Journal of Animal Ecology 2009, 78, 219–225 doi: 10.1111/j.1365-2656.2008.01475.x © 2008 The Authors. Journal compilation © 2008 British Ecological Society Blackwell Publishing Ltd Reproductive conflict delays the recovery of an endangered social species Andrés López-Sepulcre 1,2,3 *, Ken Norris 4 and Hanna Kokko 1 1 Laboratory of Ecological and Evolutionary Dynamics, Department of Biological and Environmental Sciences, University of Helsinki, FI-00014 Helsinki, Finland; 2 Evolutionary Ecology Unit, Department of Biological and Environmental Sciences, University of Jyväskylä, FI-40014 Jyväskylä, Finland; 3 Department of Biology, University of California, Riverside, CA 92521, USA; and 4 Centre for Agri-Environmental Research, School of Agriculture, Policy and Development, University of Reading, Earley Gate, Reading RG6 6AR, UK Summary 1. Evolutionary theory predicts that individuals, in order to increase their relative fitness, can evolve behaviours that are detrimental for the group or population. This mismatch is particularly visible in social organisms. Despite its potential to affect the population dynamics of social animals, this principle has not yet been applied to real-life conservation. 2. Social group structure has been argued to stabilize population dynamics due to the buffering effects of nonreproducing subordinates. However, competition for breeding positions in such species can also interfere with the reproduction of breeding pairs. 3. Seychelles magpie robins, Copsychus sechellarum, live in social groups where subordinate indi- viduals do not breed. Analysis of long-term individual-based data and short-term behavioural observations show that subordinates increase the territorial takeover frequency of established breeders. Such takeovers delay offspring production and decrease territory productivity. 4. Individual-based simulations of the Seychelles magpie robin population parameterized with the long-term data show that this process has significantly postponed the recovery of the species from the Critically Endangered status. 5. Social conflict thus can extend the period of high extinction risk, which we show to have population consequences that should be taken into account in management programmes. This is the first quantitative assessment of the effects of social conflict on conservation. Key-words: interference behaviour, IUCN Red List, population regulation, population viability, territorial conflict Introduction Conflicts between individuals over reproduction can promote behaviours that have negative consequences at the level of a group or a population (Haldane 1932; Williams 1966; Frank 1998) because natural selection maximizes relative rather than absolute fitness (Wright 1969). Consequently, investiga- tions of the behavioural and individual-level determinants of demography have highlighted a theoretical possibility that individual conflict can impact demographic processes of con- servation concern, extinction included (Dieckmann & Ferrière 2004; Parvinen 2005; Rankin & López-Sepulcre 2005). In social species, where reproduction is limited to a few individ- uals in the group, reproductive conflict and its consequences have been a common subject of study for decades (Frank 2007). Despite this, and the conservation interest of social species, the consequences of reproductive conflict in a real-life conservation context remain unexplored. Social species are assumed to fare better when sufficient numbers of nonreproductives are present, due to helping activities or other forms of group benefit (Courchamp, Gren- fell & Clutton-Brock 1999). However, sociality necessarily implies conflict between individuals over reproductive domi- nance (Frank 1998). Arguments for a positive effect of a non- breeder fraction on population dynamics (Hunt 1998; Grimm et al. 2005) ignore the fact that conflict for breeding positions can incur costs to the population (Kokko & Sutherland 1998; López-Sepulcre & Kokko 2005). Indeed, in several species of social animals, nonbreeding individuals have been shown to decrease breeder performance with aggressive and interfer- ence behaviours (Arcese et al. 1992; Bourke & Franks 1995; Iguchi & Hino 1996; Komdeur 1996; Young & Clutton-Brock *Correspondence author. E-mail: andresls@ucr.edu