Complex Landscapes

This past week I’ve been reworking a paper about a study with Anna Mosser and Craig. The study asks the question: How did lions come to live in groups? It doesn’t seem like group-living in lions would be something you would spend much time thinking about – until you realize that lions are the only cat that regularly lives in groups. What’s special about lions?

Craig’s work over the past decades has shown that seemingly intuitive ideas about why lions form groups are wrong. Lions don’t form groups in order to hunt more efficiently. Lions don’t form groups to cooperatively nurse their young. Lions don’t form groups to protect young against aggressive outsiders. Instead, it appears that the primary purpose of lion groups is to defend territories against other groups of lions.

So territorial defense appears to be the key to group living in lions. But is territorial defense the only thing that matters? That’s what we set out to investigate. We created a computer model that simulates a bunch of lions living on a landscape. The model is a simplification of what happens in real life, but it contains some essential aspects of lion living.

First, we have complex landscapes. Previous research suggests that group territoriality is more likely in complex landscapes because there are highly desirable areas that are worth defending. If you had a landscape where everything was more or less the same, then you wouldn’t need to fight your neighbor over some small patch of it; you could just wander off and find your own patch that would be more-or-less the same quality as your neighbor’s.

Second, we have various behaviors that we can turn on or off in our simulated lions. For example, we can tell them that they can live together in a territory, but they can’t cooperate to defend it. We can also tell them whether or not they can live in a territory with their parents when they grow up. And we can tell them whether they’re allowed to make their territory bigger if they recruit more lions into their group.

By manipulating the types of landscapes and the various behaviors, we explored how often our simulated lions formed groups. Our results suggest that while territorial defense is important, it’s also important to have complex landscapes with high-value real estate. If the landscape isn’t very complex, then it’s easy enough to find an area to set up a territory without fighting for it. And if the landscape is complex, but doesn’t have any areas with high value, then there’s nothing worth fighting for or defending. It’s also important that lions be able to pass their valuable territories on to their offspring, for without inheritance, the benefits of all that fighting and defending are gone in a generation.

Lions evolved on the savannas of East Africa, where the landscape is complex with patchy areas of high value (near where rivers come together, for example). Humans did too. It’s possible that the same sorts of savanna landscapes that shaped group living and territorial defense for lions did so for people, as well.

The simulation model. White areas are high-value real-estate, while black areas are low-value. Red shapes show where lions have formed a group territory, while blue shapes show where there’s a territory defended by just a single lion.