Background/Question/Methods Identifying life-history strategies is traditionally achieved by quantifying differences among individuals in their observed traits. However, there is more to individual variation than meets the eye of biologists. Although individual differences can be easily observed (sex, age, size), most differences are typically not observed or measured (frailty, maternal effects, genetic make-up). Historically, unobserved heterogeneity has been viewed as a nuisance rather than a characteristic of interest in demographic studies, and empirical evidence of the demographic consequences of individual heterogeneity on the evolution of life histories is currently lacking.
Results/Conclusions In recent years, important methodological advances have helped quantify the amount of unobserved heterogeneity in life-history traits. Such a framework could resolve - among many other questions - an intense debate on the relative contributions of individual stochasticity and individual heterogeneity to the variance in life-history outcomes. I will review methodological ways to understand the role unobserved heterogeneity plays in shaping life-history strategies and population dynamics, specifically how this role varies along the slow-fast continuum of life histories and across climatic conditions.