Available online at www.sciencedirect.com The genetic similarity between pair members influences the frequency of extrapair paternity in alpine marmots AURE ´ LIE COHAS * †, NIGEL GILLES YOCCOZ ‡, CHRISTOPHE BONENFANT * , BENOI ˆ T GOOSSENS §, CE ´ LINE GENTON * , MAXIME GALAN** , BART KEMPENAERS † & DOMINIQUE ALLAINE ´ * *Laboratoire Biome ´trie et Biologie Evolutive, UMR CNRS 5558, Universite ´ Claude Bernard Lyon 1 yDepartment of Behavioural Ecology & Evolutionary Genetics, Max Planck Institute for Ornithology, Seewiesen zDepartment of Biology, University of Tromsø xBiodiversity and Ecological Processes Group, Cardiff School of Biosciences **Centre de Biologie et de Gestion des Populations, UMR INRA e IRD e Cirad, Montpellier (Received 2 February 2007; initial acceptance 9 March 2007; final acceptance 9 January 2008; published online 12 May 2008; MS. number: 9260R) Extrapair paternity is widespread in birds and mammals. In particular, the alpine marmot, Marmota marmota, has a high frequency of extrapair paternity that seems to be explained by the genetic compati- bility hypothesis. We investigated whether the number and proportion of extrapair young depend on the heterozygosity (individual genetic diversity) of the social male, or on the genetic similarity between the social male and his mate (relatedness). Both the number and the proportion of extrapair young increased with both high similarity and dissimilarity between the social pair. In combination with previous results, our study suggests that patterns of extrapair paternity in alpine marmots can best be explained by the ge- netic compatibility hypothesis, and more precisely its optimal outbreeding variant. Our results indeed sug- gest that extrapair paternity is a mechanism to avoid both in- and outbreeding depression. We discuss which proximal mechanisms may be involved in extrapair paternity in this species. Ó 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Keywords: alpine marmot; cooperative breeding; inbreeding; Marmota marmota; mating system; miscrosatellite; related- ness; reproductive skew The genetic compatibility hypothesis states that females may benefit from extrapair paternity (EPP) if their genes are more compatible with those of the extrapair male than those of their pair mate (Zeh & Zeh 1996; Tregenza & Wedell 2000). Inbreeding is one form of genetic incompat- ibility, so females may benefit from EPP by decreasing the inbreeding level of their offspring (Zeh & Zeh 1996; Tre- genza & Wedell 2000). Although still poorly supported (Kempenaers 2007), this inbreeding avoidance hypothesis has received some support in both socially monogamous birds (Blomqvist et al. 2002; Foerster et al. 2003) and mammals (Sillero-Zubiri et al. 1996). The genetic benefits that females actually gain from their extrapair mate choice depend on their ability to bias egg fertilization in favour of the best male. Theoretically, this could be achieved via pre- and postcopulatory choice mechanisms (Pizzari & Birkhead 2002). The number and proportion of extrapair young (EPY) should then increase as the genetic quality or the genetic compatibility of the social mate decreases relative to that of other available sex- ual partners. If females are not able to adjust fertilization relative to male quality/compatibility, or if females seek Correspondence: A. Cohas, Laboratoire Biome ´trie et Biologie Evolutive, UMR CNRS 5558, Universite ´ de Lyon, Universite ´ Claude Bernard Lyon 1, 43 Bd du 11 novembre 1918, 69622 Villeurbanne cedex, France (email: cohas@biomserv.univ-lyon1.fr). N. G. Yoccoz is at the Depart- ment of Biology, University of Tromsø, N-9037 Tromsø, Norway. B. Goossens is at the Biodiversity and Ecological Processes Group, Car- diff School of Biosciences, Cardiff University, PO Box 915 Cathays Park, Cardiff CF10 3TL, U.K. M. Galan is at the Centre de Biologie et de Gestion des Populations, UMR INRA e IRD e Cirad e Montpel- lier SupAgro, Campus international de Baillarguet, CS 30016, 34988 Montferrier-sur-Lez cedex, France. B. Kempenaers is at the Max Planck Institute for Ornithology, Postfach 1564, 82305 Starnberg/Seewiesen, Gemany. 87 0003e 3472/08/$34.00/0 Ó 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2008, 76, 87e95 doi:10.1016/j.anbehav.2008.01.012