Abstract
In polygynous mammals, it is commonly observed that both sexratios at birth and dispersal are male biased. This has beeninterpreted as resulting from low female dispersal causing highfemale local resource competition, which would select formale-biased sex ratios. However, a female-biased sex ratio can beselected despite lower female than male-biased dispersal. Thiswill occur if the low female dispersal is close to the optimaldispersal rate, while the male dispersal is not close to theoptimal dispersal rate. The actual outcome depends on the jointevolution of sex-biased dispersal and sex ratio. Earlier analysesof joint evolution imply that there will be no sex-ratio nordispersal biases at the joint evolutionarily stable strategy, thusthey do not explain the data. However, these earlier analysesassume no intersexual competition for resources. Here, we showthat when males and females compete with each other for access toresources, male-biased dispersal will be associated withmale-biased birth sex ratio, as is commonly observed. A trendtoward male-biased birth sex ratios is also expected if there isintersexual local resource competition and if birth sex ratio isconstrained so that it cannot depart from balanced sex ratio.