operational sex ratio
the relative number of males and females available at the time when reproduction is possible. In nonhuman mammals with internal gestation and lactation, there are often fewer females receptive to copulation than available males, creating an operational sex-ratio bias toward males, even though adults of both sexes may be present in equal numbers. In polygynous species (see polygyny), there might be more receptive females than available males, creating an operational sex-ratio bias toward females.