Extended and cumulative effects of experimentally induced intergroup conflict in a cooperatively breeding mammal

Abstract

Conflict between rival groups is rife in nature. While recent work has begun exploring the behavioural consequences of this intergroup conflict, studies have primarily considered just the 1–2 h immediately after single interactions with rivals or their cues. Using a habituated population of wild dwarf mongooses ( Helogale parvula ), we conducted week-long manipulations to investigate longer-term impacts of intergroup conflict. Compared to a single presentation of control herbivore faeces, one rival-group faecal presentation (simulating a territorial intrusion) resulted in more within-group grooming the following day, beyond the likely period of conflict-induced stress. Repeated presentations of outsider cues led to further changes in baseline behaviour by the end of the week: compared to control weeks, mongooses spent less time foraging and foraged closer to their groupmates, even when there had been no recent simulated intrusion. Moreover, there was more baseline territorial scent-marking and a higher likelihood of group fissioning in intrusion weeks. Consequently, individuals gained less body mass at the end of weeks with repeated simulated intrusions. Our experimental findings provide evidence for longer-term, extended and cumulative, effects of an elevated intergroup threat, which may lead to fitness consequences and underpin this powerful selective pressure.

A positive effect of cumulative intergroup threat on reproductive success

“…It is now well established that outgroup conflict can lead to a range of behavioural changes [48][49][50][51][52][53][54]. Our analyses in this paper show that adult dwarf mongoose sentinel behaviour-raised guarding where an individual is dedicated to vigilance-increases in the aftermath of both IGIs and latrine events.…”

“…Such behavioural changes in response to even just the interaction with rival-group cues (e.g. faecal samples) can potentially last beyond the immediate aftermath of the interaction and into the following day [53]. Since dwarf mongoose IGIs and latrine events occur regularly (mean ± s.e.…”

Fitness consequences of outgroup conflict

“…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 ).…”

“…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.…”

Variation between species, populations, groups and individuals in the fitness consequences of out-group conflict

“…patrolling and scent-marking), movement, resting, vigilance, and intragroup affiliation and aggression [4,19,32,[36][37][38][39][40]. The result could be use of more risky areas, greater energy expenditure, reduced foraging time and lessened parental care [36,41,42]. Finally, there can be cumulative effects of multiple contests or the build-up of outsider pressure over time.…”