Why are males and females 50%-50%?

[Igel]

16. Here's a question.

One that I have no real intent to check.

Find some cross-species comparisons for birth ratios. Do all species show a fairly even distribution, male/female, at a point soon after conception? Are some skewed towards males or females? (I think I remember hearing of species that could screen sperm or zygotes for sex, but I'm not a big fan of biology.)

For those with strong skews, is there a sound reason adduced by evolution or behavior that can account for it? Perhaps the males are flashier and get eaten more often. Perhaps they're smaller. Perhaps they're a primary food source for the females.

Overall, is there a tendency towards greater complexity, biological or social, for those with flat, nearly 50/50, distributions? Perhaps body size matters. Perhaps intelligence or adaptibility?

There is a small portion of paternal mtDNA passed along to offspring (those sperm contain mtDNA, after all, and their mitochondria aren't destroyed with ovum fertilization).