Uniting against a common enemy: Perceived outgroup threat elicits ingroup cohesion in chimpanzees

Leader-follower behavioural coordination and neural synchronization during intergroup conflict

Leaders can launch hostile attacks on out-groups and organize in-group defence. Whether groups settle the conflict in their favour depends, however, on whether followers align with leader's initiatives. Yet how leader and followers coordinate during intergroup conflict remains unknown. Participants in small groups elected a leader and made costly contributions to intergroup conflict while dorsolateral prefrontal cortex (DLPFC) activity was simultaneously measured. Leaders were more sacrificial and their contribution influenced group survival to a greater extent during in-group defence than during out-group attacks. Leaders also had increased DLPFC activity when defending in-group, which predicted their comparatively strong contribution to conflict; followers reciprocated their leader's initiatives the more their DLPFC activity synchronized with that of their leader. When launching attacks, however, leaders and followers aligned poorly at behavioural and neural levels, which explained why out-group attacks often failed. Our results provide a neurobehavioural account of leader-follower coordination during intergroup conflict and reveal leader-follower behavioural/neural alignment as pivotal for groups settling conflicts in their favour.

Exploring the Influence of Perceived Ingroup and Outgroup Threat on Quality of Life in a Region Impacted by Protracted Conflict

While the detrimental effects of protracted political conflict on the wellbeing of Palestinians living in the occupied Palestinian territory (oPt) are generally recognized, the impact of perceived threat on quality of life (QoL) faced from within their community (ingroup; Palestinians) and from the outgroup (Israelis) is unexplored. This cross-sectional study examined the following: (1) The status of perceptions of QoL on four domains measured by the World Health Organization Quality of Life (WHOQoL-Bref) instrument, physical health, psychological health, social relationships, and environment, among Palestinian adults (n = 709) living in the Gaza Strip; (2) The associations between perceived ingroup threat (PIT) and QoL on the four domains; (3) The associations between perceived outgroup threat (POT) and QoL on the four domains. Multivariable linear regression models revealed PIT was negatively associated with QoL in each of the four domains (p < 0.001). POT was positively associated with QoL in three of the four domains: physical health (p < 0.001), psychological health (p < 0.001), and social relationships (p < 0.001). This study contributes valuable insights into how QoL is viewed by a group experiencing collective existential threat. The findings expand the limited recognition of the reciprocal roles of perceived threat from the ingroup and outgroup on the QoL of vulnerable populations.

The association between food insecurity and social capital under the lockdowns in COVID-hit Shanghai

As an emergency, food insecurity threatens people’s well-being, while social capital is expected to enhance their resilience in this situation. This study examined the relationship between food insecurity and social capital during the COVID-19 lockdowns in Shanghai. We collected a dataset of 1064 participants by random sampling. Structural equation modelling was used to analyse the dynamics of social capital before and after lockdowns. The results show that the level of post-lockdown social capital was higher than that of pre-lockdown social capital. Pre-lockdown social capital predicted the extent to which people suffered from food insecurity and their approaches to obtaining food. Participation in group purchases and food exchange with other residents predicted the levels of post-lockdown social capital. The results shed light on the interaction between emergencies and social capital. Our study theoretically contributes to understanding social capital through a dynamic perspective.

Parochial cooperation in wild chimpanzees: a model to explain the evolution of parochial altruism

Parochial altruism, taking individual costs to benefit the in-group and harm the out-group, has been proposed as one of the mechanisms underlying the human ability of large-scale cooperation. How parochial altruism has evolved remains unclear. In this review paper, we formulate a parochial cooperation model in small-scale groups and examine the model in wild chimpanzees. As suggested for human parochial altruism, we review evidence that the oxytocinergic system and in-group cooperation and cohesion during out-group threat are integral parts of chimpanzee collective action during intergroup competition. We expand this model by suggesting that chimpanzee parochial cooperation is supported by the social structure of chimpanzee groups which enables repeated interaction history and established social ties between co-operators. We discuss in detail the role of the oxytocinergic system in supporting parochial cooperation, a pathway that appears integral already in chimpanzees. The reviewed evidence suggests that prerequisites of human parochial altruism were probably present in the last common ancestor between Pan and Homo. This article is part of the theme issue 'Intergroup conflict across taxa'.

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.

The founder sociality hypothesis

In this review, we propose that the social dynamics of founder populations in novel and newly available environments can have critical effects in shaping species' sociality and can produce long-lasting changes in social structure and behavior. For founder populations which expand into an underexploited niche separated from the parent population, the necessity of bond formation with strangers, lack of clear territories, and initial abundance of resources can lead to altered initial social dynamics to which subsequent generations adapt. We call this the founder sociality hypothesis. After specifying the theoretical reasoning and mechanism of effect, we focus on three particular cases where the social dynamics of founder populations may have a central role in explaining their modern behavioral ecology. In particular, we develop and review evidence for three predictions of the founder sociality hypothesis in territorial, mixed-sex group forming species: relatively stronger social bonds in the dispersing sex with relatively weaker bonds in the nondispersing sex, reduced territoriality, and increased social tolerance. We briefly touch on the implications for human evolution given our species' evolutionary history marked by frequent expansion and adaptation to novel environments. We conclude by proposing several experiments and models with testable predictions following from the founder sociality hypothesis.