Social animals that live in groups often have disagreements over access to mates and food. Even fleeting in-group disputes can be costly, disrupting relationships, wasting time and energy, or causing injury if aggression escalates. So, much like humans, many social animals, including primates, birds and dogs, have evolved conflict management strategies to prevent and resolve in-group disagreements. In the immediate aftermath of a conflict, this usually involves changes in the interactions between those involved in the disagreement, or between bystander groupmates and either the victim or aggressor.

Less is known about whether social animals can recall past disputes and if they can use conflict management strategies some time after a quarrel has occurred. That is, do aggressive interactions between groupmates influence later social decisions of bystanders in the group?

To investigate, Morris-Drake et al. studied groups of wild dwarf mongooses (Helogale parvula) that have become accustomed to living alongside humans in Limpopo Province, South Africa. Dwarf mongooses live in groups of up to 30 individuals, with one dominant breeding pair and lower-ranked helpers. When disagreements arise over food, an aggressor growls deeply and hip-slams the victim away from their foraging patch, stealing the victim’s prey in the process. Victims often produce high-pitched squeals in retreat.

Using recordings of these calls, Morris-Drake et al. devised a field experiment to investigate how mongooses responded to nearby conflicts between other group members. Recordings simulating a conflict over food were played to groups of foraging mongooses over the course of an afternoon, so that group members effectively heard what sounded like repeated squabbles between two out-of-sight individuals. Similar to natural conflicts, the mongooses did not engage in any obvious conflict management behaviour immediately after hearing these disputes. But when the group returned to their sleeping burrow that evening, subordinate group members shunned the perceived aggressors from grooming, a key social activity.

This work provides evidence that dwarf mongooses keep tabs on conflicts that occur between groupmates. It shows these animals can extract information about conflicts from vocal cues alone and that bystanders use this information when making later social decisions impacting group dynamics. It also adds to growing evidence from baboons, monkeys and chimpanzees that social animals can remember past events and take these into account when interacting with groupmates.

Conflicts of interest are common in social species, with disagreements between group members arising over access to mates or food, synchronisation of group activities, and the direction of travel (Aureli et al., 2002; Conradt and Roper, 2009; Hardy and Briffa, 2013). Within-group conflict, especially if it escalates to aggression, can be costly in terms of injury and mortality, time and energy expenditure, increased stress, and disrupted social relationships (Aureli, 1997; Aureli et al., 2002; de Waal, 2000). Conflict-management strategies that minimise these costs, either by reducing the likelihood of aggressive escalation in the first place or by mitigating the consequences of such physical contests when they do arise, have therefore evolved in many species (Aureli et al., 2002; Aureli and de Waal, 2000). Much of the early work on post-conflict behaviour focussed on interactions between the protagonists (the aggressor and the target): many studies have documented increases in affiliation between former opponents in the aftermath of a contest (reconciliation; Aureli et al., 2002; de Waal, 2000; de Waal and van Roosmalen, 1979), although there are also examples of victims avoiding aggressors (wariness; Benkada et al., 2020; Kutsukake and Clutton-Brock, 2008; Sommer et al., 2002). More recently, attention has shifted to the involvement of bystanders (contest nonparticipants) in post-conflict behaviour. Considering bystanders highlights the potentially groupwide effects of dyadic within-group conflicts and a wider range of post-conflict management strategies than would be apparent from a focus on just the protagonists (De Marco et al., 2010; Schino and Sciarretta, 2015), thus providing additional insights into the dynamics of social relationships between groupmates (Aureli and de Waal, 2000). Multiple studies have now documented bystander-initiated affiliation with the victim as a means of avoiding redirected aggression (self-protection) or of providing substitute reconciliation or consolation (Fraser et al., 2009; Fraser et al., 2008; Schino and Marini, 2012; Wittig and Boesch, 2010). There is also some evidence of bystander-initiated affiliation with the aggressor, which could function as appeasement to reduce the likelihood of redirected aggression (Cordoni and Palagi, 2015; Palagi et al., 2008; Pallante et al., 2018), and group-wide post-conflict affiliation among bystanders, perhaps to reduce conflict-induced stress (De Marco et al., 2010; Judge and Mullen, 2005). However, to the best of our knowledge, this research has focussed solely on interactions that occur in the immediate aftermath (usually within 10 min) of an aggressive within-group contest; the possibility of delayed post-conflict management behaviour has not been explored.

There is increasing experimental evidence that nonhuman animals can remember past events and use information from them when making social decisions later (Carter and Wilkinson, 2013; Kern and Radford, 2018; Seyfarth and Cheney, 1984; Wittig et al., 2014). This includes conflict-management decisions about whether to get involved in an aggressive interaction. For example, baboons (Papio hamadryas ursinus) were more likely to offer support in aggressive interactions to individuals they had groomed with earlier (mean: 22 min before; range 10–55 min), evidenced by a move towards playbacks of grunt calls given during conflicts (Cheney et al., 2010). Similarly, vervet monkeys (Chlorocebus pygerythrus) were more likely to offer coalitionary support to a groupmate in a conflict if they had groomed together within the last hour (Borgeaud and Bshary, 2015). Other studies have shown that individuals can use knowledge of previous agonistic interactions to inform how best to respond in subsequent aggressive encounters. For instance, chimpanzees (Pan troglodytes) that had been involved in an unreconciled conflict earlier in the day (ca 2 hr before) reacted aversively to the playback of an aggressive bark from their former opponent’s bond partner (a third-party individual likely to offer aggressive support to the former opponent; Wittig et al., 2014). Moreover, it was recently shown that bystander wasps (Polistes fuscatus) were more aggressive towards individuals that they had observed to be less aggressive in a previous (10–30 min earlier) fight with a third party (Tibbetts et al., 2020). It is thus plausible that post-conflict decisions about the avoidance of protagonists and affiliation with groupmates could also occur some time after the relevant within-group contests. Investigating the capacity for delayed management behaviour is important because it is thought to be cognitively challenging to use social information gathered in the past, especially where there is reliance on memories of the actions of particular individuals (Frith and Frith, 2012), as other sources of personal and third-party information would likely arise in the interim (Wittig et al., 2014).

To make behavioural decisions, animals obtain information about social interactions using a variety of sensory modalities. Most research considering social monitoring of within-group conflicts has focussed on situations where individuals have seen the interaction; hence, bystanders are commonly defined as individuals who have observed the encounter (Schino and Sciarretta, 2015). But for those species living in visually occluded environments, those where group members can be scattered over large distances or those that forage in a way that prevents simultaneous vigilance, acoustic cues can be a valuable source of social information (Bradbury and Vehrencamp, 2011). Numerous species vocalise during or at the end of within-group contests (Bertram et al., 2010; Slocombe et al., 2010). For example, chimpanzees and rhesus monkeys (Macaca mulatta) produce screams when experiencing aggression (Gouzoules et al., 1984; Slocombe et al., 2010), whilst little blue penguins (Eudyptula minor) give specific calls after a contest is finished (Waas, 1990). These vocalisations likely provide bystanders with valuable information about the occurrence of within-group conflicts as well as about the group members that have potentially been involved and the outcome (Gouzoules et al., 1984; Slocombe and Zuberbühler, 2007; Szipl et al., 2017; Whitehouse and Meunier, 2020). Moreover, they can be used in playbacks to test post-conflict behaviour experimentally.

Here, we investigate post-conflict management behaviour, including the possibility that it occurs with a delay (ca 30–60 min later), in wild dwarf mongooses (Helogale parvula); the study population has been habituated to close human presence, facilitating detailed observations and field-based experiments (Kern and Radford, 2018; Morris-Drake et al., 2019). Dwarf mongooses live in cooperatively breeding groups of up to 30 individuals, comprising a dominant breeding pair (hereafter ‘dominant’ individuals) and non-breeding subordinate helpers (hereafter ‘subordinate’ individuals) of both sexes (Rasa, 1977). The most prevalent affiliative behaviour in dwarf mongoose groups is allogrooming (hereafter ‘grooming’), which underpins the strength of relationships between group members (Kern and Radford, 2021; Kern and Radford, 2016), increases following stressful situations such as intergroup interactions (Morris-Drake et al., 2019), and is traded as a reward for cooperative behaviour (Kern and Radford, 2018). Within-group aggressive interactions take two main forms: relatively rare targeted aggression, which usually acts to reinforce rank and is mainly due to reproductive conflict (Rasa, 1977), and relatively common foraging displacements, when a higher-ranking individual displaces a lower-ranking group member from a foraging patch and steals their prey (Sharpe et al., 2016; Sharpe et al., 2013). Foraging displacements generally involve the following behavioural sequence: the higher-ranking individual produces deep growls as it approaches the lower-ranking group member; the former then hip-slams the latter away from the food resource; and the displaced individual typically produces high-pitched squeals whilst it retreats (Sharpe et al., 2016; Sharpe et al., 2013). We determined whether vocal cues of within-group conflict elicit immediate or delayed behavioural responses (avoidance or changes in affiliation) by non-participant group members. We focussed on data collection of bystanders because it is not ecologically valid to consider how protagonists respond to their own calls.

Dwarf mongoose bystanders did not engage in any obvious post-conflict affiliation in the immediate aftermath of natural or simulated foraging displacements involving a dominant and subordinate group member, but did adjust their later grooming behaviour at the evening sleeping refuge following a simulated increase in within-group conflict during the afternoon. The increase in the average duration of later subordinate–subordinate grooming is in line with the increase in bystander–bystander grooming seen in some species in the immediate aftermath of a contest (Judge and Mullen, 2005). Such affiliation could reduce the group-wide social anxiety induced by aggression (De Marco et al., 2010; Judge and Bachmann, 2013; Schino and Sciarretta, 2015). The later reduction in grooming of aggressors by bystanders is, to our knowledge, the first evidence for a change in this direction; some previous studies have documented increased grooming of aggressors by bystanders in the immediate aftermath of a single contest (Cordoni and Palagi, 2015; Palagi et al., 2008; Pallante et al., 2018), whilst a few others have found no evidence for such an increase (Judge, 1991; Romero et al., 2008; Verbeek and de Waal, 1997). Subordinate bystanders could be avoiding the aggressor to reduce the likelihood of redirected aggression, which parallels the main strategy employed in the immediate aftermath of contests by meerkat (Suricata suricatta) and rook (Corvus frugilegus) targets attempting to avoid renewed aggression (Benkada et al., 2020; Kutsukake and Clutton-Brock, 2008). Our results support previous research showing that within-group conflict can affect interactions beyond those between the protagonists and highlight that bystanders can employ different conflict-management strategies depending on the identity of the group members involved. Disagreements between two individuals can thus have wide-reaching and varied implications for affiliative behaviour, which underpins dyadic relationships and social structure in dwarf mongooses (Kern and Radford, 2021; Kern and Radford, 2018) and many other group-living species (Cameron et al., 2009; Radford and Du Plessis, 2006; Silk et al., 2009).

The changes in evening grooming patterns once the group had moved from their afternoon foraging areas demonstrate that individuals can retain information relating to earlier within-group conflict and use it when making later decisions about management-related behaviours. It is possible that hearing foraging displacements generates stress in bystanders, which could still be elevated on arrival at the sleeping burrow. But the specific reduction in grooming of perceived aggressors and not of non-playback dominants suggests decision-making about which groupmates to groom rather than a general byproduct of an altered physiological state. Numerous studies on a range of species have found changes in affiliative behaviour between various combinations of protagonists and bystanders in the minutes after within-group contests (Aureli et al., 2002; de Waal, 2000). It remains unknown whether, in those species, there might be delayed effects (as we have found) of those contests that do not result in immediate changes in affiliative behaviour. By using call playbacks, we did not alter the state of the individuals who were simulated to be the aggressor and target, and thus can rule out the possibility that differences in grooming result from experimentally induced satiation effects (which might have been the case if we had caused foraging displacements with the presentation of food items; Sharpe et al., 2013). Moreover, the use of playback simulations, rather than generation of actual contests, means that the delayed grooming effects are most likely driven by subordinate bystanders behaving differently towards perceived protagonists, rather than solicitation or rejection of grooming by the latter. Previous work has shown that dwarf mongooses can use vocal information to detect earlier cooperative contributions by groupmates and then reward them later (Kern and Radford, 2018); we now provide experimental evidence for delayed post-conflict management behaviour. It is thus increasingly apparent that mongooses, as well as many other social species, are constantly monitoring the behaviour and interactions of groupmates, and are using memories of what they learn to inform later decisions (Tibbetts et al., 2020; Wittig et al., 2014). The cognitive demands of tracking individuals and their behaviours, remembering that information, and using it when making decisions explain why social interactions within (Dunbar and Shultz, 2007) and between (Ashton et al., 2020) groups are believed to be strong drivers of animal intelligence.

Our experiments show that dwarf mongooses can extract information about within-group conflicts, and the identity of at least some protagonists, from vocal cues alone. This adds to a growing body of work demonstrating the ability of social species to garner information acoustically about aggressive interactions (Gouzoules et al., 1984; Slocombe et al., 2010; Slocombe and Zuberbühler, 2007); for example, male little blue penguins had an increased heartrate after hearing vocalisations produced by winners of a contest compared to those produced by losers (Mouterde et al., 2012). Our findings also complement the small number of studies showing that social animals use vocalisations to assess the behaviour, such as the reliability (Blumstein et al., 2004) and cooperative contributions (Kern and Radford, 2018), of individually identifiable groupmates. Acoustic monitoring is beneficial as it allows information acquisition in environments where it would be difficult to do so visually (eg, in low-light and dense vegetation) or when group members are widely scattered and communication is needed over long distances (Bradbury and Vehrencamp, 2011). Moreover, acoustic information can be gathered at a relatively low cost: it can be done whilst still actively foraging (Hollén et al., 2008) and, in the case of aggressive encounters, at a safe distance that minimises the risk of the information-gatherer receiving any redirected aggression. Monitoring behaviours acoustically is likely not possible for all within-group interactions (eg, grooming) or in all social systems, but the calls commonly produced during and at the end of aggressive contests (Bertram et al., 2010; Slocombe et al., 2010) provide a valuable means for bystanders to inform subsequent decision-making.

We found clear evidence that subordinate bystanders engage in less grooming with simulated aggressors, but whether they increased their grooming with the simulated target is less clear-cut. There are several possible explanations for this difference in the strength of response exhibited to the two protagonists. First, all subordinates might be wary of the aggressor and so potentially reduce their grooming with that individual, whereas perhaps only those who are strongly bonded to the target might engage in extra grooming with it (Fraser et al., 2009; Fraser et al., 2008); any such target-related effect might be diluted by considering all subordinates in analyses. Strong within-group relationships are apparent in dwarf mongoose groups (Kern and Radford, 2021; Kern and Radford, 2016), but we do not have the power in this study to consider how relationship quality influences delayed post-conflict grooming. Alternatively, there could be selective attention towards high-ranking individuals (Chance, 1967): there may be higher selective pressure to discriminate vocalisations from dominant individuals cf those from other subordinates if the former are more important in terms of social relationships and status. In many primate species, for example, individuals focus attention on higher-ranking groupmates or those with whom they have an antagonistic relationship (Keverne et al., 1978; McNelis and Boatright-Horowitz, 1998), possibly to avoid aggression (Schino and Sciarretta, 2016). Since our simulated aggressors were dominants and our simulated targets were subordinates, the stronger effect of increased conflict on grooming with the former could reflect such an attention bias.

Another possible reason for the difference in grooming responses to aggressors and targets could be differences in the natural acoustic properties of aggressive growls and submissive squeals (Gustison and Townsend, 2015). In principle, squeals might encode less identity information than growls (Owren and Rendall, 2003; Rendall et al., 1996), although a number of studies have found that calls similar in structure and function to dwarf mongoose squeals are individually identifiable (Cheney and Seyfarth, 1980; Fischer, 2004; Gouzoules et al., 1984; Slocombe and Zuberbühler, 2005). In addition, our playback contained three growls and one squeal (to reflect natural foraging displacements), which could have made growls more salient or memorable and/or aided easier discrimination of the aggressor compared to the target. It might also be more cognitively demanding for receivers to discriminate the squeals from multiple subordinate individuals in a group, compared to growls, which are highly likely to come from one of the two dominant individuals. Finally, since contest-related vocalisations may vary depending on the severity of an attack (Gouzoules et al., 1984; Slocombe and Zuberbühler, 2007), it is possible that we used less salient squeals than growls in our playbacks (although both were recorded during natural foraging displacements). Future work is required to tease these possibilities apart.

In summary, our results demonstrate that dwarf mongooses can obtain information about within-group contests (including protagonist identity) acoustically, retain that information, and use it to inform decisions about conflict management with a temporal delay. Such a delay might be most apparent in situations where there is little opportunity for immediate post-contest affiliation (as is the case with foraging dwarf mongooses); it may also be most apparent when there is a cumulative build-up of unresolved conflict. Our results come from a single population of dwarf mongooses, but we do not believe that there were any obvious biases introduced into the data collection that would limit the generalisability of the findings (see the end of ‘Materials and methods’ for an assessment of the STRANGEness of our test sample). There is increasing experimental evidence that social animals can remember past events and take these into account when deciding whether to get involved in a contest (Borgeaud and Bshary, 2015; Cheney et al., 2010; Tibbetts et al., 2020; Wittig et al., 2014); we demonstrate that this ability extends to post-conflict affiliative behaviour. More generally, our results showcase the importance of considering group-wide consequences of dyadic within-group interactions and of looking for effects beyond the immediate aftermath.

Source data files have been uploaded for our figures and statistical tests.

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Author details

1. Amy Morris-Drake

   School of Biological Sciences, University of Bristol, Bristol, United Kingdom

   Contribution

   Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Validation, Visualization, Writing - original draft, Writing – review and editing

   For correspondence

   am9162@bristol.ac.uk

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-4243-4651

2. Julie M Kern

   School of Environmental and Rural Science, University of New England, Armidale, Australia

   Contribution

   Project administration, Resources, Supervision, Writing – review and editing

   Competing interests

   No competing interests declared

3. Andrew N Radford

   School of Biological Sciences, University of Bristol, Bristol, United Kingdom

   Contribution

   Conceptualization, Funding acquisition, Methodology, Resources, Supervision, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-5470-3463

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