REVIEW AND SYNTHESIS Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation Pat Monaghan, 1 * Neil B. Metcalfe 1 and Roxana Torres 2 1 Division of Ecology and Evolutionary Biology, Faculty of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK 2 Departamento de Ecologı ´a Evolutiva, Instituto de Ecologı ´a, UNAM, AP 70–275, CP 04510, Me ´ xico DF, Mexico *Correspondence: E-mail: p.monaghan@bio.gla.ac.uk Abstract The concept of trade-offs is central to our understanding of life-history evolution. The underlying mechanisms, however, have been little studied. Oxidative stress results from a mismatch between the production of damaging reactive oxygen species (ROS) and the organismÕs capacity to mitigate their damaging effects. Managing oxidative stress is likely to be a major determinant of life histories, as virtually all activities generate ROS. There is a recent burgeoning of interest in how oxidative stress is related to different components of animal performance. The emphasis to date has been on immediate or short-term effects, but there is an increasing realization that oxidative stress will influence life histories over longer time scales. The concept of oxidative stress is currently used somewhat loosely by many ecologists, and the erroneous assumption often made that dietary antioxidants are necessarily the major line of defence against ROS-induced damage. We summarize current knowledge on how oxidative stress occurs and the different methods for measuring it, and highlight where ecologists can be too simplistic in their approach. We critically review the potential role of oxidative stress in mediating life-history trade-offs, and present a framework for formulating appropriate hypotheses and guiding experimental design. We indicate throughout potentially fruitful areas for further research. Keywords Antioxidants, carotenoid, free radical, life-history strategies, oxidative stress, reactive oxygen species, reproduction, senescence. Ecology Letters (2009) 12: 75–92 INTRODUCTION Life-history theory has provided a powerful conceptual framework for investigating the evolution of life-history strategies. Central to this is the concept of life-history trade- offs. Such trade-offs are presumed to have a physiological basis (Zera & Harshman 2001), but we actually know rather little about the mechanisms that determine their nature and outcome, or indeed constrain the evolution of particular life- history strategies. Trade-offs are most usually thought of in terms of resource allocation. That is, where the allocation of limiting resources to one trait has negative consequences for other traits requiring the same resource (Zera & Harshman 2001). However, trade-offs can also be produced as a consequence of the performance of one activity generating negative consequences for other traits. A prime candidate mechanism that might drive this second type of trade-off, and in which there has recently been a great deal of interest amongst evolutionary ecologists, is oxidative stress. All organisms need to fuel the biological processes on which they depend. Aerobic species have evolved the capacity to use oxygen for the efficient release of energy. They also need to prevent oxidation of their body components by reactive substances produced in the process, which is what happens during oxidative stress. The need for energy efficiency therefore needs to be balanced against potential by-product toxicity. This is the basis for the suggestion that the management of oxidative stress might mediate life- history trade-offs. The extent of uncontrolled oxidative processes in animals, and their consequences for life-history traits, is still poorly understood (Balaban et al. 2005). There is no Ecology Letters, (2009) 12: 75–92 doi: 10.1111/j.1461-0248.2008.01258.x Ó 2008 Blackwell Publishing Ltd/CNRS