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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I want to preface this with the words „I am no expert“. But given that the Opossum is so much smaller than humans and that elephants -who have a gestation period of up to 22 month (almost two years!)- are so much larger than humans, it seems to me like the size of the organism plays a big role.
Funny thing…we need more than 9 months, but the human uterus cannot accommodate, so we birth this underbaked baby thing that cannot even hold its head up while deers are born running and whales swimming. Our head circumference and cervix (not cervical spine but exit chute gate) sort of came to an agreement of optimum time. Heck, we don’t even have functional lungs to breathe before 21 months. And we need a ‘parent.’ Some cats even with no ‘mom’ milk supply van survive to adulthood. A human baby exposed to nature alone has zero chance.
Also also…this ain’t even getting into placental, marsupial, or the friggin platypus, echidinae, or naked mole rat insanity of *mammal-ness*
Gestation is set by guides based on habitat and mechanical stuff as well as enviro-social developments (eg elephants, primates, whales raise the youth together versus certain mice where dad leaves to but some cigarettes and never comes back).
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.
Marsupial young are born very early. They are a more primitive type of mammal and go to the pouch to continue development. Other mammals are at various developmental levels at birth. A deer can walk minutes after birth. A human will take around 9 months to a year to walk after birth.
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