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. 2023 Sep 12;120(37):e2217973120.
doi: 10.1073/pnas.2217973120. Epub 2023 Aug 28.

Complex battlefields favor strong soldiers over large armies in social animal warfare

Samuel J Lymbery  1   2   3 , Bruce L Webber  1   2   4 , Raphael K Didham  1   2
Affiliations

Complex battlefields favor strong soldiers over large armies in social animal warfare

Samuel J Lymbery et al. Proc Natl Acad Sci U S A. .

Abstract

In social animals, success can depend on the outcome of group battles. Theoretical models of warfare predict that group fighting ability is proportional to two key factors: the strength of each soldier in the group and group size. The relative importance of these factors is predicted to vary across environments [F. W. Lanchester, Aircraft in Warfare, the Dawn of the Fourth Arm (1916)]. Here, we provide an empirical validation of the theoretical prediction that open environments should favor superior numbers, whereas complex environments should favor stronger soldiers [R. N. Franks, L. W. Partridge, Anim. Behav. 45, 197-199 (1993)]. We first demonstrate this pattern using simulated battles between relatively strong and weak soldiers in a computer-driven algorithm. We then validate this result in real animals using an ant model system: In battles in which the number of strong native meat ant Iridomyrmex purpureus workers is constant while the number of weak non-native invasive Argentine ant Linepithema humile workers increases across treatments, fatalities of I. purpureus are lower in complex than in simple arenas. Our results provide controlled experimental evidence that investing in stronger soldiers is more effective in complex environments. This is a significant advance in the empirical study of nonhuman warfare and is important for understanding the competitive balance among native and non-native invasive ant species.

Keywords: Lanchester’s laws; habitat complexity; invasive ants; social insects; warfare.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Simulated group battles in simple (blue) and complex (red) arenas. (A) Probability of nine Teutonic Knights being victorious against increasing numbers of Two-Handed Swordsmen. (B) Proportion fatalities of Knights against increasing numbers of Swordsmen. (C) Proportion fatalities of Swordsmen as the number of Swordsmen increases. Computer simulations were carried out in the real-time strategy video game Age of Empires II (30).
Fig. 2.
Fig. 2.
Group battles between ants in simple (blue) and complex (red) arenas. (A) Proportion fatalities of workers in groups of 20 meat ant workers engaged in battles against increasing numbers of Argentine ant workers. (B) Estimates of Lanchester’s θ . Points indicate estimates of θ (which describes the relationship between group fighting success and group size), based on a range of simulated values of the relative fighting strength of Argentine ant to meat ant workers ( αLh/αIp ). Fitted lines were calculated based on 10,000 iterations of random sampling from a uniform distribution around mean θ estimates. Dashed lines indicate SD of the iterated estimates.
See this image and copyright information in PMC

References

    1. Lanchester F. W., Aircraft in Warfare, the Dawn of the Fourth Arm (Constable and Company Ltd., 1916).
    1. Franks R. N., Partridge L. W., Lanchester battles and the evolution of combat in ants. Anim. Behav. 45, 197–199 (1993).
    1. Franks R. N., Partridge L. W., “Lanchester’s theory of combat, self-organization, and the evolution of army ants and cellular societies” in Behavioural Mechanisms of Evolutionary Ecology, Real L., Ed. (Chicago University Press, 1994), pp. 390–408.
    1. Adams E. S., Mesterton-Gibbons M., Lanchester’s attrition models and fights among social animals. Behav. Ecol. 14, 719–723 (2003).
    1. Clifton E., A brief review on the application of Lanchester’s models of combat in nonhuman animals. Ecol. Psych. 32, 181–191 (2020).

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