Eli5 Why is the gestation period of each mammal different? Opposum only need 12-13 days but why do we get 9months?

Question

Eli5 Why is the gestation period of each mammal different? Opposum only need 12-13 days but why do we get 9months?

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Answer 1

Opposums are an especially poor comparison because they are marsupials. Marsupials are born extremely underdeveloped by mammalian standards, but finish their development inside the mother’s pouch, which takes two to three months in the case of the oppossum. Humans (and most other mammals) are placentals; our development takes place entirely inside the uterus.

If we look at a placental mammal of similar size, like a housecat, their gestation is about two months which is comparable to the total gestation and pouch time of the opossum. Meanwhile, the largest extant marsupial, the red kangaroo, gestates for one month but needs six months before it can leave its mother’s pouch.