POPULATION ECOLOGY - ORIGINAL PAPER Sex-specific fitness returns are too weak to select for non-random patterns of sex allocation in a viviparous snake Jean-Pierre Baron • Thomas Tully • Jean-Franc ¸ois Le Galliard Received: 25 January 2010 / Accepted: 3 May 2010 / Published online: 26 May 2010 Ó Springer-Verlag 2010 Abstract When environmental conditions exert sex-spe- cific selection on offspring, mothers should benefit from biasing their sex allocation towards the sex with the highest fitness in a given environment. Yet, studies show mixed support for such adaptive strategies in vertebrates, which may be due to mechanistic constraints and/or weak selec- tion on facultative sex allocation. In an attempt to disen- tangle these alternatives, we quantified sex-specific fitness returns and sex allocation (sex ratio and sex-specific mass at birth) according to maternal factors (body size, age, birth date, and litter size), habitat, and year in a viviparous snake with genotypic sex determination. We used data on 106 litters from 19 years of field survey in two nearby habitats occupied by the meadow viper Vipera ursinii ursinii in south-eastern France. Maternal reproductive investment and habitat quality had no differential effects on the growth and survival of sons and daughters. Sex ratio at birth was balanced despite a slight female-biased mortality before birth. No sexual mass dimorphism between offspring was evident. Sex allocation was almost random apart for a trend towards more male-biased litters as females grew older, which could be explained by an inbreeding avoidance strategy. Thus, a weak selection for facultative sex allo- cation seems sufficient to explain the almost equal sex allocation in the meadow viper. Keywords Selection Á Sex allocation Á Sexual size dimorphism Á Sex ratio Á Snakes Introduction In vertebrates, differential allocation of resources to sons versus daughters (i.e. sex allocation) is a source of varia- tion in lifetime reproductive success between parents (Charnov 1982). Sex ratio theory predicts that an optimal population sex allocation is maintained by frequency- dependent selection around a level determined by the costs of producing each sex and the costs of controlling sex allocation (Fisher 1930; Pen and Weissing 2002). How- ever, when variable environmental conditions have differ- ential effects on the fitness of sons and daughters, mothers should benefit from biasing their sex allocation towards the sex with the highest fitness in each environment (e.g. Pen and Weissing 2002; Trivers and Willard 1973). Empirical studies with birds and mammals have provided support for such adaptive facultative sex allocation (reviewed by Cockburn et al. 2002). Yet, it is now increasingly apparent that patterns of facultative sex allocation are not neces- sarily adaptive in vertebrates (e.g. see Brommer et al. 2003; Cockburn et al. 2002; Ewen et al. 2004). On one hand, the adaptation of sex allocation strategies is limited by Communicated by Raoul Van Damme. J.-P. Baron CNRS/UPMC/ENS UMR 7625, E ´ cologie et E ´ volution, E ´ cole Normale Supe ´rieure, 46 rue d’Ulm, 75005 Paris, France T. Tully IUFM de Paris, Universite ´ Paris 4-Sorbonne, 10 rue Molitor, 75016 Paris, France J.-F. Le Galliard CNRS/ENS UMS 3194, CEREEP, E ´ cotron IleDeFrance, E ´ cole Normale Supe ´rieure, 78 rue du Cha ˆteau, 77140 St-Pierre-le `s-Nemours, France J.-F. Le Galliard (&) CNRS/UPMC/ENS UMR 7625, Universite ´ Pierre et Marie Curie, 7 Quai St Bernard, 75005 Paris, France e-mail: galliard@biologie.ens.fr 123 Oecologia (2010) 164:369–378 DOI 10.1007/s00442-010-1660-y