Sexual Competition among Brothers May Influence Offspring Sex Ratio in Snakes.

Most species of animals produce approximately equal numbers ofsons and daughters, apparently because the individual fitness of parents will usually be maximized by equal investment into offspring ofeach sex (Fisher, 1930; see review by Charnov, 1982). Among taxa with genotypic sex determination, the main departures from a 50/50 sex ratio in the progeny are in small populations where dispersal is limited, so that sibs compete with each other either for mating or resources (e.g., Hamilton, 1967, 1979; Clark, 1978; Werren, 1980; Bulmer and Taylor, 1980; Wilson and Colwell, I 98 I ). Ifsons compete with each other whereas daughters do not, natural selection may favor genes that cause mothers to produce an excess ofdaughters. This phenomenon is well-known in insects and mites (Hamilton. 1967;Waage, 1982), and possible examples have been described in mammals (Clutton-Brock and Albon, 1982; Cockburn et al.. 1985). Our data on adders show that brothers compete for matings, that female adders produce more daughters than sons, that the variance in litter sex ratios is less than expected under the binomial distribution, and that females facultatively adjust their progeny's sex ratio in a manner predicted by sib-competition theory. Sib-competition (or'local mate competition') should influence parental investment into sons versus daughters only in small isolated populations, because only in such cases can a single female's offspring influence the population's adult sex ratio (e.9., Hamilton, 1967, 1 979; Charnov, 19 82). Theoretical models predicting female-biased sex ratios also require occasional dispersal and the founding of new populations by dispersing individuals, but the predicted equilibrium sex ratio is affected only slightly by the number of generations ofwithin-population reproduction prior to dispersal (Bulmer and Taylor, 1980). Adders (Vipera berus) appear to fulfil the assumptions of these models. Even in large ares of suitable habitat, these small venomous snakes are highly philopatric, occurring in small sub-populations with only occasional interchange of individuals (Vitannen, 1967 ; Yozenilek, 1991; Madsen, unpubl. data). For example, during seven years of study in the Genarp region of Sweden, we have recorded only a single instance of inter-population migration among three small populations of adders (each consisting of 8 to 40 adult snakes) using hibernacula only 500 m apart. We studied a population of adders SEXUAL COMPETITION AMONG BROTHERS MAY INFLUENCE OFFSPRING SEX RATIO IN SNAKES