We are never told whether or not those breasts are mammary glands. It could just be convergent evlolution.

And I believe in one of the books it is explained that the "egg" is more a biological incubator - it photosynthsizes to nuroush the developing fetus - and grows along with the fetus until a young adult is eventually "hatched" - and I always got the impression it was an adaptation to the "dying mars"