Behavioral syndromes and population dynamics: Lessons from demographic heterogeneity

Background/Question/Methods

Within-population variation in individual behavior is now recognized as important for ecology and evolution. Some such variation can be classified as behavioral syndromes (also known as animal personality), in which suites of behavioral traits are consistently displayed across ecological contexts. For example, some individuals are consistently more aggressive than others, expressing this aggression not just in interspecific contests, but also in foraging, mating, and anti-predator behavior. Differences in such aggressive tendencies could impact population dynamics by creating heterogeneity in demographic rates.

Here we examine how the theory of demographic heterogeneity may provide insights about the population-dynamic effects of behavioral variation. This body of theory has shown that, depending on within-population correlation structure and the underlying stochastic process, heterogeneity in survival and reproduction can change the variance due to demographic stochasticity. Furthermore, persistent survival heterogeneity in long-lived organisms can increase the population’s density-independent growth rate, raise its equilibrium density, and reduce its extinction risk.

We developed and analyzed a model that incorporate a boldness-aggressiveness tradeoff: some individuals gain reproductive fitness by being more aggressive towards conspecifics, but are more bold, increasing their mortality risk.

Results/Conclusions

If an individual’s behavioral trait is determined for life and is not heritable (e.g., is a consequence of experiences or learning in early life), then the mortality cost of aggression means that the aggressive individuals die earlier, on average, than their non-aggressive siblings. The relative abundance of the aggressive type at equilibrium is controlled by the rate at which they are born into the population and the mortality cost of boldness, not by the reproductive benefit of aggressiveness. The introduction of aggressive types into a behaviorally homogeneous non-aggressive population increases the average per-capita mortality rate at equilibrium; unless aggression raises the average fecundity in the population, this reduces the equilibrium density. If, as is often the case, the reproductive benefit of aggression is frequency dependent, this reduction of equilibrium density is likely.