Behavioral Ecology Blog

The Trivers-Willard hypothesis says basically that a female in good condition should skew the sex ratio of her offspring towards males.

The assumption:

1. That only big/strong males will mate successfully- while even the homeliest of females easily acquires matings..
2. Parental investment can enhance the reproductive success of males more that that of females. Basically, that the marginal benefits of investment are greater for males.
3. Wimpy males get no (or very few) matings.

It’s like this: If you’re a female in good condition- you get most “bang for the buck” when investing in a male. This male, when raised in favorable can garnish many many matings. If you’re a dumpy female- better off to bias towards females- as even the worst will still produce offspring.

This is a pretty well known hypothesis, and has been validated in a few human studies, and by Clutton-Brock in the Red Deer of Rhum. No mechanism has been universally put forth for this effect.

In a new study on smoker-parents, a mechanism is suggested.

The Independent reported on the study, which suggests that having a male baby drops by as much as 50 percent if the parents are both smokers.

The research, done by pediatricians at the Liverpool School of Tropical Medicine, also suggests smoking raises the chances of a miscarriage. Researchers reportedly found that substances contained in cigarettes, such as nicotine, inhibit sperm carrying male chromosomes from fertilizing eggs.

The study looked at 9,000 women who gave birth between 1998 and 2003 at the Liverpool Women’s Hospital. The Independent reported that the study also found that women who were exposed to secondhand smoke during pregnancy were also less likely to give birth to male infants.

It’s pretty cool- certainly female smokers are generally speaking in poorer condition than non-smokers, and thus are predicted to have fewer male babies. The data fits the Trivers-Willard hypothesis exactly as predicted.