1 INTRODUCTION Evolutionary biologists aim to explain diversity in patterns of growth, reproduction and ageing that characterize life histories (sensu Stearns, 1992). Life history traits often vary predictably with geography and ecology (Gaston et al., 2008). However, many ecogeographic rules do not fully address the proximate mechanisms underlying variation in life history, despite repeated calls to integrate physiological mechanisms into life history theory (Ricklefs & Wikelski, 2002).