Exploitative leaders incite intergroup warfare in a social mammal

Abstract

Collective conflicts among humans are widespread, although often highly destructive. A classic explanation for the prevalence of such warfare in some human societies is leadership by self-serving individuals that reap the benefits of conflict while other members of society pay the costs. Here, we show that leadership of this kind can also explain the evolution of collective violence in certain animal societies. We first extend the classic hawk-dove model of the evolution of animal aggression to consider cases in which a subset of individuals within each group may initiate fights in which all group members become involved. We show that leadership of this kind, when combined with inequalities in the payoffs of fighting, can lead to the evolution of severe intergroup aggression, with negative consequences for population mean fitness. We test our model using long-term data from wild banded mongooses, a species characterized by frequent intergroup conflicts that have very different fitness consequences for male and female group members. The data show that aggressive encounters between groups are initiated by females, who gain fitness benefits from mating with extragroup males in the midst of battle, whereas the costs of fighting are borne chiefly by males. In line with the model predictions, the result is unusually severe levels of intergroup violence. Our findings suggest that the decoupling of leaders from the costs that they incite amplifies the destructive nature of intergroup conflict.

Keywords: aggression; animal behavior; collective violence; human evolution; social evolution.

Fig. 1.

(A and B) Evolutionarily stable probabilities of escalated fighting and (C and D) mean payoffs per individual across all encounters, as a function of the total cost c of losing a fight (expressed relative to the value of victory v, and plotted on a log scale), in the original pairwise-interaction hawk−dove game (blue curves), and in the group-interaction game (red curves). A and C show results when leaders claim a disproportionately large share of the benefits of victory, that is, when d_v > 0 (solid red curves, d_v = 0.25; dotted red curves, d_v = 0.5; dashed red curves d_v = 0.75; d_c = 0 in each case); B and D show results when leaders suffer a disproportionately small share of the cost of losing a fight, that is, when d_c > 0 (solid red curves, d_c = 0.25; dotted red curves, d_c = 0.5; dashed red curves d_c = 0.75; d_v = 0 in each case). In all cases, n = 5. (E and F) Mean payoffs to followers per encounter when leaders control group behavior (solid curves), compared to the potential maximum payoffs obtainable if a follower were to seize control of the group and impose its own preferred behavior (dashed curves), as a function of the total cost c of losing a fight (expressed relative to the value of victory v, and plotted on a log scale). Exploitative leadership occurs where the solid and dashed curves diverge. Parameter values E, (d_v = 0.5, d_c = 0, n = 5); F, (d_v = 0, d_c = 0.5, n = 5).

Fig. 2.

(A) Banded mongoose battle lines during an IGI. Image credit: Dave Seager (photographer). (B) Costs of intergroup aggression in male and female banded mongooses. Mortality rate of adult (>1 y) males and females resulting from intergroup aggression. Box, median and interquartile range; whisker, 90th percentile; points, outliers. *P = 0.018. N = 478 males, 335 females followed for 1,899 mongoose-years. (C) LRS and intergroup conflict. LEGO (Top) and LRS (Bottom) of males (blue) and females (orange) are plotted against the number of IGIs in which individuals were involved across their lifespan. Data are for 499 males and 367 female adults monitored from birth to death over 20 y.

Fig. 3.

Intergroup encounters and female reproductive state. The number of IGIs observed per day when a focal and rival group were in each of four possible combined group estrus (es) states. Points show means from the GLMM ± SE; ***P < 0.001; **P = 0.001; asterisks refer to post hoc Tukey’s all-pairwise comparison of means across all four categories.

Fig. 4.

Comparable mortality costs of intergroup aggression. Adult mortality rate from intergroup aggression in chimpanzees P. troglodytes (N = 5 populations studied for >5 y), small-scale human societies (N = 12 hunter-gatherers and N = 20 subsistence farmer societies), meerkats (N = 24 groups studied for >5 y), and banded mongooses (N = 10 groups studied for >5 y). Box, median and IQR; whisker, 90th percentile; points, outliers. Human and nonhuman primate data are redrawn from ref. .