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

•Individuals in smaller territories are more vulnerable to being killed by neighbors.•Larger territories are associated with lower death and higher birth rates.•Fewer deaths and more births can result in expanding populations and territories.•Model validation increases the likelihood that a model works as expected.•A case study illustrates the model validation process through a step-by-step procedure. 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.