As tradeoff s limit the maximum Darwinian fi tness individuals can reach, measuring reliably the strength of tradeoff s using appropriate metrics is of prime importance to understand the evolution of traits under constraints. Tradeoff s involving phenotypic traits and fi tness components, however, are diffi cult to quantify in free-ranging populations because of confounding eff ects due to environmental variation and individual heterogeneity. Furthermore, although some methods have been used previously to quantify tradeoff s, these methods cannot be applied with respect to binary traits, which are common to describe life histories (e.g. probability of reproduction, nesting success, off spring survival). Here, we demonstrate how to measure reliably the strength of tradeoff s involving binary traits using (auto)correlation estimates obtained from generalized linear (mixed) models. We fi rst propose a standardized approach that accounts for the variation in the nature of the tradeoff s being compared (e.g. continuous/binary traits, repeated/non-repeated measures), and then apply this method to longitudinal data from two contrasting species of large herbivores. Empirical estimates of tradeoff s varied among traits, and between-species comparisons suggested that reproductive tradeoff s between successive breeding attempts might only occur in capital breeders. Th e empirical results we obtained clearly demonstrate that the method we provide allows measuring reliably the strength of tradeoff s under most circumstances, including tradeoff s on binary traits. Our original approach therefore off ers an important fi rst step for comparing the strength and, hence, the relative importance of diff erent tradeoff s, and opens the door to a better understanding of the evolution of life history traits in free-ranging populations.A major goal of life history studies is to understand how natural selection shapes individual fi tness-related traits, such as growth, reproduction, and survival (Roff 1992, Stearns 1992. Without any resource limitation, a ' Darwinian demon ' , would evolve (Law 1979). Nevertheless, resources are usually limited so that individuals have to allocate resources to a given function at the cost of other functions (Williams 1966). According to this principle of energy allocation, the maximum fi tness individuals can reach is limited by tradeoff s that occur among fi tness components during a lifetime (Williams 1966). Although tradeoff s among life history traits have shaped the diversity of life history strategies we currently observe, tradeoff s involving phenotypic traits and fi tness components are diffi cult to measure in freeranging populations for two main reasons. First, tradeoff measures are subject to confounding eff ects of environmental variation in resource availability (van Noordwijk and de Jong 1986). Second, tradeoff s are usually assessed from traits measured on diff erent individuals within a population, leading to between-individual variation, and the data generally consist of repeated measure...