In the time of kings and empires, the rulers always wanted a son and an heir. So that their “line” would continue, and great things would happen in their name.
Or something like that.
So, when a bride was chosen for a prince ready to sow his oats, the search would begin for a princess from a family that had a long line of sons. That was of critical importance, and people had observed that sons of families with many sons tended to have more sons.
Anyway, the scientific basis for this was not very clear. There is an old scientific hypothesis in evolutionary biology called the Trivers and Willard hypothesis for sex allocation (modestly named after the people who proposed it), which states that parents should increase the production of the sex with the higher fitness benefits (or highest payoff for “grandchildren”). This theory basically said that healthy mammals would have more male offspring, while female mammals living in harsh conditions would have more female children. Subsequent studies went on to show that this was not really true, but collective studies suggested that the proportion of male or female offspring was in some way controlled by the mother (the females) having the offspring. Somehow, scientists did not rigorously test if males, the fathers, too had a role to play in the determining of the sex of their offspring.
Anyway, a fascinating new study has now come out challenging old theories. The authors of this study used
Quite understandable and expected.
Now, they wanted to further test if males could in some way influence the sex of the offspring.
The experiment had to be well controlled. So, to do this, they used a sample of female deer that were kept in identical environmental conditions, were of a similar age and fitness, and furthermore, to make sure that the females didn’t manipulate any results by selecting specific mates, artificially inseminated them all (at a similar time in relation to ovulation).
As well controlled an experiment as could be.
And their results were very convincing. High fertility males (who had more fast swimming, normal spermatozoa) had significantly more male offspring. So, in effect, super-males will have more male offspring, who will go on to increase their fertility, and therefore will have greater reproductive success. Low fertility males however, will have female offspring, and therefore benefit by not inheriting lower reproductive quality from their fathers.
As I read this, I wanted to ask, “so how does it work in nature, since here you are artificially inseminating females. How does nature maintain a relatively stable sex ratio, slightly favoring females? Shouldn’t the supermales eventually lead to a male excess?”.
Well, the authors start to address this question with their concluding lines.
“….creating an unforeseen evolutionary scenario that includes conflicts of interest between males and females. For instance, a fertile male may benefit from producing sons, but the costs of raising a male may be high for a female in poor physical condition….”
Read all about it in Science 1 December 2006:Vol. 314(5804) pp. 1445 – 1447.
(Also, read this fascinating old post on The Panda’s thumb if you have more time)