Larger territories reduce mortality risk for chimpanzees, wolves, and agents: Multiple lines of evidence in a model validation framework

Kristin N. Crouse, Nisarg Desai, Kira A. Cassidy, Erin E. Stahler, Clarence L. Lehman, Michael L Wilson

Research output: Contribution to journalArticlepeer-review


Validation of agent-based models is underappreciated in scientific studies even though this process is an important part of ensuring that a model is a reliable research tool. Here we propose modifications to a model validation framework and illustrate this framework with a case study of territorial behavior. In species that defend territories, larger territories provide obvious benefits, such as increased access to food, shelter, and mates. An additional potential benefit is that larger territories could provide protection from intergroup conflict. Considerations from geometry indicate that per capita risk of death from intergroup violence should decrease with increasing territory size, insofar as conflict occurs mainly at the periphery. We tested this inference using computer simulations and data from chimpanzees (Pan troglodytes) and gray wolves (Canis lupus). We designed an agent-based model that allows territories to vary freely in size and shape. We present a framework for model validation and apply it to our agent-based model to substantiate its accuracy. Simulations from the validated model confirmed the predictions from geometry. Under a broad range of parameter values, per capita mortality rate decreased in larger territories. Similarly, using published data on rates of death from intercommunity aggression in 16 chimpanzee communities, as well as new data from 38 wolf packs, we found that per capita mortality rate correlated negatively with a measure of territory size. These findings indicate that in species with lethal intergroup aggression, one simple aspect of territory geometry – size – has strong effects on mortality. Several lines of evidence indicate that the per capita rates of mortality from warfare have decreased in many human societies over time. One factor contributing to this decrease may be the increasing geographic extent of political entities.

Original languageEnglish (US)
Article number110063
JournalEcological Modelling
StatePublished - Sep 2022

Bibliographical note

Funding Information:
This study analyzes previously published chimpanzee data. Collection of these data depended on funding from many different sources, with the permission of authorities in many different countries, but no new chimpanzee data were collected for this project. The wolf data were collected as part of the long-term monitoring of wolves in Yellowstone National Park. This project is supported by the National Park Service, Yellowstone Forever, and many generous donors to wolf research in Yellowstone National Park. The authors also appreciate the tireless efforts by many volunteer observation crews and the safe piloting of Roger Stradley from Gallatin Flying Service, Mark Packila of Wildlife Air, LLC., Bob Hawkins from Hawkins and Powers, Inc. and Sky Aviation, Inc., and Jim Pope and crew from Leading Edge Aviation, LLC. We thank members of the University of Minnesota Behavior Group, the Primate Ecology Lab at Harvard University, and NIMBioS workshop on Evolution and Warfare at the University of Tennessee for helpful feedback on earlier presentations of these results, and the Institute for Advanced Study in Toulouse (IAST) for funding to M.L.W. from the French National Research Agency (ANR) under grant ANR-17-EURE-0010 (Investissements d'Avenir program).

Publisher Copyright:
© 2022


  • Agent-based modeling
  • Intergroup aggression
  • Model validation
  • Mortality rate
  • Territoriality


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