Mammals
a day ago
- Kangaroo newborns are tiny, underdeveloped, nearly transparent blobs that climb to the pouch using their front legs, with many bodily systems still incomplete.
- Marsupials have a unique reproductive system: two uteri, three vaginas, and sometimes forked penises; their urinary and reproductive tracts are tangled together and empty into a cloaca.
- Monotremes lay eggs and have a cloaca through which they pee, poop, and mate; their reproductive anatomy includes bizarre multi-headed echidna penises and a simple but janky urinary system.
- Eutherians have long pregnancies with placental implantation and produce highly developed offspring, while marsupials have short pregnancies and long lactation periods.
- Marsupial mothers often pause embryonic development and can nurse two offspring of different ages with different milk compositions from separate nipples.
- Marsupial births are cheap and easy, allowing strategies like abandoning young, but eutherians have faster maximum population growth rates, which likely helped them outcompete marsupials in many regions.
- Marsupials diversified into forms like Tasmanian devils, marsupial moles, sugar gliders, thylacines, and giant kangaroos, but no marsupial bats, seals, or horses evolved.
- Monotremes possess extraordinary traits: electroreception, biofluorescence, a complex sex chromosome system, venomous spurs in platypuses, and unique mating behaviors.