The evolution of menopause in toothed whales.

Understanding how and why menopause has evolved is a long-standing challenge across disciplines. Females can typically maximize their reproductive success by reproducing for the whole of their adult life. In humans, however, women cease reproduction several decades before the end of their natural lifespan1,2. Although progress has been made in understanding the adaptive value of menopause in humans3,4, the generality of these findings remains unclear. Toothed whales are the only mammal taxon in which menopause has evolved several times5, providing a unique opportunity to test the theories of how and why menopause evolves in a comparative context. Here, we assemble and analyse a comparative database to test competing evolutionary hypotheses. We find that menopause evolved in toothed whales by females extending their lifespan without increasing their reproductive lifespan, as predicted by the 'live-long' hypotheses. We further show that menopause results in females increasing their opportunity for intergenerational help by increasing their lifespan overlap with their grandoffspring and offspring without increasing their reproductive overlap with their daughters. Our results provide an informative comparison for the evolution of human life history and demonstrate that the same pathway that led to menopause in humans can also explain the evolution of menopause in toothed whales.

Bunching behaviour in housed dairy cows at higher ambient temperatures.

Bunching behavior in cattle may occur for several reasons including enabling social interactions, a response to stress or danger, or due to shared interest in resources such as feeding or watering areas. There is evidence in pasture grazed cattle that bunching may occur more frequently at higher ambient temperatures, possibly due to sharing of fly-load or to seek shade from the direct sun under heat stress conditions. Here we demonstrate how bunching behavior is associated with higher ambient temperatures in a barn-housed UK dairy herd. A real-time local positioning system (RTLS) was used, as part of a precision livestock farming (PLF) approach, to track the spatial position and activity of a commercial dairy herd (c100 cows) in a freestall barn continuously at high temporal resolution for 4 mo between August and November 2014. Bunching was determined using 4 different spatial measures determined on an hourly basis: herd full and core range size, mean herd inter-cow distance (ICD), and mean herd nearest neighbor distance (NND). For hourly mean ambient temperatures above 20°C, the herd showed higher bunching behavior with increasing ambient temperature (i.e., reduced full and core range size, ICD, and NND). Aggregated space-use intensity was found to positively correlate with localized variations in temperature across the barn (as measured by animal mounted sensors), but the level of correlation decreased at higher ambient barn temperatures. Bunching behavior may increase localized temperatures experienced by individuals and hence may be a maladaptive behavioral response in housed dairy cattle, which are known to suffer heat stress at higher temperatures. Our study is the first to use high-resolution positional data to provide evidence of associations between bunching behavior and higher ambient temperatures for a barn-housed dairy herd in a temperate region (UK). Further studies are needed to explore the exact mechanisms for this response to inform both welfare and production management.

Postreproductive female killer whales reduce socially inflicted injuries in their male offspring.

Understanding the evolution of menopause presents a long-standing scientific challenge1,2,3-why should females cease ovulation prior to the end of their natural lifespan? In human societies, intergenerational resource transfers, for example, food sharing and caregiving, are thought to have played a key role in the evolution of menopause, providing a pathway by which postreproductive females can boost the fitness of their kin.4,5,6 To date however, other late-life contributions that postreproductive females may provide their kin have not been well studied. Here, we test the hypothesis that postreproductive female resident killer whales (Orcinus orca) provide social support to their offspring by reducing the socially inflicted injuries they experience. We found that socially inflicted injuries, as quantified by tooth rake marks, are lower for male offspring in the presence of their postreproductive mother. In contrast, we find no evidence that postreproductive mothers reduce rake marking in their daughters. Similarly, we find no evidence that either reproductive mothers or grandmothers (reproductive or postreproductive) reduce socially inflicted injuries in their offspring and grandoffspring, respectively. Moreover, we find that postreproductive females have no effect on reducing the rake marks for whales in their social unit who are not their offspring. Taken together, our results highlight that directing late-life support may be a key pathway by which postreproductive females transfer social benefits to their male offspring.

Temporal dynamics of mother-offspring relationships in Bigg's killer whales: opportunities for kin-directed help by post-reproductive females.

Age-related changes in the patterns of local relatedness (kinship dynamics) can be a significant selective force shaping the evolution of life history and social behaviour. In humans and some species of toothed whales, average female relatedness increases with age, which can select for a prolonged post-reproductive lifespan in older females due to both costs of reproductive conflict and benefits of late-life helping of kin. Killer whales (Orcinus orca) provide a valuable system for exploring social dynamics related to such costs and benefits in a mammal with an extended post-reproductive female lifespan. We use more than 40 years of demographic and association data on the mammal-eating Bigg's killer whale to quantify how mother-offspring social relationships change with offspring age and identify opportunities for late-life helping and the potential for an intergenerational reproductive conflict. Our results suggest a high degree of male philopatry and female-biased budding dispersal in Bigg's killer whales, with some variability in the dispersal rate for both sexes. These patterns of dispersal provide opportunities for late-life helping particularly between mothers and their adult sons, while partly mitigating the costs of mother-daughter reproductive conflict. Our results provide an important step towards understanding why and how menopause has evolved in Bigg's killer whales.

Killer whales.

Michael Weiss and Darren Croft introduce Orcas (Orcinus orca) also known as killer whales.

Costly lifetime maternal investment in killer whales.

Parents often sacrifice their own future reproductive success to boost the survival of their offspring, a phenomenon referred to as parental investment. In several social mammals, mothers continue to improve the survival of their offspring well into adulthood;1,2,3,4,5 however, whether this extended care comes at a reproductive costs to mothers, and therefore represents maternal investment, is not well understood. We tested whether lifetime maternal care is a form of parental investment in fish-eating "resident" killer whales. Adult killer whales, particularly males, are known to receive survival benefits from their mothers;3 however, whether this comes at a cost to mothers' reproductive success is not known. Using multiple decades of complete census data from the "southern resident" population, we found a strong negative correlation between females' number of surviving weaned sons and their annual probability of producing a viable calf. This negative effect did not attenuate as sons grew older, and the cost of sons could not be explained by long-term costs of lactation or group composition effects, supporting the hypothesis that caring for adult sons is reproductively costly. This is the first direct evidence of lifetime maternal investment in an iteroparous animal, revealing a previously unknown life history strategy.

Social behaviour and transmission of lameness in a flock of ewes and lambs.

Introduction: Sheep have heterogenous social connections that influence transmission of some infectious diseases. Footrot is one of the top five globally important diseases of sheep, it is caused by Dichelobacter nodosus and transmits between sheep when infectious feet contaminate surfaces, e.g., pasture. Surfaces remain infectious for a few minutes to a few days, depending on surface moisture levels. Susceptible sheep in close social contact with infectious sheep might be at risk of becoming infected because they are likely to step onto infectious footprints, particularly dams and lambs, as they cluster together. Methods: High resolution proximity sensors were deployed on 40 ewes and their 54 lambs aged 5-27 days, in a flock with endemic footrot in Devon, UK for 13 days. Sheep locomotion was scored daily by using a 0-6 integer scale. Sheep were defined lame when their locomotion score (LS) was ≥2, and a case of lameness was defined as LS ≥2 for ≥2 days. Results: Thirty-two sheep (19 ewes, 9 single, and 4 twin lambs) became lame during the study, while 14 (5 ewes, 5 single, and 4 twin lambs) were lame initially. These 46 sheep were from 29 family groups, 14 families had >1 lame sheep, and transmission from ewes to lambs was bidirectional. At least 15% of new cases of footrot were from within family transmission; the occurrence of lameness was higher in single than twin lambs. At least 4% of transmission was due to close contact across the flock. Most close contact occurred within families. Single and twin lambs spent 1.5 and 0.9 hours/day with their dams, respectively, and twin lambs spent 3.7 hours/day together. Non-family sheep spent only 0.03 hours/day in contact. Lame single lambs and ewes spent less time with non-family sheep, and lame twin lambs spent less time with family sheep. Discussion: We conclude that most transmission of lameness is not attributable to close contact. However, in ewes with young lambs, some transmission occurs within families and is likely due to time spent in close contact, since single lambs spent more time with their dam than twin lambs and were more likely to become lame.

Patterns and consequences of age-linked change in local relatedness in animal societies.

The ultimate payoff of behaviours depends not only on their direct impact on an individual, but also on the impact on their relatives. Local relatedness-the average relatedness of an individual to their social environment-therefore has profound effects on social and life history evolution. Recent work has begun to show that local relatedness has the potential to change systematically over an individual's lifetime, a process called kinship dynamics. However, it is unclear how general these kinship dynamics are, whether they are predictable in real systems and their effects on behaviour and life history evolution. In this study, we combine modelling with data from real systems to explore the extent and impact of kinship dynamics. We use data from seven group-living mammals with diverse social and mating systems to demonstrate not only that kinship dynamics occur in animal systems, but also that the direction and magnitude of kinship dynamics can be accurately predicted using a simple model. We use a theoretical model to demonstrate that kinship dynamics can profoundly affect lifetime patterns of behaviour and can drive sex differences in helping and harming behaviour across the lifespan in social species. Taken together, this work demonstrates that kinship dynamics are likely to be a fundamental dimension of social evolution, especially when considering age-linked changes and sex differences in behaviour and life history.

Partial sex linkage and linkage disequilibrium on the guppy sex chromosome.

The guppy Y chromosome has been considered a model system for the evolution of suppressed recombination between sex chromosomes, and it has been proposed that complete sex-linkage has evolved across about 3 Mb surrounding this fish's sex-determining locus, followed by recombination suppression across a further 7 Mb of the 23 Mb XY pair, forming younger "evolutionary strata". Sequences of the guppy genome show that Y is very similar to the X chromosome. Knowing which parts of the Y are completely nonrecombining, and whether there is indeed a large completely nonrecombining region, are important for understanding its evolution. Here, we describe analyses of PoolSeq data in samples from within multiple natural populations from Trinidad, yielding new results that support previous evidence for occasional recombination between the guppy Y and X. We detected recent demographic changes, notably that downstream populations have higher synonymous site diversity than upstream ones and other expected signals of bottlenecks. We detected evidence of associations between sequence variants and the sex-determining locus, rather than divergence under a complete lack of recombination. Although recombination is infrequent, it is frequent enough that associations with SNPs can suggest the region in which the sex-determining locus must be located. Diversity is elevated across a physically large region of the sex chromosome, conforming to predictions for a genome region with infrequent recombination that carries one or more sexually antagonistic polymorphisms. However, no consistently male-specific variants were found, supporting the suggestion that any completely sex-linked region may be very small.

Social partner cooperativeness influences brain oxytocin transcription in Trinidadian guppies (Poecilia reticulata).

For non-kin cooperation to be maintained, individuals need to respond adaptively to the cooperative behaviour of their social partners. Currently, however, little is known about the biological responses of individuals to experiencing cooperation. Here, we quantify the neuroregulatory response of Trinidadian guppies (Poecilia reticulata) experiencing cooperation or defection by examining the transcriptional response of the oxytocin gene (oxt; also known as isotocin), which has been implicated in cooperative decision-making. We exposed wild-caught females to social environments where partners either cooperated or defected during predator inspection, or to a control (non-predator inspection) context, and quantified the relative transcription of the oxt gene. We tested an experimental group, originating from a site where individuals are under high predation threat and have previous experience of large aquatic predators (HP), and a control group, where individuals are under low predation threat and naïve to large aquatic predators (LP). LP, but not HP, fish showed different behavioural responses to the behaviour of their social environment, cooperating with cooperative partners and defecting when paired with defecting ones. In HP, but not LP, fish brain mid-section oxt relative transcription varied depending on social partner behaviour. HP fish experiencing cooperation during predator inspection had lower oxt transcription than those experiencing defection. This effect was not present in the control population or in the control context, where the behaviour of social partners did not affect oxt transcription. Our findings provide insight into the neuromodulation underpinning behavioural responses to social experiences, and ultimately to the proximate mechanisms underlying social decision-making.

Sensory-based quantification of male colour patterns in Trinidadian guppies reveals no support for parallel phenotypic evolution in multivariate trait space.

Parallel evolution, in which independent populations evolve along similar phenotypic trajectories, offers insights into the repeatability of adaptive evolution. Here, we revisit a classic example of parallelism, that of repeated evolution of brighter males in the Trinidadian guppy (Poecilia reticulata). In guppies, colonisation of low predation habitats is associated with emergence of 'more colourful' phenotypes since predator-induced viability selection for crypsis weakens while sexual selection by female preference for conspicuousness remains strong. Our study differs from previous investigations in three respects. First, we adopted a multivariate phenotyping approach to characterise parallelism in multitrait space. Second, we used ecologically-relevant colour traits defined by the visual systems of the two selective agents (i.e., guppy, predatory cichlid). Third, we estimated population genetic structure to test for adaptive (parallel) evolution against a model of neutral phenotypic divergence. We find strong phenotypic differentiation that is inconsistent with a neutral model but very limited support for the predicted pattern of greater conspicuousness at low predation. Effects of predation regime on each trait were in the expected direction, but weak, largely nonsignificant, and explained little among-population variation. In multitrait space, phenotypic trajectories of lineages colonising low from high predation regimes were not parallel. Our results are consistent with reduced predation risk facilitating adaptive differentiation, potentially by female choice, but suggest that this proceeds in independent directions of multitrait space across lineages. Pool-sequencing data also revealed SNPs showing greater differentiation than expected under neutrality, among which some are found in genes contributing to colour pattern variation, presenting opportunities for future genetic study.

Reduced older male presence linked to increased rates of aggression to non-conspecific targets in male elephants.

Males in many large mammal species spend a considerable portion of their lives in all-male groups segregated from females. In long-lived species, these all-male groups may contain individuals of vastly different ages, providing the possibility that behaviours such as aggression vary with the age demographic of the social environment, as well as an individual's own age. Here, we explore social factors affecting aggression and fear behaviours in non-musth male African elephants (Loxodonta africana) aggregating in an all-male area. Adolescent males had greater probabilities of directing aggressive and fearful behaviours to non-elephant targets when alone compared to when with other males. All males, regardless of age, were less aggressive towards non-elephant targets (e.g. vehicles and non-elephant animals) when larger numbers of males from the oldest age cohort were present. The presence of older males did not influence the probability that other males were aggressive to conspecifics or expressed fearful behaviours towards non-elephant targets. Older bulls may police aggression directed towards non-elephant targets or may lower elephants' perception of their current threat level. Our results suggest male elephants may pose an enhanced threat to humans and livestock when adolescents are socially isolated, and when fewer older bulls are nearby.

Function of Trunk-Mediated "Greeting" Behaviours between Male African Elephants: Insights from Choice of Partners.

A common behavioural interaction between male African elephants is for an actor to direct his trunk to contact a same sex conspecific's mouth, temporal gland, or genital region. Such behaviours are often referred to as "greetings". Along with its inherent tactile element, these behaviours also likely provide olfactory information to actors concerning aspects of the target's phenotype, including sexual status, feeding history, individual identity, and emotional state. Here, we explore whether the age and novelty of potential interactors affect the choice of individuals targeted by male African elephants for these trunks to scent emitting organ (SEO) behaviours at social hotspots in a male-dominated area. Male elephants of all ages, except older adolescents aged 16-20 years, preferentially targeted elephants of the same age class for trunk-to-SEO behaviours. Elephants younger than 26 years did not direct trunk-to-SEO behaviours to mature bulls (26+ years) more than expected by chance, suggesting these behaviours are not primarily used for younger males to establish contact with, or obtain information from or about older, more experienced individuals. We also found no evidence that males directed these behaviours preferentially to new individuals they encountered at male aggregations (compared to those they arrived in groups with), suggesting these behaviours are not primarily employed by males as a reunion display to establish relationships between new individuals or update relationships between familiar individuals separated over time. Age-mates may be preferentially targeted with these behaviours as a means to facilitate further interaction with partners (e.g., for sparring activity), or as a safe way to assess relative dominance rank in similarly aged and hence, size and strength, matched dyads. Our results suggest male African elephants use close contact trunk-to-SEO behaviours continuously over time, to facilitate positive relationships, test willingness to interact, and assess aspects of phenotype, between males occupying the same ecological space.

Kinship dynamics: patterns and consequences of changes in local relatedness.

Mounting evidence suggests that patterns of local relatedness can change over time in predictable ways, a process termed kinship dynamics. Kinship dynamics may occur at the level of the population or social group, where the mean relatedness across all members of the population or group changes over time, or at the level of the individual, where an individual's relatedness to its local group changes with age. Kinship dynamics are likely to have fundamental consequences for the evolution of social behaviour and life history because they alter the inclusive fitness payoffs to actions taken at different points in time. For instance, growing evidence suggests that individual kinship dynamics have shaped the evolution of menopause and age-specific patterns of helping and harming. To date, however, the consequences of kinship dynamics for social evolution have not been widely explored. Here we review the patterns of kinship dynamics that can occur in natural populations and highlight how taking a kinship dynamics approach has yielded new insights into behaviour and life-history evolution. We discuss areas where analysing kinship dynamics could provide new insight into social evolution, and we outline some of the challenges in predicting and quantifying kinship dynamics in natural populations.

A long postreproductive life span is a shared trait among genetically distinct killer whale populations.

The extended female postreproductive life span found in humans and some toothed whales remains an evolutionary puzzle. Theory predicts demographic patterns resulting in increased female relatedness with age (kinship dynamics) can select for a prolonged postreproductive life span due to the combined costs of intergenerational reproductive conflict and benefits of late-life helping. Here, we test this prediction using >40 years of longitudinal demographic data from the sympatric yet genetically distinct killer whale ecotypes: resident and Bigg's killer whales. The female relatedness with age is predicted to increase in both ecotypes, but with a less steep increase in Bigg's due to their different social structure. Here, we show that there is a significant postreproductive life span in both ecotypes with >30% of adult female years being lived as postreproductive, supporting the general prediction that an increase in local relatedness with age predisposes the evolution of a postreproductive life span. Differences in the magnitude of kinship dynamics however did not influence the timing or duration of the postreproductive life span with females in both ecotypes terminating reproduction before their mid-40s followed by an expected postreproductive period of about 20 years. Our results highlight the important role of kinship dynamics in the evolution of a long postreproductive life span in long-lived mammals, while further implying that the timing of menopause may be a robust trait that is persistent despite substantial variation in demographic patterns among populations.

Age and sex influence social interactions, but not associations, within a killer whale pod.

Social structure is a fundamental aspect of animal populations. In order to understand the function and evolution of animal societies, it is important to quantify how individual attributes, such as age and sex, shape social relationships. Detecting these influences in wild populations under natural conditions can be challenging, especially when social interactions are difficult to observe and broad-scale measures of association are used as a proxy. In this study, we use unoccupied aerial systems to observe association, synchronous surfacing, and physical contact within a pod of southern resident killer whales (Orcinus orca). We show that interactions do not occur randomly between associated individuals, and that interaction types are not interchangeable. While age and sex did not detectably influence association network structure, both interaction networks showed significant social homophily by age and sex, and centrality within the contact network was higher among females and young individuals. These results suggest killer whales exhibit interesting parallels in social bond formation and social life histories with primates and other terrestrial social mammals, and demonstrate how important patterns can be missed when using associations as a proxy for interactions in animal social network studies.

Network-level consequences of outgroup threats in banded mongooses: Grooming and aggression between the sexes.

Animal groups are heterogeneous assemblages of individuals with differing fitness interests, which may lead to internal conflict over investment in group territorial defence. Differences between individuals may lead to different behavioural responses to intergroup conflict, particularly between the sexes. These potential impacts have been little studied. We used social network analysis to investigate the impact of simulated intergroup conflicts on social relationships in groups of wild banded mongooses Mungos mungo, in which intergroup fights are more costly for males than females. We predicted that social cohesion (specifically male-to-male and female-to-male grooming) would increase after conflict, and aggression would decrease, to minimize conflict between the sexes. Simulated intergroup conflicts were performed by exposing banded mongoose groups to scents, 'war cry' playbacks, and live intruders from a rival group. All grooming and aggression interactions between individuals were recorded, and grooming and aggression social networks were created for the 2 days preceding a simulated intergroup conflict (pre-conflict network) and the 2 days after (post-conflict network). We found no evidence of an increase in social cohesion after simulated conflicts, measured as grooming eigenvector centrality. Male-to-male, male-to-female and female-to-male grooming strength decreased after simulated intrusions compared to female-to-female grooming strength. However, male-female aggression decreased in intrusion trials compared to other interaction types, consistent with the hypothesis that intergroup encounters reduce the level of intragroup conflict between males and females. Males were more affected socially by intergroup encounters than females, which may be because they are investing in defence rather than internal relationships. Focusing on individual relationship changes, using social network analysis, can reveal changes in the directionality of behaviour in response to intergroup encounters, and highlight how individual responses to conflict may scale up to affect social networks and, potentially, group performance. This study highlights the importance of studying both group-level behaviours and individual relationships to more fully understand responses to intergroup encounters.

Intraspecific variation in inhibitory motor control in guppies, Poecilia reticulata.

Inhibitory control (IC) is the ability to overcome impulsive or prepotent but ineffective responses in favour of more appropriate behaviours. The ability to inhibit internal predispositions or external temptations is vital in coping with a complex and variable world. Traditionally viewed as cognitively demanding and a main component of executive functioning and self-control, IC was historically examined in only a few species of birds and mammals but recently a number of studies has shown that a much wider range of taxa rely on IC. Furthermore, there is growing evidence that inhibitory abilities may vary within species at the population and individual levels owing to genetic and environmental factors. Here we use a detour-reaching task, a standard paradigm to measure motor inhibition in nonhuman animals, to quantify patterns of interindividual variation in IC in wild-descendant female guppies, Poecilia reticulata. We found that female guppies displayed inhibitory performances that were, on average, half as successful as the performances reported previously for other strains of guppies tested in similar experimental conditions. Moreover, we showed consistent individual variation in the ability to inhibit inappropriate behaviours. Our results contribute to the understanding of the evolution of fish cognition and suggest that IC may show considerable variation among populations within a species. Such variation in IC abilities might contribute to individual differences in other cognitive functions such as spatial learning, quantity discrimination or reversal learning.

Common datastream permutations of animal social network data are not appropriate for hypothesis testing using regression models.

1. Social network methods have become a key tool for describing, modelling, and testing hypotheses about the social structures of animals. However, due to the non-independence of network data and the presence of confounds, specialized statistical techniques are often needed to test hypotheses in these networks. Datastream permutations, originally developed to test the null hypothesis of random social structure, have become a popular tool for testing a wide array of null hypotheses in animal social networks. In particular, they have been used to test whether exogenous factors are related to network structure by interfacing these permutations with regression models. 2. Here, we show that these datastream permutations typically do not represent the null hypothesis of interest to researchers interfacing animal social network analysis with regression modelling, and use simulations to demonstrate the potential pitfalls of using this methodology. 3. Our simulations show that, if used to indicate whether a relationship exists between network structure and a covariate, datastream permutations can result in extremely high type I error rates, in some cases approaching 50%. In the same set of simulations, traditional node-label permutations produced appropriate type I error rates (~ 5%). 4. Our analysis shows that datastream permutations do not represent the appropriate null hypothesis for these analyses. We suggest that potential alternatives to this procedure may be found in regarding the problems of non-independence of network data and unreliability of observations separately. If biases introduced during data collection can be corrected, either prior to model fitting or within the model itself, node-label permutations then serve as a useful test for interfacing animal social network analysis with regression modelling.

The dynamics of social cohesion in response to simulated intergroup conflict in banded mongooses.

Intergroup conflict is widespread in nature and is proposed to have strong impacts on the evolution of social behavior. The conflict-cohesion hypothesis predicts that exposure to intergroup conflict should lead to increased social cohesion to improve group success or resilience in future conflicts. There is evidence to support this prediction from studies of affiliative responses to outgroup threats in some animal societies. However, most of these studies have focused on behavioral changes over short time periods (minutes and hours after exposure to an outgroup), and hence very little is known about the dynamics and durability of responses to intergroup conflict over the longer term. We investigated this question by simulating intergroup encounters in wild banded mongooses (Mungos mungo) and measuring social behavior before, during, and after these encounters over a 5-day period. We also ran control trials with non-threatening stimuli. Banded mongooses reacted immediately to intrusion stimuli by vocalizing, grouping together, and advancing on the stimulus. In the first 5 min after simulated intrusions, we saw an elevation in grooming levels, but in the hour after exposure grooming rates declined sharply, contrary to our expectation. In the two subsequent days, grooming rates remained at this depressed rate. In control trials, the initial increase in grooming was not seen, but grooming declined compared to the longer-term time periods. Grooming changed across time, but not in the same pattern as during intrusions, suggesting that intrusions had an impact above and beyond that of the experimental setup. The dynamics of grooming responses were short lived and more complex than we initially expected. We suggest this unexpected result may be linked to the frequency of aggressive intergroup encounters in this system. As control and experimental trials were run at different times of year, future work would be needed to confirm that these relative patterns are replicable. Our results indicate short-lived impacts of outgroup threat on measures of social cohesion in this species, but cannot confirm longer-term changes.