Experimental evidence that chronic outgroup conflict reduces reproductive success in a cooperatively breeding fish

Abstract

Conflicts with conspecific outsiders are common in group-living species, from ants to primates, and are argued to be an important selective force in social evolution. However, whilst an extensive empirical literature exists on the behaviour exhibited during and immediately after interactions with rivals, only very few observational studies have considered the cumulative fitness consequences of outgroup conflict. Using a cooperatively breeding fish, the daffodil cichlid (Neolamprologus pulcher), we conducted the first experimental test of the effects of chronic outgroup conflict on reproductive investment and output. ‘Intruded’ groups received long-term simulated territorial intrusions by neighbours that generated consistent group-defence behaviour; matched ‘Control’ groups (each the same size and with the same neighbours as an Intruded group) received no intrusions in the same period. Intruded groups had longer inter-clutch intervals and produced eggs with increasingly less protein than Control groups. Despite the lower egg investment, Intruded groups provided more parental care and achieved similar hatching success to Control groups. Ultimately, however, Intruded groups had fewer and smaller surviving offspring than Control groups at 1-month post-hatching. We therefore provide experimental evidence that outgroup conflict can decrease fitness via cumulative effects on reproductive success, confirming the selective potential of this empirically neglected aspect of sociality.

A positive effect of cumulative intergroup threat on reproductive success

“…In contrast to previous studies of banded mongooses, chimpanzees and cichlid fish, which report negative effects of outgroup conflict on reproductive success [27][28][29], and a study of Tasmanian native hens (Tribonyx mortierii) that found no effect [56], our results indicate the possibility of a positive impact. Such interspecific variation might be related to differences in, for instance, reproductive ecology (e.g.…”

“…The conflict-related improvement in dwarf mongoose pup survival is not counteracted by more infanticide or abortion, as these latter events occur only very rarely in our study population: infanticide has been observed only once, and abortion only three times, in more than 120 breeding attempts (DMRP 2011–2021, unpublished data). Our study therefore highlights the possibility that conflict with rival groups could have positive effects, not just the negative reproductive consequences previously documented [27–29]; better offspring survival might arise as a by-product of conflict-induced increases in vigilance behaviour. Moreover, it showcases the need to think beyond individual contests and to assess cumulative threat when considering outgroup conflict as a social driver of fitness.…”

“…Outgroup contests, especially those that escalate to physical violence, are well known to have substantive immediate and delayed fitness consequences [10]. By showing that dwarf mongoose pups survived better when groups were more frequently engaged in IGIs, we add to the small body of evidence that the cumulative threat of outgroup conflict can also affect reproductive output [27–29]. The conflict-related improvement in dwarf mongoose pup survival is not counteracted by more infanticide or abortion, as these latter events occur only very rarely in our study population: infanticide has been observed only once, and abortion only three times, in more than 120 breeding attempts (DMRP 2011–2021, unpublished data).…”

“…Using long-term observational data, two studies have found negative effects of an increased level of outgroup threat: in banded mongooses ( Mungos mungo ), pup survival was lower [27]; while in chimpanzees ( Pan troglodytes ), fetal survival was lower and inter-birth intervals were longer [28]. Similarly, recent experimental work with the daffodil cichlid ( Neolamprologus pulcher ) showed that elevated levels of outgroup threat resulted in longer inter-clutch intervals and the production of fewer and smaller offspring at one month post-hatching [29].…”

Fitness incentives to male fighters undermine fighting performance in intergroup contests

“…Our measure of the location of a contest on each group’s territory involved calculating how much time each group spent at the contest location before the contest occurred. Using default parameters in the ctmm package (11) we first built utilization distributions (UDs) for the GPS location data of each group involved in the contest for the 90 days preceding the date of the contest (a timeframe representing a full reproductive period (12)). These UDs, which accurately account for autocorrelation in the telemetry data, represent a “familiarity index” of each grid cell (average grid cell size was 5 x 5m).…”