Although maternal dominance and good condition are frequently associated with raised offspring sex ratios in mammals, the key factor may be female testosterone, which not only underpins the behavioural indicators but could also provide a pathway to a possible proximate mechanism for sex determination. By taking into account the fact that female testosterone levels rise in response to environmental stressors, it is possible to re-interpret the findings of atypical sex ratios in mammals in a way that reconciles seemingly conflicting results and reveals instead what could be a coherent, adaptive system of sex allocation in mammals.