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Sex ratio and sexual dimorphism of Borderea pyrenaica, a long-lived dioecious geophyte endemic to the Pyrenees (north-east Iberian Peninsula), were examined in three alpine populations. In this species, age can be estimated and the sex of nonreproductive adult plants identified. Male plants attain sexual maturity earlier, flower more frequently and grow faster than female plants, whereas females allocate a higher biomass to reproduction than males. These results support the hypothesis that female plants incur a higher cost of sexual reproduction and that this higher cost is measurable as reduced vegetative growth and lower flowering frequency. Variation of sex ratio among young, intermediate and old adults within populations suggests, however, that this higher female reproductive investment does not result in sexual differences in mortality. The overall male-biased sex ratio in B. pyrenaica is mainly a consequence of the tendency of males to reproduce at an earlier age and more frequently than females.

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