It takes a full 12 months for the fetus to develop into a fully grown baby dolphin. While the fetus is inside the uterus of the mother, its tail flukes and its dorsal fin remains folded and soft until the time of birth. Two weeks after birth, the fin begins to stiffen.
Kangaroos usually give birth to one joey a year. After they are born, they crawl into their mother’s pouch and attach themselves to one of her 4 teats. After a few weeks, the young kangaroo spends more and more time out of the pouch until eventually leaving for good at seven to ten months of age.
Mating season for bats occurs in the late summer to early autumn and the female bats store the male’s sperm until the following spring. Their gestation period lasts anywhere from 40 days to six months and they only give birth to one pup.
The opossum gives birth only 12-13 days after mating, which is the shortest gestation period of all of the mammals in North America. They can give birth to up to 20 babies at a time and newborn opossums are usually the size of a grain of rice.