5 The Shortest Pregnancy
To the humble (and rather funny-looking) opossum goes the title of the shortest gestational period of the entire mammal class. (We are talking about both the common American Opossum and its southern relative, the Yapok of South and Central America, for the record, not the possums of various Pacific regions.) The opossum is a marsupial so much of its offspring’s development comes in the mother’s “pouch,” but as for the actual pregnancy, a newborn opossum typically emerges after 12 days in utero. Sometimes they require fewer than 10.
4 The Longest Pregnancy
Elephant pregnancies last more than 240% longer than the gestation period of a human mother. That’s right: elephants carry their offspring for twenty two grueling months. That epic gestation affords the calf ample time to develop both its body and brain while in utero, which makes the newborn elephant far better equipped to deal with the outside world than other newborn mammal species. In many cases, within hours of birth a calf can use its trunk to find its mother’s breasts and can even begin to perceive the social structure of its herd.
3 The Largest Gestational Spread
Expectant sperm whale parents have to have their newborn announcements ready after around sixteen months of pregnancy, but they might not be mailing anything out to all their whale friends and relatives until as many as twenty months have elapsed. While in human beings, an OBGYN won’t let a woman go much more than a week past her due date, a sperm whale’s average gestation is all over the map, sometimes matching the giraffe, sometimes rivaling the elephant!
2 The Egg Layers
The platypus and the echidna are the most common monotremes, a clade (or subclass) of mammals that buck the live birth trend and instead lay eggs! Now, these Australian animals are still very much considered mammals, what with their milk, their hair, their inner ear bone structure, and so on, but their gestation and natal periods sure stretch the boundaries of class definition. Monotreme pregnancy lasts about three weeks, after which the mother will lay a small clutch of eggs. She then incubates these eggs with her body heat for another week or so, then nurses her hatchlings just like any other mammal mother.
1 The Strangest Gestation of All
Giant pandas are complicated creatures when it comes to reproduction. And from a propagation point of view, they’re kind of bad at it. First of all, the females only have one “estrous” period a year, a few day stretch during which conception is even possible. The panda embryo will frequently undergo delayed implantation, wherein a fertilized zygote does not implant on the uterine wall, thus beginning development, for long after conception has occurred. Thus while a panda’s fetal development period is always around the same length of time, the mother panda may give birth anywhere from 95 to 160 days after mating! And one more note: largely due to the low nutritional value in the panda’s bamboo diet, a panda fetus does not develop nearly as much as most other mammal species, and is born proportionally smaller than any other non-marsupial mammal; a newborn panda usually weighs less than half a pound, while a full grown panda often weighs well over 200 pounds.