IKOS 101: 376–384, 2003 Causes of sex-biased adult survival in ungulates: sexual size dimorphism, mating tactic or environment harshness? Carole Toı ¨go and Jean-Michel Gaillard Toı ¨go, C. and Gaillard, J.-M. 2003. Causes of sex-biased adult survival in ungulates: sexual size dimorphism, mating tactic or environment harshness? – Oikos 101: 376–384. Using both a conventional and a phylogenetic approach, we tested whether sexual size dimorphism, mating tactic and environmental conditions influenced the between- sex differences in adult survival among 26 populations of polygynous ungulates. As a general rule, male survival was both lower and more variable among species than female survival. Whatever the method we used, sexual size dimorphism had no direct influence on male-biased mortality. In food-limited environments, the survival of males relative to that of females was lower than in good environments, suggesting a cost of large size for males facing harsh conditions. On the other hand, the survival of males relative to that of females tended to increase with sexual size dimorphism in good environments, indicating that large size may be profitable for males facing favourable conditions. Lastly, we found that the between-sex differences in adult survival did not vary with sexual size dimorphism in harem-holding or tending species, but tended to increase with sexual size dimorphism in territorial species. Our analyses indicate that sexual size dimorphism does not lead directly to a decrease in male survival compared to that of females. Thus, environmental conditions rather than the species considered could shape between-sex differences in adult survival observed in ungulate populations. C. Toı ¨go and J.-M. Gaillard, Unite ´ Mixte de Recherche N° 5558 Biome ´trie et Biologie Eolutie, Uni. Claude Bernard Lyon 1, 43 bouleard du 11 noembre 1918, FR-69622 Villeurbanne Ce ´dex, France (gaillard@biomser.uni -lyon1.fr). Present address for CT: Office National de la Chasse et de la Faune Sauage, Centre National d’Etudes et de Recherches Applique ´es sur la Faune de montagne, 8 impasse Champ Fila, FR-38380 Eybens, France. In ungulates, previous studies have reported that adult survival is remarkably constant despite environ- mental variations (reviewed by Gaillard et al. 1998, 2000), and that sex appears to be the most important factor accounting for variation in adult survival. In most species of ungulates, adult male survival is both lower and more variable over time than adult female survival (e.g. De Bie and Van Wieren 1976 on rein- deer, Rangifer tarandus ; Clutton-Brock et al. 1982 on red deer, Cerus elaphus ; Boer 1988 on moose, Alces alces ; Owen-Smith 1993b on greater kudu, Tragela - phus strepsiceros ; Jorgenson et al. 1997 on bighorn sheep, Ois canadensis ). Gender differences in adult survival are generally interpreted as a result of sexual selection, and a positive correlation between male-bi- ased mortality (or female-biased adult sex ratio) and the magnitude of sexual size dimorphism has often been reported in comparative studies (Ralls et al. 1980, Promislow 1992, but see Berger and Gompper 1999). However, several exceptions to this general rule have been recently highlighted. Males survive equally well as females in the highly dimorphic alpine ibex, Capra ibex ibex (Toı ¨go et al. 1997) and in the mouflon, Ois gmelini (Cransac et al. 1997), whereas males survival is much lower than that of females in the slightly dimorphic roe deer, Capreolus capreolus (Gaillard et al. 1993) and isard, Rupicapra pyrenaica (Loison 1995). Accepted 25 September 2002 Copyright © OIKOS 2003 ISSN 0030-1299 OIKOS 101:2 (2003) 376