Outgroup conflict is widely discussed as a powerful selection pressure in social evolution, but studies quantifying fitness consequences have been somewhat limited in scope. In social species across a broad range of taxa (Figure 1), conspecific outsiders (see Glossary) threaten the valuable resources (e.g. territories, food, mates, and breeding positions) of groups and their members (Christensen and Radford, 2018; Beehner and Kitchen, 2007; Radford et al., 2016). Threats may come from single outsiders, same-sex coalitions, or rival groups; when groups compete, all or just a subset of individuals may participate. We use ‘outgroup’ conflict to encompass all scenarios involving a threat from conspecific outsiders, with ‘intergroup’ conflict the subset of those involving conflict between rival groups. Theoretical work indicates the selective importance of outgroup conflict in the evolution of within-group dynamics, cooperation, territoriality, and social structure (Alexander and Bargia, 1978; Choi and Bowles, 2007; Gaston, 1978; Rusch, 2014). By contrast, the traditional focus of empirical research has been on behaviour during contests between rivals (e.g. how different group members contribute, the costs and benefits of participation, what factors affect the level of escalation and who wins; Arseneau-Robar et al., 2016; Batchelor and Briffa, 2011; Beehner and Kitchen, 2007; Radford, 2003) and, more recently, broader behavioural changes arising as an immediate consequence of outgroup interactions (e.g. changes in individual vigilance and foraging decisions, within-group affiliation, and group movement patterns; Birch et al., 2019; Braga Goncalves and Radford, 2019; Crofoot, 2013; Morris-Drake et al., 2019; Preston et al., 2021; Radford, 2008). Fitness assessments are crucial in general to elucidate selection pressures; in the case of outgroup conflict, they would also help to bridge the gap between theoretical work considering lasting evolutionary differences between species and empirical work considering plastic and ephemeral behavioural responses within species. However, studies that explicitly quantify fitness consequences in the context of outgroup conflict have, to date, focused mostly on those arising immediately from physical contests (for exceptions, see Kerhoas et al., 2014; Lemoine et al., 2020; Rudolph and McEntee, 2016; Thompson et al., 2017).

Figure 1

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Beyond the immediate negative impacts of physical contests, broader attention needs to be paid to delayed, cumulative, and third-party fitness consequences, both negative and positive, that arise from living in a landscape of outgroup threat. Adversarial interactions with outsiders, especially if they escalate to physical violence, can lead to immediate direct costs to survival or reproductive success (e.g. loss of life or breeding position; Batchelor and Briffa, 2011; Thompson et al., 2017; Wrangham et al., 2006). However, we argue that consideration of fitness consequences should expand more systematically in three non-exclusive ways. First, as with predation (Creel and Christianson, 2008), the influence of outsiders is likely not restricted to confrontations between animals. For instance, there can be behavioural and hormonal effects of encountering secondary cues (e.g. faecal deposits) of rival presence (Christensen et al., 2016; Morris-Drake et al., 2019) and as a result of the overall risk of outgroup conflict (e.g. the number of territorial neighbours or the likelihood of intrusions; Lemoine et al., 2020; Radford, 2010; Samuni et al., 2020; Schoof and Jack, 2013), which could translate into fitness consequences. Second, in addition to the immediate effects of individual contests, there could be knock-on consequences from contest-related occurrences such as injuries and takeovers (Packer and Pusey, 1983; Schneider-Crease et al., 2020), from changes in behaviour or space use (Crofoot, 2013; Mares et al., 2012) and from the cumulative effects of multiple events (Isbell et al., 1990; Mosser and Packer, 2009). These could affect the fitness of both those directly involved and third-party individuals in the current and subsequent generations (Brunton, 2013; Goldstein et al., 1998; Noguera et al., 2017). Third, a particular event or scenario can have different consequences (including some that are positive) for different individuals within a group. For example, the takeover of a breeding position is most costly to the usurped individual but is positive for the incomer and may affect third-party opposite-sex (e.g. through improved reproductive opportunities) and same-sex (e.g. due to eviction) group members (Balshine et al., 1998; Clutton-Brock et al., 2001).

Here, we describe many of the myriad ways that outgroup conflict could have fitness consequences and what we believe is needed to increase our understanding of this relatively neglected aspect of sociality. There is increasing recognition that not all interactions with conspecific outsiders necessarily entail conflict (Furuichi, 2020; Pisor and Surbeck, 2019): in some species, such as bonobos (Pan paniscus), many intergroup encounters are described as peaceful (Fruth and Hohmann, 2018; but see Cheng et al., 2021); whilst in other species, some intergroup interactions at least are characterised by tolerance and/or are about just information exchange (Hashimoto et al., 2020; Radford and du Plessis, 2004). However, we know far less about these types of encounters and their consequences (Van Belle et al., 2020), and there may be elements of conflict even in seemingly tolerant or peaceful exchanges. Thus, our primary focus is a conflict perspective.

Our review has two main parts. In the first part, we describe the full range of potential fitness consequences that could arise from outgroup conflict. We begin by documenting the potential survival and reproductive consequences of single contests, which is the most-commonly considered scenario. Then, we step beyond contests to describe fitness consequences that can also result from interactions with cues of rival presence and the general landscape of outgroup threat, and beyond single interactions to describe cumulative effects of territorial pressure and elevated outgroup-induced stress. We discuss which individuals are affected negatively and positively, using illustrative examples from a range of taxa; we do not present a comprehensive review of the literature. To complement the text presentation of the core ideas, we split the concepts into examples of fitness consequences that arise directly to individuals (immediately, with a delay and cumulatively; Table 1) and those that arise to third-party group members (of the same or subsequent generations; Table 2). In the second part of the review, we provide suggestions about how to move the research field forward. First, we highlight the importance of considering how different types of outgroup conflict can generate different selection pressures and of investigating variation in fitness consequences within and between species. Then, we discuss the value of both theoretical modelling and empirical work, including long-term studies of natural populations, experimental manipulations, and interspecific meta-analyses.

Consequences of single contests

Outgroup contests can have immediate fitness consequences for those involved (Table 1a): there can be loss of life, extra-group mating, transfer of females, and replacement of breeders. The most extreme example is targeted killing: in chimpanzees (Pan troglodytes), for instance, coalitions of group members undertake coordinated incursions into neighbouring territories seemingly with the intention of attacking rivals (Goodall et al., 1979; Wilson et al., 2014), whilst raids to kill offspring in rival groups occur in species such as banded mongooses (Mungos mungo) and greater anis (Crotophaga major) (Cant et al., 2016; Strong et al., 2018). More commonly, the death of participating adults or juveniles accidentally caught in the melee arises as a by-product of escalated physical contests (Dyble et al., 2019; Thorne et al., 2003; Zahavi, 1990). There is also the possibility that engagement in fighting behaviour incurs an increased predation risk as participants are distracted (Hess et al., 2016; Jakobsson et al., 1995). Contest-related extra-group mating, which have negative reproductive consequences for the cuckolded male but are positive for the outsider and for the female if she gains better genes or access to an unrelated partner, can arise in two main ways. In some species, such as alpine marmots (Marmota marmota) and meerkats (Suricata suricatta), males ‘rove’ between groups specifically seeking mating opportunities (Lardy et al., 2015; Young et al., 2007); in other species, such as banded mongooses and common marmosets (Callithrix jacchus), individuals from different groups sneak matings whilst others are occupied in outgroup contests (Johnstone et al., 2020; Lazaro-Perea, 2001). Longer-lasting reproductive consequences result from the transfer of females between groups, as seen in various primate species (Breuer et al., 2016; Pines and Swedell, 2011), and enforced takeovers of breeding positions by outsiders (Figure 2), which occur across many taxa (primates: Beehner and Bergman, 2008; carnivores: Packer and Pusey, 1983; ungulates: Rubenstein and Nuñez, 2009; rodents: Hackländer and Arnold, 1999; birds: Ridley, 2011). A takeover follows a particular contest event, though that can sometimes be the culmination of a series of skirmishes over an extended period (Dunbar, 1987; Sicotte et al., 2017), with the usurped individuals losing future (and potentially current; see below) reproductive success unless they themselves can take over another group in turn.

Figure 2

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Outgroup contests can also generate a variety of knock-on fitness consequences. First, there can be delayed consequences for contest participants specifically (Table 1b). For instance, physical confrontations with outsiders can lead to injuries in a wide range of species (e.g. primates: Aureli et al., 2006; Rosenbaum et al., 2016; carnivores: Jordan et al., 2017; Mosser and Packer, 2009; mongooses: Dyble et al., 2019; Thompson et al., 2017; birds: Hannon et al., 1985; insects: Batchelor and Briffa, 2011; Rudolph and McEntee, 2016). Injured animals likely have a greater mortality rate and reduced reproductive performance (Bernardo and Agosta, 2005; Krause et al., 2017; Wilson, 1992). The second category of knock-on fitness consequences are those resulting in third-party individuals; that is, adult group members (Table 2a) and offspring (Table 2b) who were not necessarily directly involved in the original outgroup contest. For instance, in primate species where females may be kidnapped or voluntarily move to a rival group during contests, their own-group males may aggressively herd or coerce them to remain (Breuer et al., 2016). Another possibility is that lethal fights (see above) create a breeding vacancy that a subordinate group member benefits from filling (Johns et al., 2009). In at least one case, current non-breeders increase the likelihood of this occurrence: dry wood termite (Cryptotermes secundus) workers are believed to tunnel through to the next colony to incite an intergroup conflict, increasing the prospect of their king and queen being killed and a breeding vacancy arising (Korb and Roux, 2012).

Other striking examples of third-party consequences follow a change in breeder (Figure 2). This event may be beneficial to members of the opposite sex if there is now an unrelated dominant individual with whom they can mate. For example, in geladas (Theropithecus gelada), subordinate females are more likely to mature sexually in the two months following a male breeder takeover than in the preceding two months (Beehner and Lu, 2013), although accelerated sexual maturity may not always be beneficial. There are also potential negative consequences for existing group members. For instance, the remaining breeder may suffer a decrease in reproductive output; newly established pairings may produce fewer offspring compared to those who have been together a long time, as seen in alpine marmots, Azara’s owl monkeys (Aotus azarai), and pied babblers (Turdoides bicolor) (Fernandez-Duque and Huck, 2013; Lardy et al., 2015; Wiley and Ridley, 2018). The loss of a valuable ally could, in principle, result in a lower position in the dominance hierarchy and thus fewer mating opportunities or lowered survival (Cheney and Seyfarth, 1987). In a range of species, including African lions (Panthera leo), ursine colobus (Colobus vellerosus), and geladas, incoming male breeders kill dependent offspring sired by their predecessors to bring females into oestrus sooner (Packer and Pusey, 1983; Schneider-Crease et al., 2020; Sicotte et al., 2017); females may also exhibit higher abortion rates (Roberts et al., 2012; Zipple et al., 2017) and reduced fertility (Packer and Pusey, 1983) following male takeovers. Infanticide and feticide are costly for the reproductive success of the parents but benefit the new breeding males. Incoming breeders of both sexes sometimes evict existing group members: former breeders and other adults may be evicted as potential rivals, as seen in the cichlid Neolamprologus pulcher and Florida scrub-jays (Aphelocoma coerulescens) (Balshine et al., 1998; Goldstein et al., 1998), whilst independent offspring may be driven to disperse early (Elliot et al., 2014). In principle, some existing group members may also choose to disperse if the presence of a new breeder reduces inclusive fitness benefits (Eikenaar et al., 2007; Rowley and Russell, 1990; Spong et al., 2008). Individuals that leave or are evicted can suffer fitness consequences because being alone or in a small splinter group likely results in increased predation, reduced foraging success, and fatal encounters with rival groups (Cant et al., 2001; Kingma et al., 2016; Ridley et al., 2008); there is clear evidence that spending time alone has a negative effect on longevity in lions and meerkats (Cram et al., 2018; Elliot et al., 2014).

Outgroup contests can also have knock-on fitness consequences for all group members, rather than just specific individuals, because of group-size changes (Table 2). Contest-related deaths of adults and offspring, infanticide by incoming breeding males, female transfers, and dispersal or eviction following a change in breeder (details above) can all lead to a reduction in group size. Conversely, group-size increases can arise when a breeding individual is usurped by a same-sex coalition (Bygott et al., 1979; Ridley, 2011), when there is kidnapping of young from rival groups as seen in white-winged choughs (Corcorax melanorhamphos), banded mongooses and pied babblers (Heinsohn, 1991; Müller and Bell, 2009; Ridley et al., 2022), and when intraspecific slave-raiding occurs in insects such as the honey ant Myrmecocystus mimicus and the fire ant Solenopsis invicta (Bartz and Hlldobler, 1982; Tschinkel and Howard, 1983). Changes in group size can have a variety of fitness consequences, with the positive effects of increased group size the reverse of the negative ones arising from a reduced group size that we describe here. Individuals in smaller groups may suffer a general increase in mortality risk both from predation and starvation, although smaller groups might be less easily detected by predators and may have less competition among group members for limited resources (Krause and Ruxton, 2002). Fewer group members can also mean a reduced fighting strength when competing with conspecific rivals; relative group size is known to play an important role in intergroup contest outcomes, as evidenced from meerkats and green woodhoopoes (Phoeniculus purpureus) (Dyble et al., 2019; Radford and du Plessis, 2004). Moreover, loss of offspring can reduce the motivation to fight (Dyble et al., 2019). In cooperatively breeding species, the loss of helpers likely has negative effects on the inclusive fitness of breeders and other group members because helper number is often positively related to reproductive success (Brown et al., 1982; Russell et al., 2007; Taborsky et al., 2007). The loss of adults or offspring is especially costly for small groups that are more sensitive to a reduction in group size; in cooperatively breeding species, the maintenance of a critical size is vital to avoid group extinction (Courchamp et al., 1999; Taborsky et al., 2005).

Beyond contests

Some fitness consequences of outgroup conflict may arise not only from engagement in contests but also from interactions with secondary cues of rivals (Figure 3; Table 1b). Close encounters with outsiders and inspection of rival faeces, urine, and other secretions, for instance, could lead to disease and parasite transmission (Brown and Brown, 2004; Craft et al., 2011; Drewe, 2010; Nolan and Delaplane, 2016). As a specific example, modelling of disease transmission in African lions demonstrates the importance of contact between neighbouring prides (Craft et al., 2011). Susceptibility may be further increased in the context of outgroup conflict because social stress is a key factor in the pathogenesis of disease (Padgett et al., 1998; Quan et al., 2001). There are clear survival costs for animals who have contracted a disease or carry a high parasite load: these individuals may be more prone to, for example, lethal infections, starvation, and predation (Milinski, 1985; Robar et al., 2010). Individuals compromised in these ways may also have a lowered reproductive output (Fitze et al., 2004; Scott, 1988).

Figure 3

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Interactions with rivals and cues of their presence (e.g. olfactory or auditory indicators) can result in a range of behavioural responses that could have fitness consequences (Figure 3; Table 1b). For instance, there can be alterations in space use: groups might avoid areas where contests typically occur (Markham et al., 2012; Mech and Harper, 2002; Seiler et al., 2017) or might spend more time in border zones to guard against potential intrusions, as seen in green woodhoopoes that are more likely to roost where an intergroup conflict occurred earlier in the day (Radford and Fawcett, 2014). In both cases, this might mean more time spent in areas with fewer food resources, more predators and/or less-preferred sleeping sites, and thus an increased risk of starvation or predation (Crofoot, 2013). In addition to changes in space use, outgroup interactions can cause other behavioural responses. For instance, defensive actions can include greater patrolling and scent-marking (Amsler, 2010; Christensen et al., 2016; Jordan et al., 2007), whilst there may be more general increases in vigilance and intragroup affiliation (Morris-Drake et al., 2019; Radford, 2008; Lemoine et al., 2020; Walker et al., 2016), altered speed of movement, and reduced time spent resting (Christensen et al., 2016; Crofoot, 2013; Mirville et al., 2020). In dwarf mongooses (Helogale parvula), for example, presentation of rival-group faeces compared to control faeces resulted in more scent-marking, vigilance, and grooming (Christensen et al., 2016; Morris-Drake et al., 2019). Losing groups of white-faced capuchins (Cebus capucinus) moved further and faster, stopped less frequently and were active until later in the evening than groups that won intergroup encounters (Crofoot, 2013). These behavioural changes have likely knock-on consequences in terms of greater energy expenditure and reduced time for foraging, and thus lower body mass, and lessened parental care (Crofoot, 2013; Mares et al., 2012; Morris-Drake et al., 2021), which could influence the reproductive success and survival chances of both adults and dependent young (Table 2).

Many of the studies considering behavioural effects of outgroup conflict have focused on the period immediately following a single interaction (up to 1 or 2 hr in the aftermath), when responses are most likely due to elevated stress (Culbert et al., 2021; Samuni et al., 2020; Schoof and Jack, 2013) or exclusion from territorial areas (Crofoot, 2013; Mirville et al., 2020). Fitness consequences from such short-term, single occurrences might be relatively small, in at least some instances. However, there is also some evidence for longer-lasting behavioural effects of single events (Dyble et al., 2019; Radford and Fawcett, 2014) and that the overall threat of outgroup conflict (a ‘landscape of fear’) may cause avoidance of likely conflict areas (Markham et al., 2012; Seiler et al., 2017) and behavioural changes when in such locations (Radford, 2010). Moreover, there could be a cumulative build-up from multiple outgroup events which results in behavioural changes not just in the immediate aftermath of each interaction, but also more generally to baseline activity (Morris-Drake et al., 2021; Thompson et al., 2020). As these scenarios all increase the frequency and magnitude of behavioural changes, they enhance the likelihood of fitness consequences arising from them.

Beyond single interactions

In addition to cumulative behavioural effects (see above), a build-up of outsider pressure over time can lead to changes in territory ownership or size. In extreme cases, a group might be usurped from its whole territory either by neighbours or groups from further afield (Isbell et al., 1990; Ligon and Ligon, 1990; Mitani et al., 2010). For instance, Goodall, 1986 famously documented how the Kasekela community of chimpanzees at Gombe National Park took over the territory of the neighbouring Kahama community after a series of lethal attacks. More commonly, a group may lose part of its territory to a stronger neighbour, as seen in rattling cisticolas (Cisticola chiniana), vervet monkeys (Chlorocebus pygerythrus), chowchillas (Orthonyx spaldingii), and lions (Carlson, 1986; Isbell et al., 1990; Jansen, 1999; Mosser and Packer, 2009). Losing groups then have access to areas of reduced quality and/or less familiarity, and thus individuals have potentially lessened survival chances and reproductive success (Table 1c). Survival might be reduced due to reliance on poorer quality food resources, more time where there is a higher predation risk (in terms of predator numbers and less familiarity with escape options) or limited availability of safe sleeping sites (Crofoot, 2013; Isbell et al., 1990; Markham et al., 2012; Mosser and Packer, 2009). Reduced access to valuable resources could also have negative impacts on both current and future reproductive success, as documented for mud crabs (Panopeus herbstii), Seychelles warblers (Acrocephalus sechellensis), and chimpanzees (Griffen and Norelli, 2015; Komdeur and Edelaar, 2001; Thompson et al., 2007). These consequences are qualitatively similar to at least some arising from temporary avoidance of areas (see ‘Beyond contests,’ above). However, when there is a permanent change in territory ownership, there are also benefits to those groups gaining additional resources, who likely experience positive effects on survival and reproductive success. One further benefit of increasing territory size in some bird species, such as Seychelles warblers and Florida scrub jays, is that sons can ‘bud’ off part of the territory and so begin reproducing independently (Komdeur and Edelaar, 2001; Stallcup and Woolfenden, 1978).

The cumulative build-up of outgroup threat likely generates chronic stress (Samuni et al., 2019), with survival and reproductive consequences for the affected individuals (Figure 4; Table 1c). Chronic stress is associated with reduced body condition and increased chances of mortality (Campos et al., 2021; Pride, 2005; Wey et al., 2014) due to, for example, increased susceptibility to predation (Romero et al., 2009; Vuarin et al., 2019). Chronic stress may also reduce reproductive investment and success: it can have negative effects on courtship activity (Romero-Diaz et al., 2019; Schreck, 2010), breeding rates (Dulude‐de Broin et al., 2020; Mileva et al., 2011), fecundity (O’Brien et al., 2018; Schreck, 2010), egg size and composition (Ensminger et al., 2018; Henriksen et al., 2013), and hatching and fledging success (Cyr and Michael Romero, 2007; Eriksen et al., 2015; Kleist et al., 2018). Many of these effects can occur in the same species (Zanette et al., 2011). There has been only a limited amount of research examining the reproductive consequences of outgroup conflict directly. A recent observational study on chimpanzees found that an increased cumulative pressure from intergroup conflict is correlated with longer inter-birth intervals and reduced offspring survival (Lemoine et al., 2020). Experimental work with cichlid fish has demonstrated that an increase in outgroup threat can drive longer inter-clutch intervals, cause females to produce relatively smaller eggs with less protein, and result in fewer offspring surviving to one month post-hatching (Braga Goncalves & Radford, In revision). The consequences are not necessarily always negative, however, as outgroup threat has been found to be correlated with reduced foetal mortality in crested macaques (Macaca nigra) and banded mongooses (Kerhoas et al., 2014; Thompson et al., 2017) and with increased pup survival in dwarf mongooses (Morris-Drake, 2021).

Figure 4

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As with individual contests (see ‘Consequences of single contests,’ above), there can be knock-on consequences of cumulative outgroup effects for third-party individuals – most notably, in this case, for offspring (Figure 4; Table 2b). These could arise through maternal effects, due to conflict-induced stress in mothers (Brunton, 2013; Culbert et al., 2021). In general, offspring from smaller eggs and those with, for example, higher levels of corticosterone might be smaller, be of lower quality, and have learning difficulties (Dantzer et al., 2019; McCormick, 1998; Roche et al., 2012). Stress, as well as engagement in conflict-related activities, could also decrease the quality of offspring care by parents and, in the case of cooperative breeders, non-breeding helpers (Mares et al., 2012; Stein and Bell, 2012). Carers might abandon nests, whilst offspring could be at greater risk of starvation if there is reduced provisioning and at greater risk of predation as a direct consequence of decreased protection or indirectly if smaller offspring are more vulnerable (Ahnesjo, 1992; Ridley, 2016; Vitousek et al., 2014; Vitousek et al., 2018). Moreover, early-life stress on offspring and lower growth has lasting effects: there is increasing evidence that developmental trajectories shape the physiology and behaviour of adults, with major effects on survival and reproductive success (Cram et al., 2017; English et al., 2013; Marshall et al., 2017; Noguera et al., 2017; Royle et al., 2005).

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Author details

1. Ines Braga Goncalves

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

   Contribution

   Conceptualization, Visualization, Writing – original draft, Writing – review and editing

   Contributed equally with

   Amy Morris-Drake, Patrick Kennedy and Andrew N Radford

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-0659-9029

2. Amy Morris-Drake

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

   Contribution

   Conceptualization, Visualization, Writing – original draft, Writing – review and editing

   Contributed equally with

   Ines Braga Goncalves, Patrick Kennedy and Andrew N Radford

   Competing interests

   No competing interests declared

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

3. Patrick Kennedy

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

   Contribution

   Conceptualization, Visualization, Writing – original draft, Writing – review and editing

   Contributed equally with

   Ines Braga Goncalves, Amy Morris-Drake and Andrew N Radford

   Competing interests

   No competing interests declared

4. Andrew N Radford

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

   Contribution

   Conceptualization, Funding acquisition, Visualization, Writing – original draft, Writing – review and editing

   Contributed equally with

   Ines Braga Goncalves, Amy Morris-Drake and Patrick Kennedy

   For correspondence

   andy.radford@bristol.ac.uk

   Competing interests

   No competing interests declared

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

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