Kangaroo

The front arms of kangaroos look small compared to its powerful hind legs. At birth, though, the tiny kangaroo will use those front arms to pull itself from birth canal to mother's pouch.
Thorsten Milse/Robert Harding Picture Library/AGE Fotostock

Think you know all about kangaroos? They have a pouch and get around by hopping. That's it, right? Think again.

The kangaroo pouch is necessary to provide shelter for their tiny, immature young--born blind, hairless, and at a stage when placental mammals would still be fetuses growing safely inside their mother's womb.
 
Red kangaroos can have babies in three different stages of development all relying on mom at the same time. An inch-long (2.5 cm) newborn receives one type of milk from a nipple inside the pouch while a larger, older sibling drinks from a nipple that supplies milk with a higher fat and protein content. In the meantime, a fertilized egg can be kept in suspension in the womb, ready to start growing when one of the other babies stops nursing.
 
Strangely enough, that journey from birth to pouch as a youngster is thought to be the reason marsupials like kangaroos are restricted to locomotion on land.
 
Even though marsupials are tiny and immature at birth, their front arms must already be developed enough to pull them from the birth canal to the pouch. At that point, it's either eat or die. The need for strong front arms at this stage of development is thought to have prevented the evolution of flippers and wings--which is why there are no flying or ocean-dwelling marsupials.