Parental investment in male and female offspring in polygynous mammals

Two evolutionary theories predict how mammalian parents would be expected to allocate the resources at their disposal to their male and female progeny. Where reproductive success varies more widely among males than females and variation in success among adults is influenced by parental investment1, parents would maximize their reproductive success by allocating a greater proportion of their resources to individual sons than to individual daughters2–5. However, because the benefits of producing offspring of one sex are inversely related to the total investment allocated to them, parents should, on average, divide their total investment equally between their male and female progeny, rearing fewer of whichever sex is individually more expensive to produce6–10. Here we examine the extent to which parental investment in red deer (Cervus elaphus) and other polygynous mammals matches these predictions. We conclude that, in several mammals, mothers invest more heavily in individual sons than daughters but that, contrary to prediction, there is no indication that fewer male offspring are reared in these species.