Journal of Animal Ecology 2008, 77, 927–935 doi: 10.1111/j.1365-2656.2008.01416.x © 2008 The Authors. Journal compilation © 2008 British Ecological Society Blackwell Publishing Ltd Longevity, early emergence and body size in a pollinating fig wasp – implications for stability in a fig–pollinator mutualism Derek W. Dunn 1,2,3 *, Douglas W. Yu 2 , Jo Ridley 2 and James M. Cook 1,3 1 Division of Biology, Imperial College London, Silwood Park, SL5 7PY, UK; 2 School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK; and 3 School of Biological Sciences, University of Reading, Reading RG6 6AH, UK Summary 1. Fig trees (Ficus) are pollinated only by agaonid wasps, whose larvae also gall fig ovules. Each ovule develops into either a seed (when pollinated) or a wasp (when an egg is also laid inside) but not both. 2. Ovipositing wasps (foundresses) favour ovules near the centre of the enclosed inflorescence (syconium or ‘fig’), leaving ovules near the outer wall to develop into seeds. This spatial stratification of wasps and seeds ensures reproduction in both partners, and thereby enables mutualism persistence. However, the mechanism(s) responsible remain(s) unknown. 3. Theory shows that foundresses will search for increasingly rare inner ovules and ignore outer ovules, as long as ovipositing in outer ovules is sufficiently slow and/or if inner ovules confer greater fitness to wasps. The fig–pollinator mutualism can therefore be stabilized by strong time constraints on foundresses and by offspring fitness gradients over variation in ovule position. 4. Female fig wasps cannot leave their galls without male assistance. We found that females in outer ovules were unlikely to be released. Inner ovules thus have added value to foundresses, because their female offspring are more likely to mate and disperse. 5. For those offspring that did emerge, gall position (inner/outer) and body size did not influence the order in which female pollinators exited syconia, nor did early emerging wasps enjoy increased life spans. 6. We also found that the life spans of female wasps nearly doubled when given access to moisture. We suggest that conflict resolution in the fig–pollinator mutualism may thus be influenced by tropical seasonality, because wasps may be less able to over-exploit ovules in dry periods due to time constraints. Key-words: cooperation, Ficus, fig wasp, mutualism, optimal foraging, Pleistodontes imperialis Introduction Mutualisms are interspecific interactions in which both partners benefit (Janzen 1985; Herre et al. 1999; West, Griffin & Gardner 2007a) and are important contributors to global biodiversity at multiple trophic levels (e.g. Christian 2001; Brundrett 2004; Harrison 2006). Although hosts and symbionts both profit from their association, benefits are usually obtained by imposing costs onto the other partner. Why, then, does one partner not destabilize the mutualism by exploiting the other unsustainably (Herre et al. 1999; Anstett 2001; Yu 2001; Frank 2003; Foster & Wenseleers 2006)? The diversity of mutualisms has meant that no all-encompassing explanation for their maintenance has been forthcoming until recently (see Foster & Wenseleers 2006; West, Griffin & Gardner 2007b). In some mutualisms hosts can often favour those symbionts that are cooperative and impose sanctions on cheats (e.g. Kiers, Rousseau, West & Denison 2003; Edwards et al. 2006). More generally, it has been shown that a high benefit : cost ratio of cooperation, high within-species relatedness and high between-species fidelity are important factors contributing to mutualism stability (Foster & Wenseleers 2006). Fig trees (Ficus spp.) are important in maintaining diversity in tropical and subtropical forests (Janzen 1979; Cook & West 2006; Harrison 2006). Their association with pollinating wasps (Hymenoptera: Agaonidae) is an obligate mutualism *Correspondence author: E-mail: d.w.dunn@reading.ac.uk