Personality, plasticity, and resource defense

In many animal taxa, behavior varies both among individuals (animal “personalities”) and within individuals (“plasticity”). Personality and plasticity may co-vary if individuals differ in responsiveness to changes in their environment (“I × E” interaction). The nature and fitness implications of individual differences in behavioral plasticity in the wild are poorly understood. In territorial animals, fitness depends fundamentally on resource-defense behavior—their response to various threats from conspecific competitors. Such systems thus offer an ideal opportunity to investigate individual differences in behavioral plasticity. We used male superb fairy-wrens (Malurus cyaneus) to test for individual differences in the intensity and plasticity of territorial defense behavior. Using captive assays, we identified behavioral types on the basis of fast versus slow exploration of a novel environment (the “proactive–reactive” axis of behavior). Then we simulated territorial intrusions in the wild, using playback of experimentally manipulated songs that differed in trill length in a pair-wise design. Although previous work suggests that proactive individuals are more aggressive and less responsive than reactive individuals, we found no support for these differences in the context of territorial defense in the wild. Fast explorers (proactive) and slow explorers (reactive) responded equally aggressively to a simulated territorial intrusion overall. Furthermore, although fast explorers responded equally strongly to the two different playback stimuli, there was little evidence that slow explorers differentiated between the two stimuli, and this “I × E” interaction was not statistically significant. Further work is needed to determine the prevalence in the wild of personality-related differences in behavioral plasticity.