Giant pandas lack the genes necessary to completely digest bamboo.
It is commonly known that giant pandas maintain a diet of almost strictly bamboo.
However, recent research has revealed that they are not physically capable of fully digesting this plant.
Geneticists have successfully sequenced the Panda's genome and have learned that pandas lack the genes necessary to digest bamboo.
At the genetic level, pandas are actually bears and they are carnivorous, but their actual diet is almost exclusively herbivorous.
One explanation for this is that mutations in the pandas' genes may have caused them to lose the ability to taste meat.
As for why they eat bamboo when they can't digest it?
While their bodies can't completely digest bamboo, scientists theorize that the digestion may be taken care of by microbes in it's digestive tract.
Despite all that, apparently pandas can digest cake better than I can. :(
(source)
