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Unknowns comments on Open Thread, Jul. 13 - Jul. 19, 2015 - Less Wrong Discussion
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It actually is not very odd for there to be a difference like this. Given that there are only two sexes, there only needs to be one hormone which is sex determining in that way. Having two in fact could have strange effects of its own.
Sex determination in placental mammals turns out to be really complicated, which is probably why there are so many intersex conditions. It's much simpler in marsupials, which is why male kangaroos don't have nipples. (Where would they keep them?)
If you think it's complicated in placental mammals, it's REALLY fun in zebrafish... all embryos start off building an ovary and dozens of loci all over the genome on autosomes rather than sex chromosomes alter the probability of the ovary spontaneously regressing then transforming into a testis. Immature egg cells are vital to both the process by which it becomes an ovary and by which it becomes a testis. Every breeding pair of zebrafish will produce a unique sex ratio of offspring depending on their genotypes at many loci and what they pass on to their offspring.