In polygynous mammals, it is commonly observed that both sex ratios at birth and dispersal are male biased. This has been interpreted as resulting from low female dispersal causing high female local resource competition, which would select for male-biased sex ratios. However, a female-biased sex ratio can be selected despite lower female than male-biased dispersal. This will occur if the low female dispersal is close to the optimal dispersal rate, while the male dispersal is not close to the optimal dispersal rate. The actual outcome depends on the joint evolution of sex-biased dispersal and sex ratio. Earlier analyses of joint evolution imply that there will be no sex-ratio nor dispersal biases at the joint evolutionarily stable strategy, thus they do not explain the data. However, these earlier analyses assume no intersexual competition for resources. Here, we show that when males and females compete with each other for access to resources, male-biased dispersal will be associated with male-biased birth sex ratio, as is commonly observed. A trend toward male-biased birth sex ratios is also expected if there is intersexual local resource competition and if birth sex ratio is constrained so that it cannot depart from balanced sex ratio.