several previously hidden features of zebrafish fighting strategies. We found strong correlations between the velocity of the attacker and the defender, indicating a dynamic matching of approach and avoidance efforts. While velocity matching was ubiquitous, the spatial dynamics of attacks showed phase-specific differences. Contest-phase attacks were characterized by a paradoxical sideways attraction of the retreating animal towards the attacker, suggesting that the defender combines avoidance manoeuvres with display-like manoeuvres. Post-resolution attacks lacked display-like features and the defender was avoidance focused. From the perspective of the winner, game-theory modelling further suggested that highly energetically costly post-resolution attacks occurred because the winner was trying to increase its relative dominance over the loser. Overall, the rich structure of zebrafish motor coordination during fighting indicates a greater complexity and layering of strategies than has previously been recognized.

Male-male aggression is unlikely to stabilize a poison frog polymorphism

Phenotypic polymorphism is common in animals, and the maintenance of multiple phenotypes in a population requires forces that act against homogenizing drift and selection. Male-male competition can contribute to the stability of a polymorphism when males compete primarily with males of the same phenotype. In and around a contact zone between red and blue lineages of the poison frog Oophaga pumilio, we used simulated territorial intrusions to test the nonexclusive predictions that males would direct more aggression towards males of (i) their own phenotype and/or (ii) the phenotype that is most common in their population. Males in the monomorphic red and blue populations that flank the contact zone were more aggressive towards simulated intruders that matched the local coloration. However, males in the two polymorphic populations biased aggression towards neither their own colour nor the colour most common in their population. In sympatry, the rarer colour morph gains no advantage via reduced male-male aggression from territorial males in these O. pumilio populations, and so male aggression seems unlikely to stabilize colour polymorphism on its own. More broadly, these results suggest that the potential for divergent male aggression biases to maintain phenotypic diversity depends on the mechanism(s) that generate the biases and the degree to which these mechanisms persist in sympatry.