A female field mouse is able to mate two to four times a year and, after a short gestation period, give birth to up to nine babies, which are blind, pink, and completely hairless at birth. These "pinkies," which are usually only a few centimeters long, will be completely weaned and able to leave the nest just 21 days after birth, and females can have their first litter at the age of thirteen weeks.
Kangaroos are the world's largest marsupials, but at birth are about the size of a cherry. Born blind, hairless, and less than an inch (only a few centimeters) long, the joey immediately climbs into the mother's pouch to nurse, and remains there for up to six months. The joey will spend a few weeks just poking its head out of the pouch, and then progress to spending increasingly more time in the outside world. It won't be until almost a full year after birth that the young kangaroo will leave the pouch for good.
These tiny owls are smaller than a pint glass as adults. Screech owls are generally monogamous, mating for life. They nest in hollows of trees, and often recycle the nests of other animals. In the nest, screech owl hatchlings fiercely fight each other for food and resources, often killing the smaller, weaker siblings.