A few facts about Kangaroo reproduction:
- Joeys are 'born' basically in a fetal state. The blind, furless, miniature newborn, the size of a jelly bean, crawls across its mother's fur to make its way into the pouch, where it latches onto a teat for food.
- Interestingly enough, newborn joeys are not even developed to the point where their Superior region (head) and Inferior region (tail end) are differentiable. Thus, the joey is capable of latching onto its mother's teat with either its "mouth" or "anus", which are actually identical openings connected to a simple two-way digestive system. Whichever end latches onto the teat becomes the head and thorax, the other becomes the abdomen. The lack of differentiation can be clearly seen in this photograph:
This remarkable physiological phenomenon is is possible through the kangaroo's unique embryological development. The joey possesses two clusters of nerve cells (one at either end of its body), one of which becomes the brain and the other the spinal cord, which develops after the spine itself does. Each cluster is connected to 2 proto-limblets, which will develop into front or back legs, depending on whether the cluster in question becomes a brain or spine. - Occasionally, both ends of the newborn joey will latch onto their own teat. The resulting double-headed joey (or "dubby") generally dies within a few days. Extremely rarely the dubby will survive and develop for several weeks, resulting in a grotesque animal that is its own mirror image:
