[pirunner (OP)]

I understand that the human male:female ratio for the entire world is roughly 1:1. I'm pretty sure this must be the case due to the randomness of the reproductive process. What is the actual ratio however and does this ratio fluctuate in different regions of the world?

Also, do we know the sex ratios of any animal species? Has research been done in this area? Again, I presume that most species will have a similar close-to-1-1 ratio...but there are always those odd cases.

[another_someone]

The actual birth ratio for humans is very slightly off from 1:1 - normally quoted at around 104:100 male to female.

There are variables that can influence sex ratios.

One factor that is speculated upon is the time of conception - the closer conception is to the time of ovulation, the greater the probability of a male being conceived (my understanding is that male sperm, rather like males in later life, are faster, and so will reach the egg first, but have less stamina than their female counterpart, so if they have to wait around for the egg to arrive, they will probably die off).

Another factor that has been observed in various species (I have heard of this in some species of dear, although I have heard it is also true of humans) that the parents of higher social status are more likely to conceive male offspring (the theory is that most females will have similar numbers of offspring, whereas males can have very variable number of offspring, so if you have high status parents, then there is a higher probability that the offspring will also be high status, and thus the males will be more successful but the females will have no better reproductive success than their lower status counterparts; but for low status parents, the offspring will more likely be low status, so the males will be much less successful in reproducing, and so there is more to be gained in having female offspring).

The sex rations of reptiles and insects can be very far off 1:1 (many reptiles depend on the environmental temperature to determine the sex of the offspring).  Some fishes actually change sex during their life, starting as small males, but as they grow bigger they change to become females.

[_Stefan_]

Here's a lecture about sex-ratio theory and sexual selection that Richard Dawkins gave on a trip to the Galapagos Islands.

http://video.google.com/videoplay?docid=8712792765758299597
or

[pirunner (OP)]

Great! Thanks!