Maybe Baby

The news report (http://news.bbc.co.uk/1/hi/health/7358384.stm) that first time mothers who have a high calorie diet are more likely to give birth to a boy than a girl is a simple extrapolation from animal studies with beasts like the Red deer. In such animals, it has been shown that dominant does (who are presumably well-nourished) are more likely to produce a stag rather than give birth to another female. The (rather speculative but intriguing) presumed reason for this is that a successful male offspring in this polygynous species (stags compete for entire herds of females) will pass on more copies of his mother's genes by fertilising them all than a female offspring ever could (she generally would produce only one offspring per year of her breeding life). So it pays for the female to invest more in rearing a male offspring. There are one or two difficulties in the interpretation of this recent human data. The first is that it is not absolutely certain that the animal phenomenon depends only on nutrition (there could be hormonal influences). Secondly, humans (unless they have a harem) are not generally such obviously polygynous species as are Red deer (you could get the same effect by producing promiscuous male offspring who were attractive to females in general). It used to be claimed that more male children were initially implanted than girls but that the mortality in boys was higher. Perhaps the good diet reduces this male mortality to produce an apparent surplus of boys? It would be interesting to see if the phenomenon applies also to subsequent pregnancies.